Mould in a Rented Flat: Rights, Duties and Securing Evidence (Germany)

A dark patch behind the wardrobe, black spots in the window reveal, a musty smell in the bedroom: mould in a rented flat is common – and it creates uncertainty on both sides. Tenants ask whether they may reduce the rent and who pays for the remediation; landlords and property managers, whether the cause lies in the building or in how the flat is used.

This article sets out how German tenancy law treats mould, shows how to report it correctly, and explains which evidence counts – for tenants with mould in their flat and for property managers who want to handle mould reports in a traceable way. It is not legal advice.

In short: report mould without delay and in writing, document it with dated photographs, and do not paint over anything before the cause is known. Whether and by how much the rent may be reduced depends on the individual case – get advice before withholding anything. A laboratory analysis of the indoor air gives you a dated, objective finding; cause and liability are for an expert to settle.

Mould found: the first 48 hours

The first few days decide how well the case can be reconstructed later. Four things should happen straight away – whether you are the tenant or the owner:

• Photograph with a date. First an overview of the room, then close-ups with something for scale (a folding rule, a coin), plus the gap between furniture and wall, the windows and the radiators. Smartphone photos store date and time – do not delete the originals.
• Do not paint over, do not scrape off. Removing the growth before it is documented destroys the evidence. Small areas can be cleaned after documentation and reporting; anything larger belongs in competent hands.
• Record the room climate. With a simple thermo-hygrometer, note temperature and humidity morning and evening for a few days, plus when and for how long you ventilated and heated.
• Report in writing. Not just a phone call: an e-mail or letter with the date, the room, the size of the growth and photographs (details further down).

What not to do: watch the patch for weeks, treat large areas with chlorine-based products, or push furniture in front of it. With mould, time works against everyone – the growth spreads, and the right to a rent reduction can be lost for the period before the report.

Who is responsible? Structural cause or occupant behaviour

Mould needs moisture, and where it comes from usually decides the case – “mould: landlord or tenant?” is first of all a question about the cause. Broadly there are two groups:

• Structural: thermal bridges, defective insulation, leaking roofs and windows, burst pipes, rising damp. These are the landlord's responsibility – the landlord must keep the flat in a condition fit for the contractual use (§ 535 para. 1 sentence 2 of the German Civil Code, BGB).
• Occupant-related: too little ventilation or heating, laundry dried indoors, furniture tight against cold external walls, many plants or aquariums. Then the responsibility generally lies with the tenant.

The two often act together: a thermal bridge only becomes a problem when little ventilation happens in winter. For older buildings the Federal Court of Justice (BGH) has ruled that thermal bridges are not a defect if the building complies with the regulations that applied when it was built; what ventilation can reasonably be expected of the tenant depends on the individual case (BGH, judgments of 5 December 2018 – VIII ZR 271/17 and VIII ZR 67/18).

In a dispute the burden of proof is staged: the landlord must first prove that the cause does not come from their sphere of responsibility – the building. If the landlord succeeds, the tenant must prove that they are not responsible for the defect – that they heated and ventilated properly (BGH, judgment of 1 March 2000 – XII ZR 272/97). Both sides therefore need robust records, not opinions.

Reporting mould to the landlord: step by step

As a tenant you must report a defect without delay (§ 536c para. 1 BGB). If you fail to, you can lose your right to a rent reduction for the period in which the landlord did not know, and you may be liable for consequential damage (§ 536c para. 2 BGB). A good defect notice is factual, complete and provable:

1. Put it in writing. An e-mail is usually enough; a letter by registered post is safer. Address it to the landlord or the property manager named in the tenancy agreement.
2. Describe what you found. Which room, which wall, how large, since when, whether there is a smell or visible damp. Stick to facts – no guesses about the cause, no accusations.
3. Attach photographs. Two to four dated pictures are enough; keep the full documentation for yourself.
4. Ask for remedy and set a deadline. Ask for a site visit and for the cause to be fixed. About two weeks for a response is usually reasonable; less for large growth or health complaints.
5. Reserve your rights. A sentence such as “I reserve the right to reduce the rent” protects your position without committing you to a figure.
6. Keep copies. The notice, proof of sending, the reply, dated notes of phone calls and site visits – in one folder.

If there is no reply, send one written reminder. After that, the tenants' association (Mieterverein) or a lawyer specialising in tenancy law is the next step – not a unilateral reduction.

Rent reduction for mould: principles and risks

If a defect reduces the flat's fitness for use, the rent is reduced by operation of law for that period (§ 536 para. 1 BGB); there is nothing to apply for. Insignificant impairments are disregarded, and for residential lettings the tenancy agreement cannot restrict this right to the tenant's disadvantage (§ 536 para. 4 BGB).

The amount depends on the individual case: the rooms affected, the size of the growth, how much the use of the flat is restricted, any health risk. German courts have accepted very different rates depending on the case – from small deductions for a minor patch in a secondary room to high rates for uninhabitable rooms. Reduction tables found online are therefore only a rough guide, not an entitlement.

The biggest risk is reducing too much. If the cause was occupant behaviour or the rate was too high, the amount withheld is rent arrears. Once the arrears reach two months' rent – or exceed one month's rent on two consecutive payment dates – the landlord can generally terminate without notice (§ 543 para. 2 no. 3, § 569 para. 3 BGB).

A mistake about the cause is no protection (BGH, judgment of 11 July 2012 – VIII ZR 138/11). The safer route: keep paying the full rent under reservation of rights and announce that you will claim the difference back. That preserves your claim without putting the tenancy at risk, and it buys time for advice.

Securing evidence: lab report and expert opinion

Photographs show that visible growth existed on a given day – not how heavily the indoor air is loaded, whether spores are reaching other rooms, or where the moisture is coming from. For securing evidence there are therefore two tools that complement each other.

What a lab report delivers

An air sample with the AirTrap XL captures 75 litres of room air on a slide in five minutes. The ISO/IEC 17025-accredited Sporecyte laboratory counts and identifies the spores (total spore count, DIN ISO 16000-20); the report arrives by e-mail within 48 hours of the samples reaching the lab. It contains:

• spores per cubic metre of air by genus – for instance Aspergillus/Penicillium, Cladosporium or Stachybotrys
• a comparison of every indoor sample with the outdoor air and an elevation rating (none, slight, major)
• images from the slide plus the date, room and time of sampling – a finding that can still be followed months later

The outdoor sample is essential: without it an indoor value cannot be assessed, because there is no legal limit value. In addition, surface tests show whether a surface carries growth or only settled dust. How to read the figures is explained in Understanding your mould lab report; whether a DIY kit is enough is covered in DIY mould test or lab.

What only an expert delivers

The lab report says what is in the air – not why. The cause is established by a mould or building-physics expert (Sachverständiger) using moisture readings inside the building element, thermography and an assessment of how the flat is used. Only such an expert opinion carries a statement on responsibility that has weight in court.

In practice: photographs and report immediately, an air sample with a complete set in the first few days as an objective baseline, an expert opinion once the cause remains disputed. The report supports the assessment of a competent person; it does not replace it.

Costs: who pays for the test, the expert and the remediation?

“Removing mould – who pays, tenant or landlord?” The answer follows the cause and is usually only certain at the end. As a guide:

• Air sample and lab analysis. Whoever orders the test pays for it first. If the cause is shown to lie in the building, tenants may in some cases be able to claim the cost as damages – that is for the tenants' association or a lawyer to check.
• Expert opinion. A privately commissioned opinion is paid by whoever commissions it. In court proceedings the court appoints an expert, and the costs are generally borne in the end by the losing party.
• Remediation. Where the cause is structural, the landlord pays for fixing the cause and for the remediation. Where the mould is occupant-related, the landlord can generally claim the costs back from the tenant. Where the causes are mixed, the costs are often shared – usually by settlement rather than judgment.

Regardless of who pays: the sooner the cause is known, the smaller the damage for both sides.

For property managers: a standard process for mould reports

For a property manager a mould report is a recurring event. Handling it the same way every time means deciding on data rather than impressions and taking the heat out of the conversation with the tenant. A process in six steps:

1. Log and acknowledge the report. Record the date received, the tenant's photographs and information; confirm in writing and offer a site visit within a few days.
2. Site visit. Visual inspection, photographs with scale, surface moisture on the building element, room climate, furniture position, windows. First impression of the cause – without committing to it yet.
3. Triage with an air sample. One AirTrap XL sample per affected room, plus outdoor air and ideally an unaffected reference room. Within a few days the lab report shows whether the indoor air is conspicuous and whether neighbouring rooms are affected – and therefore whether a minor measure, a specialist contractor or an expert is the next step.
4. Document. Report, photographs, readings, lab report and correspondence in the property file – the basis for the owner, the insurer and, if it comes to it, the court.
5. Fix the cause and remediate. Clean small areas properly; have larger damage remediated by a specialist contractor in line with the UBA guideline. Without removing the source of moisture, the mould comes back.
6. Clearance test and handover. After the remediation, a documented verification with air samples – the procedure is described in Clearance testing after mould remediation. The report goes to the tenant with a short note.

The benefit lies in comparability: the same method, the same documentation, the same report for every case – and the initial measurement later serves as the baseline for the clearance test. For managers with several properties we are happy to adapt the process – get in touch.

Frequently asked questions

May I reduce the rent immediately when I find mould?
The right arises with the defect, but the amount is unclear as long as the cause is open. Report the growth, keep paying under reservation of rights and get advice on the rate – that way no arrears build up that could later become grounds for termination.

The landlord says I ventilated wrongly. What now?
That can be resolved. The landlord must first show that the building is not the cause; only then do you have to demonstrate your heating and ventilation habits. Your records of temperature, humidity and ventilation times help here; if it remains disputed, an expert settles the building physics.

Is a photograph enough as evidence?
A dated photograph proves visible growth on one day – not the load in the indoor air, affected neighbouring rooms or the cause. Photograph, report, lab report and, if needed, an expert opinion together form a robust chain of evidence.

Do I have to remove the mould myself?
Document and report first. Small patches with a known cause can usually be cleaned by you afterwards – the UBA guideline takes the same view. Larger areas or recurring mould belong in competent hands.

Can I take an air sample myself as a tenant?
Yes. With a complete set you take one sample in the affected room and one outdoors and post the cassettes in the prepaid return envelope. The report is a dated laboratory finding that the landlord can follow too.

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Written by the AirTrap team. Sources: §§ 535, 536, 536c, 543, 569 of the German Civil Code (BGB); BGH, judgment of 1 March 2000 – XII ZR 272/97; BGH, judgment of 11 July 2012 – VIII ZR 138/11; BGH, judgments of 5 December 2018 – VIII ZR 271/17 and VIII ZR 67/18; UBA guideline on the prevention, detection and remediation of mould in buildings (2017, updated 2024); DIN ISO 16000-20. This article does not replace legal advice from a tenants' association or a lawyer.