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Mould and moisture as a construction defect: cause, proof and claims

Mould and moisture after moving in: when a construction defect exists, how the burden of proof under sec. 924 ABGB is distributed and which claims arise.

BRANDAUER Rechtsanwälte
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BRANDAUER Rechtsanwälte

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2 July 2026 · Mag. Bernhard Brandauer, Rechtsanwalt

A few weeks after moving in, dark patches appear in the corner of the room, the wallpaper bubbles, a musty smell hangs in the air. Mould and moisture are among the most distressing defects in construction, because they affect living quality and health. For those affected the question quickly arises whether this is a construction defect or due to their own behaviour.

This article explains when mould and moisture establish a construction defect, how the burden of proof is distributed and which claims come into consideration. The focus is on warranty under sections 922 and 932 of the Austrian Civil Code (ABGB), the presumption rule of section 924 ABGB and damages under section 933a ABGB.

Particularly with moisture damage the evidence position decides. Those who secure the condition early and avoid premature repairs protect their claims. From a lawyer’s perspective the dispute is almost always conducted through the expert opinion.

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01 Question 1

Is the cause of the moisture clarified?

Mould is only a symptom. Decisive for your claims is whether the cause lies in the building fabric or in the way the home is used.

All paths at a glance

Overview of all answers.

01

If the defect appears within six months, the presumption of sec. 924 ABGB helps you.

If the mould shows within six months after handover, it is presumed that the defect already existed at handover. In this period the contractor must prove the opposite. This considerably strengthens your position in enforcing warranty claims.

Secure the condition at once with photographs and document the timeline since handover. A legal assessment clarifies whether repair, price reduction or damages is the right path and how to use the period.

02

After the first six months you bear the burden of proof for the defect.

If the mould appears only after the first six months, you must prove that the cause is a construction defect and that it was already present at handover. As a rule this succeeds only with an expert opinion that distinguishes the structural cause from the use of the home.

The warranty period for immovable property is three years from handover. Have it checked quickly whether the period is preserved and whether an opinion proves the defect. The earlier the condition is secured, the better your evidence position.

03

Where the cause is disputed, the expert opinion decides.

Where it is disputed between you and the contractor whether the mould comes from a construction defect or from the use of the home, there is no way around expert evidence. An opinion clarifies whether a thermal bridge, faulty sealing or a planning error exists, or whether ventilation and heating are the cause.

Beforehand secure the condition with evidence and avoid premature repairs that destroy the findings. Legal guidance clarifies whether judicial preservation of evidence is sensible and how to prepare your claims.

04

If the cause lies in how the home is used, there are generally no warranty claims.

If the mould is clearly due to how the home is used, for example too little ventilation or insufficient heating, there is no construction defect. Warranty and damages claims against the contractor are then excluded, because the work was produced free of defects.

The distinction is, however, often harder than it seems, because structural and use-related causes can act together. Before you give up a claim, a technical clarification of the actual cause is worthwhile.

When mould is a construction defect

A work is defective if it lacks an agreed or a customarily presupposed quality. This follows from sections 922 and 1167 ABGB. A flat or a house must be dry and suitable for living. If moisture escapes from the building fabric and mould forms, this presupposed quality is missing.

Mould itself is only a symptom. The legally decisive question is the cause. If it lies in the building fabric, for example in a thermal bridge, a missing or faulty sealing or a planning error, it is a construction defect. If by contrast it rests exclusively on the use of the home, there is no defect of the work.

In practice structural and use-related causes often act together. A thermal bridge can favour mould that might have been avoidable through particularly intensive ventilation. This distinction is the core of almost every dispute and can only be judged reliably with technical help.

Burden of proof and the six-month period under section 924 ABGB

For the enforcement of your claims the burden of proof is decisive. Under section 924 ABGB it is presumed that a defect already existed at handover if it comes to light within six months thereafter. In this period the contractor must prove the opposite.

If the mould therefore appears in the first six months, this relief on proof works to your benefit. You do not have to show that the cause was already present at handover. The contractor bears the burden of proving freedom from defects at handover.

After the six months the burden of proof reverses. Then you must prove that a construction defect exists and that it was already present at handover. As a rule this succeeds only with an expert opinion. The warranty period for immovable property is three years from handover.

Warranty and damages

If a construction defect exists, the warranty remedies under section 932 ABGB are available to you. Primarily you can demand repair, that is the elimination of the cause of the defect. Only where this is impossible or disproportionate or is refused do price reduction or, for a defect that is not minor, rescission come into consideration.

In addition a damages claim under section 933a ABGB can exist where the contractor is at fault. This covers not only the defect itself but also consequential damage, for example damaged furnishings or substitute costs. Here the injured party in principle bears the burden of proof for the fault.

Mould often also affects health. Have affected spots properly assessed and removed, independently of the legal clarification. The repair of the cause and the question of compensation are to be treated separately but should be documented from the outset.

Place the cause

Construction defect or use of the home as cause

The following overview names typical indications. A reliable allocation is provided only by the expert opinion.

Indications of the cause of moisture and mould
Feature Points to a construction defect Points to use of the home
Location of the growth At thermal bridges, junctions or ground-contact building parts Behind furniture on external walls without a structural weak point
Timeline Appears quickly after moving in or after rain Develops gradually with little ventilation
Accompanying circumstances Visible cracks, soaking, missing sealing High room humidity without a structural finding

These features are indications, not proof. Structural and use-related causes can act together.

Important: do not repair prematurely. Whoever removes the mould and closes the wall before the cause is documented often destroys the decisive proof. First secure the condition with photographs and call in an expert. If you would like to discuss your case, you can arrange an initial consultation (EUR 72).

What you should do as an affected person

Document the growth at once and comprehensively: photographs of the affected spots, the date of the first appearance and the timing in relation to handover. Also record in which weather and at which building parts the mould appears. These records are the basis of every legal assessment.

Give notice of the defect in writing to your contractual partner and set a period for repair. For now refrain from a repair of your own, as long as the cause is not secured. Call in an expert who distinguishes the structural cause from the use of the home.

Where the cause is disputed or the period is running out, early legal advice is sensible. A legal assessment clarifies whether judicial preservation of evidence comes into consideration and which warranty remedy carries. An overview is provided on our focus page on construction defects and warranty.

Frequently asked questions

Mould and moisture as a construction defect.

Is mould automatically a construction defect? +

No. Mould is only a symptom. A construction defect exists if the cause lies in the building fabric, for example in a thermal bridge, a missing sealing or a planning error. If the mould rests exclusively on how the home is used, there is no defect of the work. The distinction is regularly clarified by an expert.

Who has to prove that a construction defect exists? +

If the mould appears within six months after handover, it is presumed under section 924 ABGB that the defect already existed then. The contractor must then prove freedom from defects. After this period you bear the burden of proof and usually need an expert opinion.

May I remove the mould myself before the cause is clarified? +

For health reasons proper removal is sensible. From a legal perspective you should first document the condition with secured evidence, because a premature repair can destroy the decisive proof of the cause. First secure photographs and call in an expert.

Further reading

Related topics around construction defects and evidence

Construction defects and warranty

The focus page on defects in construction, from the notice to the choice of the right remedy.

Construction defects after handover

Which rights you have when defects only show after moving in.

Preserving evidence before litigation

How to secure the condition before a repair changes the findings.

Expert opinion in a construction dispute

Which role the opinion plays in clarifying the cause and in the proceedings.

Topics
mouldmoistureconstruction defectwarrantyABGB

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