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Mould in Your New Build Flat — What Causes It and What You Can Do
You moved into a brand-new flat six months ago. Now there is black mould in the corner of the bathroom, behind the sofa, around the window frames. You have been told it is because you dry clothes indoors, or cook with the door open, or do not ventilate properly.
In most cases, that is not true. Mould in a new-build flat is almost always a ventilation defect — and the developer is legally obligated to fix it.
Why new-build flats are especially prone to mould
New-build flats are built to very high air-tightness standards to meet Part L of the Building Regulations (energy efficiency). This means the envelope — walls, windows, floor, ceiling — is sealed much more effectively than an older property. Draught means energy loss; airtightness means efficiency.
The problem is that sealing a building against air leakage removes the 'natural ventilation' that older properties rely on to manage moisture. In a sealed new-build, every shower you take, every meal you cook, every breath you exhale deposits moisture into the air. That moisture has to go somewhere — and if the mechanical ventilation system is not working properly, it goes onto cold surfaces as condensation. Condensation feeds mould.
Part F of the Building Regulations requires adequate ventilation in all residential properties. A developer cannot legally hand over a flat where the ventilation does not meet this standard. If your flat has mould because the ventilation is inadequate, the developer is in breach of their obligations.
How to test your ventilation right now
- Find your ventilation system switch (usually in the hallway). Turn it on.
- Take a sheet of A4 paper to each ceiling vent in every room.
- Hold the paper flat against the vent from below.
- If the system is working, the paper will stick to the vent by suction and stay there when you let go.
- If the paper falls away, the vent is not extracting. Document this with a video.
Failed vents = ventilation defect = developer's obligation to fix, at no cost to you.
What causes ventilation failures in new builds?
Fan not connected to ducting
The fan is installed, the switch works, but the connection to the external duct is missing or blocked during construction. Air circulates locally but goes nowhere. Common on internal bathrooms.
Ducting kinked or obstructed
Flexible ducting runs through ceiling voids and partition walls. If it is kinked, crushed, or looped incorrectly, airflow is severely restricted. The fan runs but extracts almost nothing.
MVHR system not commissioned
More expensive developments use Mechanical Ventilation with Heat Recovery. These systems must be commissioned and balanced after installation. If the commissioning is not done — or done incorrectly — performance is poor even when everything appears to be working.
Wrong fan specification
The fan installed does not meet the extraction rate required for the room size. Developers sometimes substitute specified units with cheaper alternatives that do not meet Part F requirements.
What to do if you have mould in your new build
Step 1 — Photograph everything
Photograph the mould itself, all ventilation grilles, and the results of the paper test (video is better). Date-stamp if possible. This is your evidence.
Step 2 — Report in writing
Email your developer's customer care team. State clearly that you have mould caused by inadequate ventilation, that you have tested the vents (describe results), and that you require written confirmation of when this will be investigated and fixed. Keep the email chain.
Step 3 — Quote the regulations
Reference "Building Regulations Part F — Ventilation" and "Building Regulations Part C — Resistance to Moisture" in your written report. Developers are much more responsive when buyers demonstrate knowledge of their legal obligations.
Step 4 — Get a professional inspection
A professional snagging inspection creates an independent written record of the ventilation defect, its likely cause, and the estimated cost of remediation. This is significantly more powerful than a buyer complaint — and it often reveals additional defects the mould masked.
Step 5 — Escalate if needed
If the developer fails to act within a reasonable period (28 days is standard), escalate to the New Homes Quality Board or the NHBC Resolution Service. You can also take the matter to the Housing Ombudsman. Always escalate in writing.
What the fix costs — and who pays
If the mould is caused by a ventilation defect, the developer pays — including the cost of mould remediation, repainting, and any damaged fixtures. In our experience, the typical cost of ventilation investigation and repair by a specialist is £2,000–£4,000. Mould remediation on top: £300–£800 depending on extent.
Do not treat the mould yourself and then claim the developer has resolved the issue — you will lose your evidence. Document first, report second, remediate only after the developer has acknowledged the defect in writing.
Found mould? Get an independent inspection.
A professional BluntSnag inspection identifies every ventilation defect and documents it in a written report the developer cannot ignore. From £299, report in 24 hours.