Damp and mould are the most common bathroom complaints in UK homes, and inadequate ventilation is almost always the culprit. Building Regulations Part F sets minimum extract rates for every wet room, and any bathroom created or substantially altered since 2006 must comply. Getting this right is not just a question of comfort — a failing extractor can void a warranty, cause condensation damage to ceilings and joinery, and flag up as a defect on a home survey.
This guide covers what the regulations actually require, how different fan types compare, what you should expect to pay, and how to choose the right unit for your bathroom.
What Building Regulations Say
Approved Document F (Ventilation) requires intermittent extract ventilation in bathrooms at a minimum rate of 15 litres per second (54 m³/hr). For a separate WC without a bath or shower the minimum drops to 6 l/s. These figures apply to new dwellings and material change of use; extensions and alterations are expected to meet the same standard where “reasonably practicable.”
Key requirements:
- The fan must have a 15-minute overrun (run-on timer) to clear residual moisture after you leave the room.
- Fans must not recirculate air back into the same room — ducted extract to outside is required.
- If the bathroom has no openable window, mechanical ventilation is mandatory regardless of build date.
- In new builds, fans must also meet background ventilation requirements under Part F 2021 (applicable for work started after June 2022).
Electrically, extractor fans in bathrooms fall within Zone 2 (or Zone 1 if above the bath). Any fan in Zone 1 or 2 must be rated IP44 minimum; fans directly above a bath or shower should be IP65 or higher. Work in these zones must be carried out by a Part P-registered electrician or notified to Building Control.
Types of Bathroom Extractor Fan
Intermittent Axial Fans
The most common type — a wall- or ceiling-mounted fan that runs when switched on (or triggered) and continues for a timed run-on period. These are inexpensive and straightforward to install, making them suitable for most renovation projects.
Fans with Humidistat
A humidistat sensor triggers the fan automatically when relative humidity exceeds a set threshold (typically 70–80 %). This eliminates the need to remember to switch the fan on and provides better moisture control. Ideal for families or rental properties.
Fans with PIR Motion Sensor
PIR-triggered fans switch on when someone enters the room and continue for a set period after the space is vacated. Convenient but less effective than humidistats for controlling peak moisture during showers.
Continuous Low-Rate (MVHR / dMEV)
Decentralised mechanical extract ventilation (dMEV) fans run continuously at a very low rate (around 8 l/s) with a boost mode (15 l/s+) triggered by humidity or occupancy. These are required in some new-build specifications and perform best in airtight modern homes. They are quiet but more expensive to buy.
Inline/Remote Fans
For bathrooms where a wall or ceiling fan is impractical — often en-suites in converted lofts — a remote fan sits in the loft or duct run, connected to the room by flexible ducting. Quieter than axial units and easier to service.
Performance: Comparing Fan Types
| Fan Type | Min Extract Rate | Typical Noise (dB) | Typical Unit Cost | Installed Cost |
|---|---|---|---|---|
| Basic axial (timer) | 15 l/s | 35–45 dB | £20–£60 | £80–£160 |
| Axial with humidistat | 15 l/s | 33–42 dB | £50–£120 | £120–£220 |
| Axial with PIR | 15 l/s | 34–44 dB | £45–£110 | £110–£210 |
| dMEV continuous | 8–15 l/s | 22–30 dB | £90–£200 | £180–£350 |
| Inline/remote fan | 15–30 l/s | 20–28 dB (at grille) | £80–£250 | £200–£450 |
Prices as at mid-2026. Inline fan costs vary significantly with duct run length.
Ducting: What You Need to Know
Extract fans must duct to outside air — routing into a loft void or wall cavity is not compliant and encourages mould within the structure. Options include:
- External wall termination — shortest duct run, highest efficiency, simplest to retrofit.
- Roof termination — used when an external wall is not accessible; requires a suitably rated roof tile or ridge vent.
- Soffit vent — common in modern houses; duct exits through the soffit. Ensure the soffit vent is not blocked by loft insulation.
Use rigid ducting (100 mm or 125 mm diameter rigid PVC or galvanised steel) wherever possible. Flexible duct is acceptable for short connections but adds resistance, reducing effective extract rate. Keep bends to a minimum and use swept bends rather than sharp right angles.
Duct runs exceeding 3–4 m will require a more powerful fan to maintain the required extract rate. Specify a fan by its installed performance at system resistance, not just the free-air rating on the box.
Cost to Supply and Install
| Scenario | Estimated Cost |
|---|---|
| Like-for-like fan replacement (existing wiring, same position) | £80–£160 |
| New fan installation, existing circuit (notifiable work) | £150–£280 |
| New fan + new wiring from consumer unit | £250–£500 |
| Inline fan with duct run through loft (up to 5 m) | £350–£600 |
| dMEV continuous fan, new install | £250–£450 |
All electrical work in bathrooms is notifiable under Part P. A Part P registered electrician will self-certify; if you use an unregistered contractor, you must notify your local authority Building Control (fee typically £300–£400 for a minor building notice).
Choosing the Right Fan
- Extract rate: Always confirm the installed performance at your estimated system resistance — most fan datasheets include a performance curve.
- Noise: For en-suites adjacent to bedrooms, target below 30 dB(A). Inline fans typically achieve this; axial fans rarely do.
- IP rating: IP44 minimum for Zone 2 (within 600 mm horizontally of bath/shower edge up to 2.25 m height); IP65 for Zone 1 (directly above).
- Timer vs humidistat: A humidistat is almost always worth the extra cost. It works when the light switch is off, catches prolonged showers accurately, and reduces energy use versus a long fixed timer.
- Duct diameter: 100 mm is standard for most axial fans; 125 mm improves performance for longer runs or higher-capacity units.
Common Mistakes to Avoid
- Buying on free-air rating — fans are tested without ductwork; installed performance is always lower.
- Flexible duct throughout — adds resistance, creates condensation traps, and reduces extract rate significantly.
- Ducting into the loft — non-compliant and leads to structural damp.
- No overrun timer — a fan that switches off with the light will not comply with Part F and fails to clear post-shower moisture.
- Skipping notification — bathroom fan wiring is notifiable under Part P; buyers’ solicitors routinely ask for the certificate.
A correctly specified and installed bathroom extractor fan is one of the best-value investments in a bathroom renovation. It protects the room’s finishes, prevents structural damage, and satisfies Building Control — small things that matter enormously when you come to sell.
