Rent reduction for defects: what mould, construction noise and a heating failure are really worth
Mould, a broken heater in January, a building site right outside the window: anyone who tolerates a defect pays full rent for half a flat for months. Yet the law gives you a clear right — and the courts long ago put a percentage on what each defect is worth.
⚖️ Your four rights when there is a defect — the reduction is only one
As soon as a defect appears in your flat that you neither caused nor have to fix yourself, the law gives you not one right but a whole bundle. Art. 259a CO lists them — and they combine. Most tenants know only the first and give away the rest.
The threshold is decisive: a trifle — a loose blind handle — does not entitle you to a reduction, only to repair. The defect must impair the agreed use noticeably (practice draws the line at around 5 % impairment of use). From there the claim runs — from the moment the landlord knows about it.
Do I even have a claim to a reduction?
Three questions – the traffic light tells you whether the letter is worth it
Does the defect noticeably impair the use of the flat (more than a trifle)?
Did you cause the defect yourself (e.g. by poor ventilation)?
Have you already reported the defect to the landlord in writing?
📊 The ConvivaPlus rent-reduction table
How much is a defect worth in percent? No statute says — it is decided case by case. Other guides leave you alone with that question. We dig deeper: the ConvivaPlus rent-reduction table bundles around 130 published Swiss court decisions (collected by Dr Martin Züst / mietrecht.ch, supplemented by Federal Supreme Court and cantonal practice) into benchmark ranges per defect type. You see at a glance what was actually awarded before conciliation authorities and courts — not a gut feeling, but documented practice.
| Defect type | Range | Benchmark | Example from practice | Source |
|---|---|---|---|---|
| Heating failure / too cold🌡️ | 15–50 % | 20 % | 16 °C in winter 20 %; heating + hot water off 50 %; flat uninhabitable 100 % | BGE 97 II 58 · CJ GE 1986 |
| Mould / damp | 20–30 % | 25 % | Furniture rotting 22 %; breach of hygiene 26 %; persistent damp with odour 30 % | BezGer Rorschach 1988 · TC VS 1987 |
| Water damage | 10–50 % | 25 % | Stained ceiling/wallpaper 10 %; 1½ of 3 rooms affected 42 %; parquet to replace 50 % | CJ Genève 1984 / 1985 |
| Works / renovation in the building | 10–35 % | 20 % | Façade/lift 10–15 %; works in the flat above 25 %; entire building 35 % | Mietgericht ZH · MRA 2/2000 |
| Building site nearby | 10–35 % | 18 % | Large site, massive noise 16 %; 4 of 5 rooms disturbed 25 %; severe immissions 35 % | Mietgericht ZH 1999, MRA 2/2000 |
| Noise (neighbours, premises, lift) | 5–25 % | 15 % | Lift/heating noise in one room 5–15 %; bar in the building 15 %; concerts making it uninhabitable 40 % | mietrecht.ch-Kasuistik |
| Lift failure | 8–15 % | 10 % | Lift on the 4th floor 10 %; on the 6th + attic/laundry 15 % | Tribunal des baux GE 1975/76 |
| Odour immissions | 10–20 % | 12 % | Occasional smoke/sewage 10 %; rotten eggs for hours 12 %; butcher next door 20 % | mietrecht.ch-Kasuistik |
| Faulty fittings | 3–10 % | 5 % | Dishwasher broken 3 %; hot water off at times 5 %; washing machine fails 10 % | ASLOCA-Merkblatt |
| Cosmetic defects | 2–10 % | 5 % | Torn entrance carpet 2 %; water stains & peeling wallpaper 8–10 % | ASLOCA-Merkblatt |
🌡️ Seasonal: heating and fireplace defects only take effect in the winter half-year, loss of garden or terrace only in summer — the reduction accordingly applies only to the affected months.
Methodology: the percentages are benchmarks from published court and conciliation practice, not guaranteed entitlements — each case is assessed individually by the relative method (value of the defective vs. the defect-free object). According to the ConvivaPlus analysis of the casuistry, the most frequent corridor for residential defects is between 10 % and 30 %; only the impossibility of use leads to 100 %. Source: decisions collected by Dr M. Züst (mietrecht.ch), Federal Supreme Court, cantonal courts.
🧮 The reduction calculator
Choose your defect type, estimate the severity and enter your gross rent. The calculator takes the matching benchmark from the ConvivaPlus table and shows you how many francs per month — and over the whole duration of the defect — are at stake.
What is my defect worth?
Benchmark in % and CHF, derived from Swiss case law
Benchmark from case law, not a guaranteed entitlement. In a dispute the exact amount is set by the conciliation authority or the court.
✍️ Step 1: report the defect correctly
The reduction only runs from the moment the landlord learns of the defect — not from the day the mould began to grow. The notice is therefore the most important step of all: in writing, dated, ideally by registered mail, with a reasonable deadline for repair. The ConvivaPlus generator builds the letter ready to send.
Defect notice & demand to repair
Sets the deadline and secures your reduction claim from the date
📨 Step 2: quantify and demand the reduction
If the defect lasts or the landlord stalls, quantify the reduction concretely. Take the percentage from the calculator above — the second ConvivaPlus generator turns it into a clear demand letter. If nothing helps, there is still terminating the lease; more templates are gathered in the tenancy-template hub, and at move-out the handover protocol counts.
Demand for rent reduction
Quantifies the reduction in % and CHF — straight from the calculator
🔐 The pressure tool: depositing the rent (259g–i)
What if the landlord simply sits out the defect? Then you have the sharpest instrument of tenancy law: you deposit the future rents with the office designated by the canton. The money is not gone — it sits with the official office and counts as paid. The landlord therefore cannot terminate you for default, but only gets their money once the defect is fixed. That works.
⚠️ The five most expensive mistakes
Most tenants do not lose their legitimate claim in court, but to one of these five avoidable mistakes:
Reporting too late. The reduction only runs from the landlord's knowledge. Every month you stay silent is lost — nothing is retroactive.
Complaining only verbally. In a dispute the burden of proof that the landlord knew of the defect is on you. Without a dated letter (registered) you are left empty-handed.
Cutting the rent on your own. Reducing the rent unannounced is risky: if you pay too little, you risk a termination for default. The correct move is to assert the reduction or formally deposit — not simply transfer less.
Causing the defect yourself. Poor ventilation that encourages mould, or damage you cause yourself, gives no claim. So document the cause (photos, construction defect, exterior wall).
Missing the 30-day deadline for the deposit. Depositing without applying to conciliation in time = money gone. The two steps belong inseparably together.
A defect only costs the landlord something once you pin them down in writing. Until then you pay the full price for half a flat.
The reduction applies to the gross rent of the affected flat — not just to the single room. If a water leak makes one room unusable and its furniture must be stored in another, practice (mietrecht.ch, criterion 6) holds that this room is impaired too — so the reduction is higher than the mere floor-area share. This is exactly where most tenants give away percentage points.
Have you ever had a defect that would have justified a rent reduction?
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Frequently asked questions about rent reduction for defects
The essential answers on entitlement, amount, deadline and procedure — based on the CO and case law.
People also ask
Related questions from our magazine
All information without guarantee. Found an error? → support@conviva-plus.ch
For a room temperature of 16 °C in winter months, the Tribunale d'Appello TI awarded a rent reduction of 20 %. A heating failure that makes the flat uninhabitable even leads to 100 % — the tenant then owes no rent at all.
What goes with it
More from the ConvivaPlus tenancy-law cluster
Discussion
8 voices from the community
Bei uns ist im Januar die Heizung drei Wochen ausgefallen, Wohnung knapp 17 Grad. Die Tabelle hat mir gezeigt, dass 20 Prozent für die Wintermonate realistisch sind. Habe den Brief geschickt, Verwaltung hat ohne Murren reduziert.
Stark, dass es ohne Schlichtung ging. Wichtig für andere: Die Reduktion läuft ab dem Tag, an dem der Vermieter vom Mangel weiss — darum die Meldung immer schriftlich und datiert.
Seit Monaten Baustelle direkt vor dem Fenster, Lärm ab sieben Uhr. Ich dachte, da kann man nichts machen, weil es ja nicht der Vermieter ist. Dass auch Immissionen von aussen zählen, war mir neu.
Genau, entscheidend ist die Beeinträchtigung des vertragsgemässen Gebrauchs, nicht wer den Lärm verursacht. Bei Grossbaustellen hat die Praxis 16 bis 25 Prozent zugesprochen.
Schimmel im Schlafzimmer, die Verwaltung hat ein halbes Jahr nur vertröstet. Erst als ich die Hinterlegung angedroht habe, kam plötzlich der Maler. Diese Druckwaffe kennt fast niemand.
Frage: Ich habe den Mangel erst nach drei Monaten gemeldet. Bekomme ich die Reduktion rückwirkend für die drei Monate auch?
Leider nein. Die Herabsetzung läuft erst ab dem Zeitpunkt, in dem der Vermieter vom Mangel erfährt (Art. 259d OR). Genau deshalb: sofort melden, nicht warten.
Der Rechner ist gut, aber ich hätte nicht gedacht, dass ein defekter Lift im obersten Stock auch zählt. 10 bis 15 Prozent klingt fair für vier Stockwerke ohne Lift.
ConvivaPlus Editorial
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