Problems

As a foreigner, you may run into problems with housing or your landlord that are not easy to resolve. Here are some of the most common problems and some info on what you can do.

Scams when looking for apartments

In some large cities, scammers try to take advantage of foreigners’ ignorance of how the German rental system works to scam them out of money. Here are some signs that indicate that an apartment listing is a scam:

Price is too good to be true
Photos of the apartment include typically American features such as window air conditioning units, ceiling fans, sash windows, or American-style electrical wall sockets.
The landlord offers to rent the apartment without you having seen the apartment, and without meeting you.
The apartment is offered on Craigslist. Craigslist is not used in Germany.
The landlord promises that you will get the flat immediately after you contact them, and requires some sort of deposit to show that you are “serious”. German law is very much on the side of tenants in case of conflicts; therefore landlords try to choose their tenants carefully and will not pick you as their one and only candidate, based on a short message in which you expressed your interest in the place.
The landlord is “out of the country”, and offers to send you the keys by mail once you’ve paid the deposit. Sometimes the landlord claims that a trusted third party such as AirBnB will act as “escrow”. Any emails from “AirBnB” you receive will be fake; any payment address you are given will be the scammer’s.
The landlord claims that they will travel far to show the apartment to only one person (you). Therefore you are required to pay a deposit to show that you are “serious”. In reality, landlords or their agents show a place to as many people as they can schedule, sometimes in large groups. They pick from those applicants according to their instincts and the information applicants give on their personal and financial situation. In most locations, landlords will have many (dozens to hundreds of) applicants for a single flat. They do not need (or want) to “sell” their flat to one particular tenant whom they selected in advance.
The process involves you making payments via Western Union, cryptocurrency, or other services uncommon in Germany where you cannot recover your money or trace who received it. Sometimes these payments are made by you to a family member or vice versa; the “landlord” merely demands a picture of the receipt - the data on the receipt will enable the scammer to withdraw the money.
The contract is in English.
The landlord claims to be a native German or English speaker, but their grammar and spelling do not support that claim.
The landlord offers to use some kind of escrow service to hold your money until they have signed the contract.

There have also been recent reports about a scam where a scammer rents an apartment on AirBnB and then tries to rent it to you - in person, with a written rental agreement and with the keys. This scam is harder to detect, but warning signs include:

This scam only applies to fully furnished apartments which is the first yellow light since more than 99% of legitimate apartments for rent in Germany are not fully furnished.
The scammer does not want to show you their Ausweis (German ID card) when you ask for it, the real landlord would have no problem with that - yellow light
If the scammer claims to work for an agency but the apartment is not advertised on the website of the agency, the scammer has no business card or corporate email address - red light
You ask to pay the deposit (Kaution) in three monthly installments (which is your right as a tenant in Germany) but the scammer wants the full deposit now - yellow light
You offer the landlord that you will buy deposit insurance (Mietkautionsversicherung) and give him the certificate from the insurance company. This allows the landlord to get the money from the insurance if there is any damage to the apartment and it allows you to not pay any deposit. Landlords are not required to accept this insurance instead of a deposit payment but if the landlord accepts the insurance then it is guaranteed to be not a scam since scammers are out to get the deposit.
The scammer asks for cash - red light
With the majority of legitimate apartments you get at least two sets of keys but the scammer has only one set of keys - yellow light
Check on AirBnB if the apartment is on offer there - red light if it is
The scammer asks to pay the money to a foreign bank account (IBAN does not start with DE) - red light
The scammer does not want to see your Schufa (German credit rating) or proof of income - dark yellow light
You seem to be the only person interested in the apartment so that the scammer offers to rent it to you immediately - yellow light
A fully furnished apartment should be more expensive than other apartments on the market but somehow the price for this apartment is just - too good to be true

How to avoid scams

Never pay any money until you have seen the apartment in person, have signed the rental contract, and have the keys to the apartment in your hand. That is how renting an apartment works in Germany. Everyone who asks you to pay earlier is either trying to scam you, or they are a real landlord unaware of tenants’ rights, or they are a real landlord aware of tenants’ rights but hoping you are unaware. Neither of those options are good.

Moving in and out

The Übergabeprotokoll (a description of the state of the apartment when you move in or out) is very important. Don’t skip it, and take your time filling it out with your landlord. Don’t let the landlord fill it out by themselves, and make sure you agree with all of it before you both sign it (and you both get a copy of it). If the landlord seems reticent, insist - if necessary fill it out yourself and present it to them for their signature (here is a example template).
When moving out, the Übergabeprotokoll must record all issues/damages which the landlord will pay for using your deposit. Therefore, make sure that all such issues are recorded correctly - this prevents the landlord from later claiming to have discovered “additional damages” which they will let you pay for (using your deposit). An agreed Übergabeprotokoll is also very important in getting your deposit back on time (see below).

Notice periods for moving out

The standard notice period for leaving an “normal” (indefinite) apartment/house lease is 3 months if the tenant wants to move out (it’s longer if the landlord gives the tenant notice). Some immigrants are caught out by this - they want to move out earlier, and are shocked when their landlord demands that they pay the rent until the end of their notice period. Therefore, always read your contract carefully, and observe the notice periods.

Notably, you can’t bypass the notice period even if you have to unexpectedly move due to finding a job or a family emergency, for example. While there is nothing stopping you from moving out earlier, you’re still liable for the rent until the end of your notice period.

If you do need to move out earlier, you can try to reach some kind of compromise with your landlord. For example, you could find a replacement tenant to take over the apartment, you could sublet the apartment until your notice period is up, or you and the landlord could agree to shorten the notice period due to exceptional circumstances. However, to be clear: the landlord is not obliged to accept any of these proposals (it’s a common misconception that landlords have to accept a replacement tenant if you show them at least three candidates; they do not).

Additionally, in many larger cities leases include a clause prohibiting a tenant from giving notice at all within the first (usually) 1-2 years of the lease. As above, there is almost no way out of these arrangements, so read your lease carefully before you sign - you are expected to abide by what you sign (as is the landlord, obviously).

Time-limited leases are almost impossible to get out of ahead of time (for either tenant or landlord), so keep that in mind before you sign.

Getting your deposit back

After you’ve moved out of your apartment/house, the landlord must return your deposit (minus any costs for damages, unpaid bills etc). However, this can take some time - so you should expect this, but also know your rights in case the landlord takes too long to return the deposit.

TL;DR: You should get around 80% of your deposit back within about 2-6 months, and the remainder back after about 18 months.

Now, in detail. The deposit is usually returned in two steps:

The first step accounts accounts for all the damages, unpaid rent, and other unpaid bills related to the apartment. The tenant and the landlord agree (in the Übergabeprotokoll - see above) which damages are attributable to the tenant, the landlord then repairs these, deducts the appropriate amount from the deposit, and returns most of (see below) the deposit to the tenant. This usually takes between one to three months (e.g. the time it takes for the landlord to hire a painter to repaint a wall, for the painter to bill the landlord for their work, for the landlord to deduct that amount from the deposit, and to return the rest), but no more than six months (as, by law, the statute of limitations for open claims related to a rental property is six months).

Obviously, if the Übergabeprotokoll shows no damages attributable to the tenant, and no unpaid bills, then the tenant can insist that the landlord return this part of the deposit sooner. This again undelines how important it is to fill out and sign an Übergabeprotokoll with your landlord when you leave, and immediately get a copy.

The second step is to account for the utility bills for the last billing period. Recall that the tenant pays an estimate of their utility bills to the landlord every month (the Nebenkosten), and that, once a year, the landlord gets all of the actual bills, compares the actual costs to the pre-payments (the Nebenkostenabrechnung), and sends the tenant a bill or a refund.

The billing period varies (but is usually January-December), but the bills for a billing period often only come nine to twelve months after the end of the period. So if you left an apartment in (say) February 2021 with a Jan-Dec billing period, then the final bills for that period would only arrive in late 2022 (about one year after the end of the billing period, which ended in December 2021), and only then could the landlord determine whether your utility pre-payments for 2021 were too low or too high - even if that’s nearly two years after you left the apartment.

Therefore, after the landlord has returned most of your deposit (step 1, see above), they still have the right to keep a small amount to cover any potential utility costs. They can’t choose this amount arbitrarily; they must justify it in some way (e.g. extrapolating from previous Nebenkostenabrechnungen). Once the landlord gets all of the utility bills, they can do the Nebenkostenabrechnungen for your final billing period, deduct any open claims from the rest of your deposit, and send you the rest (or send you the rest with a refund for any money you overpaid in utility pre-payments).

This second step assumes that you were renting a “normal” apartment. Some short-term apartments are rented “utilities included” (Warmmiete) - in this case there is no need to wait for the Nebenkostenabrechnung for your final billing period, and you can expect to have your deposit fully returned after the first step.

If your landlord doesn’t return your deposit in time, and the deadlines explained above have passed, then you should first send them a letter (not an e-mail), through registered mail (Einschreiben), giving them a deadline for returning your deposit (e.g. two weeks). Make it clear in the letter that should they fail to do so, you will take legal action against them.

If they still don’t reply (or fail to return your deposit), then you need to hire a lawyer, specialised in Mietrecht, to enforce your demands. If you were a member of a tenants’ association (Mieterverein/Mieterbund, see below), then you should contact them for advice first. Note that your claim to have your deposit returned lapses after the end of the third year after you moved out of your apartment (e.g. if you moved out in February 2021, then it lapses on the 1st of January 2025), so make sure to act before then.

Mieterschutzverein

Most large-ish cities/towns have a Tenants Rights Protection Office (the German titles vary. Google for one of “Mieterbund”, “Miterschutzbund”, “Mieterverein”, “Mieterschutzverein” + your city). A Germany-wide search option for the German Renter’s Association is here (you can search by postcode for the next office - “Mieterverein vor Ort”)

If you plan on renting an apartment/house in Germany it’s highly recommended to become a member there. The annual cost is very low benefit - especially in case of problems with your landlord - can be great. They offer advice and help in legal matters concerning your rental situation and have lawyers specialised in German Renter’s Law (“Mietrecht”) at your disposal.

Multiple tenants

As explained on the main housing wiki page, there are three ways in which several tenants can rent an apartment (e.g. a WG, but this could also apply to a couple moving in together) together:

All tenants sign a single contract with the landlord.
Every tenant signs their own contract with the landlord.
One tenant signs the contract with the landlord, and all other tenants sign a contract with the first (“prime”) tenant.

(1) is by far the most common (and the easiest for the landlord), but it’s also the most fraught with difficulties for the tenants. In this case, these people are jointly liable for all their obligations as tenants (e.g. paying rent, keeping the apartment in good order, etc). This means that, should one tenant (among several) renege on these obligations (e.g. stop paying rent), then the other tenants have to make up the difference. Additionally, a rental contract can only be terminated for all tenants at once - therefore, if one person in a WG (or one half of a couple who have had a falling out) wants to leave an apartment with multiple tenants, then every other tenant and the landlord must agree (and they often don’t) to amend the contract. Therefore, it can be very difficult to get “out” of such an arrangement, particularly at short notice. It’s also not uncommon for one tenant to simply “disappear” (i.e. they pack up their things and leave, presumably abroad, and stop paying rent), leaving the remaining tenants on the hook for the entire rent.

Therefore, if you’re looking at a WG with contractual arrangement (1), make sure to clarify as many aspects as possible before signing, ideally in writing, including:

How is the rent split between the different tenants?
How are the running costs split? This includes both the Nebenkosten paid to the landlord, and the costs which the tenants pay directly to the provider (e.g. internet).
Who is responsible for paying when something is damaged?
What happens when someone wants to leave? Who is responsible for finding a replacement tenant?
Who gets what part of the deposit back when they leave? How is this handled if not everyone leaves at the same time?

Here is an excellent example of some of the issues which can arise with joint leases.

Additionally, irrespective of what contractual arrangement your WG has, the Rundfunkbeitrag (“TV tax”) also causes friction in some WGs. Since it’s payable once per apartment, deciding who pays on behalf of the entire WG, and how the cost is split internally (and sometimes you have a flatmate who refuses to pay outright), can lead to conflict if you don’t agree with everyone else on how to handle it.

You’ll also want to join the Mieterschutzverein (see above), as there is a low but nonzero chance that you may have to involve a lawyer at some point (or at least you’ll want to have someone to turn to for legal advice).

Problems with neighbours (noise)

If you live in an apartment building (or otherwise in close proximity to other people), then you should be aware of the noise rules in Germany. Generally, “quiet periods” are 22:00-06:00, and all day Sunday - although these can vary, so check the Hausordnung for your building and/or the noise ordinances for your city.

During this time you may only make noise up to Zimmerlautstärke, which roughly translates to “the noise should stay within your apartment”. However, unavoidable noise is always allowed, and this includes:

Flushing the toilet
Having a shower
Walking around
Running a washing machine
Speaking at conversational volume
Children playing (noise from children playing has to be really excessive for it to be grounds for a complaint - German laws are very protective of childrens’ right to play)

Sometimes apartment buildings are so poorly designed that even these “normal” activities generate a lot of noise. There isn’t much you can do about this, other than ask your landlord to add some noise insulation (which they are, frankly, unlikely to do).

Other kinds of avoidable noise are forbidden during these “quiet hours”, including

Playing a musical instrument or turning a stereo up too loud
Conversation levels which go beyond “normal” (i.e. yelling and shouting)
Running and jumping (except for children, see above)
Drilling or other kinds of DIY (yes, even if you only just moved in)
Mowing the lawn or otherwise using outdoor power tools
Parties (a common myth is that “one party a month is allowed” - that’s not true)

Note that outside of these “quiet hours” there is essentially no limit on noise - so you can’t demand that your neighbours not practice the trombone at 2 in the afternoon while you’re working from home.

If a neighbour disturbs you by making a lot of noise or repeated avoidable noise (drilling at 23:00, party/music deep in the night, yelling in the communal staircase…) then you can demand that they stop.

As always when dealing with humans: First, try to talk to them, depending on the noise, they might not be aware they are causing a disturbance. If they ignore you, or otherwise don’t listen to you, you can call the police, but they will not treat it as an emergency, and possibly have better things to do. Still, loud parties until dawn every second day - they will come. If they have to come repeatedly to the same address in the same night, they will take action.

If this doesn’t work either, you should contact your landlord and make it his responsibility. You can demand he acts on your complaint in a reasonable time and otherwise you can lower the rent (“Mietminderung”). This is a delicate process however, so before you bring out the big guns, try to get your landlord to cooperate through less confrontational means. In the meatime, start a “Lärmprotokoll”, writing down when/what/how long a noise disturbance happened. Ideally with witnesses (e.g. neighbours on the same floor that get disturbed as well, friends you invite over when you know there will be the usual Thursday night techno festival upstairs, etc). This makes it so that it’s not a he-said-she-said situation.

When you’ve got solid written evidence, you then ask your landlord to effect changes in the offending neighbours behaviour within 2-4 weeks. Use this time and get a consultation at your Mieterschutzverein (see above). No member yet? It’s about time to become one. They can tell you, what your next steps are, how to word the second letter to your landlord (providing he has not taken action) and by how much you might be able to lower the rent. Also they know all about what is an actionable noise “offence” and what you have to live with.

Locksmiths

If you lose your keys (or forget them inside your apartment), then you often have no choice but to call a locksmith (Schlüsseldienst) to open the door for you. A simple Google search will turn up a dozen services, who will promise to turn up quickly and open your door for you. However, when they do turn up, you’re often confronted with an outrageous bill (close to a thousand Euros is not unheard of), and companies using very shady tactics to pressure you into paying on the spot, in cash. To be clear: these tactics are illegal, but getting your money back through legal means can be a long and drawn-out process.

To avoid becoming a victim, you should:

Deposit a copy of your apartment key with a trusted friend, preferably one who lives nearby. Note that, by German law, your landlord may not have a copy of your apartment key (unless you specifically authorise them to).
Soon after moving in (and before losing your keys! :) ), ask around to find a trustworthy locksmith in your area. Ask your neighbours, your landlord, or the Hausmeister of your building. Save their phone number, so that you have it with you in case you need it. Consider visiting their shop ahead of time.
If you do find yourself needing a locksmith, and didn’t research one ahead of time, avoid the top results in your search engine at all costs, particularly any sponsored links. Scroll further down to find a trustworthy provider (you may have to scroll through several pages of results). Unfortunately there is no easy way to tell a trustworthy website from a less trustworthy one, but one tip is to look for a business which does more than open locks (i.e. they also sell locking systems).

The German Verbraucherzentrale (consumer advocacy association) has tips about how to find a reputable locksmith.

Here are a few first-hand experiences:

Air conditioning

Maybe you come from a country where indoor temperatures are controlled through brute force (i.e. air conditioning)? This is simpler for the end user, but also extremely energy intensive. For instance, air conditioning in the US consumes 240 TWh of electricity per year, which is about half of the total electricity produced in Germany (484 TWh).

This isn’t the case in Germany; indoor thermal control in German housing is different than, say, American housing. Instead of a single brute-force approach you have to use several levers at your disposal to control your indoor temperature and improve your comfort. This also means that you won’t achieve the same effect of always having exactly the same indoor temperature, irrespective of whether it’s boiling or freezing outside - so you’ll have to get used to a broader range of indoor temperatures. This alone will take some adjustment if you’ve been living in air-conditioned apartments all your life. Also, it’s a bit more “work” for you.

The upside: massively reduced energy consumption, a lower energy bill, and knowing that you’re not wrecking the planet (even more than you already are) for your descendants.

Some of the “levers” you can use:

Open your windows during the times of the day and night when it’s coolest, and shut them completely otherwise. This is generally the early morning, especially for a west-facing apartment. If it’s feasible, leave them open from late at night until morning if you can. Ignore the “kipp” (half-open) setting on your window - open or close them all the way.
Consider getting an indoor-outdoor thermometer. You should then open the windows whenever the outdoor temperature is cooler than the indoor temperature, and close them otherwise.
Shade your windows when the sun shines on them. Close the curtains (or, even better, shutters if you’ve got them). Deploy (open) any sunshades or awnings you may have.
Consider installing reflective foil on the windows.
Use a fan to move the air around.
Wear light, loose-fitting clothes if you can.
Drink plenty of water.
Take cool showers, and keep your skin and hair moist afterwards.
Turn off as many heat sources as you can. For example, a powerful PC or a large TV can easily put out several hundred Watts of heat, which will raise the temperature of a room by a few degrees. See if you can avoid baking a cake or preparing a big meal in the middle of the day.
If you think you’ll be living in that apartment for a while, and it gets uncomfortably warm in the summer, talk to your landlord about installing sunshades or shutters (Rollläden) to shade your windows from direct sunlight. In extreme cases you may also be able to lower your rent unilaterally to “force” your landlord to install such measures, but this is generally a last resort, and you should definitely consult with a lawyer (as well as trying to resolve things amicably) before breaking out the big guns, so to speak.

Since handling the warmer months almost always involves leaving windows open for longer periods of time, bugs may get into your house/apartment. While Germany doesn’t have anywhere near the variety of ferocious flying insects that some other countries have, this can still get annoying. The simplest solution (especially for tenants) is to install insect screens using velcro strips (such as this example - although there are obviously plenty of others to choose from). These are cheap, easy to install, and easy to remove when you move out. Fixed or hinged insect screens are rare.

You can find “evaporative cooling fans” (they go by various names) in shops and online. I would urge caution - while these may lower your temperature by a few degrees, they will also increase the humidity, which may make you even less comfortable, as your body won’t be able to cool down as effectively through sweating.

While it’s possible to install (portable) air conditioning in some German apartments, it’s generally a lot of hassle, as the buildings aren’t designed for it. You’ll have to find some way of piping the hot exhaust air outside, and unless you drill a proper hole through an exterior wall (which creates a whole host of other problems, particularly in winter), all of the “DIY” solutions (e.g. window kits) will probably let in a fair amount of hot outside air, which means your A/C will have to work even harder to keep your room cool. Plus, the high cost of electricity in Germany means you’ll be paying through the nose for a slightly lower air temperature (a room-sized A/C unit probably costs around 100 € / month in electricity). In most cases it’s simply not worth it.

It’s also important to note that German houses tend to be built of material with high thermal mass (e.g. stone, concrete), and are extremely well insulated, so that (provided you don’t open the windows at lunchtime!) they are much slower to warm up when it’s hot outside than houses made of lighter materials (such as wooden American houses). While even the best-insulated house will eventually warm up if it’s constantly hot outside, by that time in Germany the heat wave will usually have passed.

Some threads with more information:

Humidity, mold, and window condensation

German houses and apartments are, in general, very well-built, with well-insulated walls and high-quality windows and doors. This keeps the heat inside, your heating cost low, and reduces your environmental footprint. But it also encloses the humidity you produce by breathing, taking a shower, cooking, or drying clothes. You have to regulate the humidity, otherwise it will condense on your walls or windows, and sooner or later you have mold (Schimmel) growing there. Mold can be fairly dangerous - the spores released by mold can cause asthma and other respiratory diseases. People who have not lived in colder climates before, who are used to houses which leak far more than German houses, or who come from countries where constant air-conditioning is the norm may not know how to properly regulate the humidity inside their house/apartment.

Note that while very modern German houses and apartments often have a central ventilation system (which ensures a regular exchange of air and takes care of the problem for you), most do not.

If you discover mold in your apartment, then you should immediately notify the landlord. However, it’s more than likely that the mold is caused by you (the tenant) incorrectly ventilating the apartment, so there isn’t much the landlord can do. You should therefore remove the mold (using fairly aggressive chemical sprays you can get in many shops - make sure to read the instructions, as they can cause nasty skin irritations), but you absolutely need to combat the underlying causes of the mold growth, otherwise it will just come back.

Proceed similarly if you have excessive condensation on the insides of your windows. While this can also be caused by (or at least exacerbated by) insufficiently well-insulated windows, the root cause is more commonly high indoor humidity. While condensation on windows isn’t usually a problem per se, water can leak into the floorboards (damaging them and causing them to warp), and mold can grow on the silicone window insulation surrounding the windowpane. As a stopgap measure, put some rolled-up towels under the windows to catch any water run-off. If you have outside shutters, then closing them may also help, as they improve the window’s insulation.

Simple instructions

To properly regulate the humidity in your apartment and avoid mold growth:

Open all windows in your apartment fully for at least 5-10 minutes, twice a day. Open the windows fully while doing this - don’t just use the part-open Kippstellung. 2-3 minutes is usually enough in winter; in summer, you may need to leave them open for 15 minutes or so. Ideally open windows on both sides of the apartment, so that the air can pass all the way through - if your interior doors are being slammed shut by the air currents, then you’re doing it right. The German term for this is Stoßlüften, and it’s very important that you do this all the time, even if your apartment doesn’t have mold.
Don’t leave windows open slightly (Kippstellung) for extended periods of time. Leave the windows closed if you’re not actively ventilating (see above).
Fully open the window in your bathroom and kitchen for at about 5-10 minutes after showering, bathing, or cooking. If your bathroom doesn’t have a window, make sure the extractor fan runs for long enough after you’ve finished showering/bathing (at least 15-30 minutes).
Don’t turn down the heating in rooms you’re not using (including leaving for work during the day) more than a few degrees at worst.
If possible, avoid drying clothes indoors (or, if you do, ventilate even more intensively than usual).

Longer explanation

For anyone who isn’t satisfied with the explanation above, here is a longer one.

Causes of mold

Broadly speaking, mold grows on walls when:

The relative humidity of the air at the wall is above 80%, and
The air temperature at the surface of the wall is above 0°C , and it grows fastest between about 20 and 35°C.

The humidity in the air comes from two main sources:

From within the wall, i.e. from a source of water on the other side of the wall. This is usually a leaky pipe or a flaw in the roof (or other rain protection) in the building, through which water can infiltrate the wall. If this is the cause, then it’s (broadly speaking) the landlord’s responsibility to fix the problem.
From the occupants of the apartment and their activities. This is, by far, the most common source of humidity, and it’s the responsibility of the tenant(s). The rest of this explanation will focus on this second cause.

Absolute and relative humidity

Air can hold a certain amount of water vapour, and the warmer the air, the more water vapour it can hold. For example, at 20°C, 1 m³ of air can hold up to 17 g of water vapour, but at 15°C, it can only hold 13 g. 20°C air with 17 g/m³ of water vapour is said to be “saturated”, or to have a relative humidity of 100%. Similarly, 20°C air with 8.5 g/m³ of water vapour is said to have a relative humidity of 50%, as it’s holding half as much water vapour as it theoretically could at that temperature. Typical indoor air has a relative humidity of 40-60%. (Yes, this is an oversimplified explanation of relative humidity, but it’s good enough for the purposes of this discussion).

When air is cooled, the amount of water it can hold decreases. So if air at 20°C containing 13 g/m³ of water vapour (13/17 = 76% relative humidity) is cooled down to 15°C, its relative humidity increases to 100%, as it’s now containing as much water vapour as it possibly can. If it’s cooled further, some of the water vapour in the air will condense, i.e. it will go from a gas to a liquid, either staying as fine mist in the air, or (more likely) forming small droplets of water on a nearby surface.

Sources of water vapour

Many activities add water vapour to the air of an apartment. As German houses and apartments are generally very air-tight, the water vapour cannot escape outside, and so it builds up in the indoor air. Eventually, the air in the apartment becomes saturated (i.e. it is carrying as much water vapour as it possibly can, given its temperature), and the water vapour condenses on nearby walls, where the moisture makes for an ideal breeding ground for mold.

Additionally, outer walls are (particularly in winter) colder than the interior air, as is the air right next to them. So even if the air in the middle of a room (at, say, 20°C) still isn’t saturated, the air near a wall (which could be at 16°C) could very well be. Spots in the wall with less insulation than their surroundings tend to be the first spots where mold forms, as they are the coldest in winter, and therefore the place where water vapour condenses first.

The main contributors to indoor humidity include:

People breathe out water vapour with every breath. The more active they are, the more they breathe out.
Drying clothes on drying racks.
Cooking.
Showering or bathing.

As the examples in this thread show, two people simply breathing in an apartment can bring the air in the apartment to saturation in about a day, and that’s not accounting for showering, cooking, or drying clothes.

Stoßlüften

The principle behind Stoßlüften is to exchange as much of the humid, indoor air with dryer, outside air as possible. This is particularly effective in winter - the cold outside air can hold very little water vapour, so when it’s warmed to indoor temperatures it’s very dry (i.e. it has very low relative humidity). For example, fairly saturated winter air (0°C at 90% relative humidity) only holds 4 g/m³ of water vapour. When warmed to 20°C, the relative humidity drops to 25%.

Many people don’t ventilate in this way, as they are worried about driving up their heating bills. It’s true that the (probably colder) outside air has to be warmed up again after Stoßlüften. However, air has a very low thermal capacity - i.e. it doesn’t take much energy to heat up air. As an order of magnitude, warming up all the air in a 60 m² apartment from 10°C to 20°C takes about 0.5 kWh of energy, which costs around 0.03 €. Most of the thermal energy in a building is in the walls - therefore, while there is a bit of heating energy lost when ventilating, the losses are comparatively small. To save a bit of extra energy, turn all the radiators off (to the “frost” symbol) before opening the windows, and turn them back on again afterwards. The warm walls (and the heating system) will warm up the indoor air in a short time.

Therefore, you should ventilate for long enough for the relative indoor humidity to drop below about 40-50%. This will happen faster in winter, but take longer in summer. Opening windows on both sides of the apartment accelerates this process. However, you don’t want to leave the windows open so long that the walls start to cool down (which need a lot more energy from the building heating system to warm up again). A portable hygrometer (here is an example, but there are many different models on the market) can be very handy here. A hygrometer can also be used to know when it’s time to ventilate - as a rule of thumb, open all the windows when the relative humidity indoors exceeds about 60% (recall that mold needs about 80% relative humidity to grow, and the walls may be a bit colder than the indoor air).

Kippstellung

It may seem tempting to leave your windows half-open (the so-called Kippstellung, where the top of the window is tilted slightly inwards, letting air in) for extended periods of time. However, this is a bad idea, particularly in spring, autumn, and winter, when the outside air is colder than the indoor air.

Firstly, while leaving several windows half-open does exchange enough air between the outside and inside, reducing the humidity levels of the indoor air, it’s an inefficient way of doing it, and it costs a fair amount more in heating energy when compared with Stoßlüften. The constant stream of cold air must be constantly offset through additional heating, increasing your energy use and heating bill (whereas with Stoßlüften you only use a little energy a few times a day, see above).

Secondly, and more importantly, leaving windows half-open actually increases the risk for mold. The constant stream of cold outside air passes over the wall right above and next to the window. This cools down this section of the wall considerably (and does so from the inside, irrespective of how well insulated the wall is). The moment you close the window, the humidity in the indoor air condenses on it (as it’s easily the coldest part of the room), making it a prime candidate for mold growth.

Therefore, use the Kippstellung only sparingly, and only when the outside air is relatively warm (e.g. in summer).

Extraction fans

Interior rooms without windows where water vapour is produced (primarily bathrooms) always have an extraction fan. These fans extract the air from the room (and with it, the water vapour it contains) and dump it outside. The best models operate with a humidity sensor (i.e. they turn on when the humidity in the room is too high, and turn off once the humidity levels have returned to normal), but most operate on a timer, or are turned on and off with a switch.

Showering or bathing produces a lot of water vapour, and if that water vapour distributes itself throughout an apartment or house, it can easily raise the humidity by double digits. Therefore, always shower or bathe with the bathroom door closed, and fully open the bathroom window when you’re done. For interior bathrooms, make sure the extraction fan runs for long enough to get all the water vapour out - when you’ve finished showering, turn on the extraction fan (if it’s not already running), and leave the bathroom door slightly ajar. The extraction fan probably has to run for longer than you might expect - 15 to 30 minutes is not unusual. Most fans on a timer only run for a few minutes, which likely isn’t enough. Again, you can use a portable hygrometer to get an idea of how long it needs to run for.

Extraction fans have filters in them, and these progressively collect dirt. The more dirt they hold, the less air the fan can extract, and the longer is needs to run for - and, eventually, when the filter is completely choked with dirt, the fan may not be able to extract much at all. Therefore, once a year, check the filters inside the fan, and if they are completely clogged with dirt, inform your landlord so that he can replace them.

Not heating unused rooms

Some people turn down the heating in rooms they don’t use often (e.g. storage rooms, guest rooms). While a few degrees cooler is perfectly OK (a bedroom will always be cooler than a living room, for example), don’t turn the heating down all the way in unused rooms. If you do this, the walls of that room will start to cool down, and when you open the door to that room, the warm, humid air from the rest of the apartment will come streaming in, and will condense on the cold walls, making them prime locations for mold growth. Therefore, keep the heating in all your rooms running - you wouldn’t be saving much in terms of heating costs anyway, as the interior walls barely insulate.

Something similar happens if you leave a window open in a room (and the door to that room closed). The air in that room will cool down significantly, as will the interior walls. The moment you close the window and open its door, the warm humid air from the rest of the apartment will rush in and its humidity will condense on the frigid walls.

Dehumidifiers

If all else fails and you can’t get your indoor humidity down to acceptable levels, then you can consider running a dehumidifier (“Entfeuchter”). These come in two broad varieties: chemical and electrical. The former are relatively cheap (and silent), but they’re only useful for very small spaces, and you have constantly buy new desiccant.

Electrical ones are the most common, and they’re very effective - but they’re also loud (you wouldn’t want to sleep next to one), and absolute energy hogs, which, given Germany’s relatively high electricity prices, means you’ll be paying through the nose to run one. As an order of magnitude, a one-room dehumidifier uses around 400W and probably needs to run about half the time (depending on how much humidity you produce). That’s about 5 kWh (1.50 €) per day or 140 kWh (45 €) per month.