bathroom

How to Install a Bathroom Extractor Fan That Meets Building Regulations

Gabriel

28 Nov 2025 — 3 min read

Bathroom condensation is the leading cause of mould growth in UK homes, and an adequately sized, properly installed extractor fan is the most effective solution. But bathroom electrical work falls under Part P of the Building Regulations, meaning installation must either be carried out by a registered electrician or notified to Building Control. This guide explains the requirements, the options, and how to specify the right fan for your bathroom.

Building Regs Requirements

Part F (Ventilation) requires bathrooms to have either a window or a mechanical extractor fan. The minimum extraction rate for a bathroom is 15 litres per second (54 m³/h) for an intermittent fan, or 8 l/s (29 m³/h) if continuous running. The fan should run for a 15-minute overrun after the light is turned off or after the occupant leaves.

Part P (Electrical Safety) classifies bathrooms as a special location. Any electrical work in a bathroom — including fitting a fan — must be carried out by a Part P-registered electrician or self-certified through a competent persons scheme, or notified to Building Control. DIY electrical work in bathrooms is legal but requires notification and inspection.

Fan Specification: What to Look For

Extraction Rate

For a standard UK bathroom up to 8m², a fan rated at 15–22 l/s (54–80 m³/h) is appropriate. For larger bathrooms, en-suites with showers, and wet rooms, go higher — 22–30 l/s (80–110 m³/h). Undersized fans are the most common specification error; they run continuously but can't keep up with condensation from a hot shower.

Fan Type

Axial fans: Basic, inexpensive (£15–40). Suitable for direct through-wall or short duct runs (up to 1–2 metres). Significant noise at high extraction rates.
Centrifugal fans: More powerful, much quieter at equivalent extraction rates, handle longer duct runs (up to 4–5 metres). Essential for internal bathrooms without direct external wall access. Brands: Manrose, Vent-Axia, Airflow iCON — expect to pay £40–120.
Humidity sensors: The most useful upgrade for any bathroom fan. The fan runs automatically whenever humidity rises above a set threshold (typically 70–80% RH) regardless of whether a light switch is operated. This ensures the fan runs throughout a shower even if the light isn't linked, and prevents it running unnecessarily. A humidity-sensing centrifugal fan: £60–150.

Duct Routing

All bathroom fans must vent to outside air — discharging into a loft space is not permissible under Part F and creates serious condensation problems in the loft. Options:

Through external wall: Simplest and most effective. Fit a through-wall grill on the outside face. Core drill a 100mm or 125mm hole through the wall. Maximum efficiency — no duct run loss.
Through soffit: Acceptable if the soffit is well ventilated but not recommended — moist air discharging into a confined soffit space can cause deterioration.
Through roof: Used for internal bathrooms. Requires a rigid duct run (semi-rigid or solid aluminium) rather than the corrugated flexible duct that loses significant performance over length. Use the shortest and straightest route possible; each 90° bend reduces effective extraction rate by the equivalent of approximately 1 metre of duct.

Wiring: The Three Main Configurations

Switched live from light switch: Fan runs with the light — simple but doesn't provide overrun control unless the fan has a built-in timer.
Separate switched live with overrun timer: Fan controlled independently from the bathroom light, with an internal timer module providing the required overrun. Most flexible configuration.
Humidity sensor (no switch required): Fan runs autonomously based on humidity — requires only a permanent live and neutral connection.

Use appropriately rated cable (1.0mm² twin and earth for a fan circuit), and ensure all connections in the ceiling void are made in accessible junction boxes, not buried in the plaster.

IP Rating in Bathrooms

Bathroom fittings must meet IP ratings determined by their proximity to water (the Zone system). Most ceiling-mounted bathroom fans should be minimum IP44 if within Zone 1 (above the bath or shower, up to 2.25m from the floor) or Zone 2 (0.6m beyond Zone 1). Outside these zones, IP20 is permissible. Most fans sold for bathroom use meet at least IP44 — check the specification sheet before purchasing.