Bathroom Extractor Fan Regulations: What Building Regs Require

Under UK Building Regulations Part F (ventilation), a bathroom without an openable window must have mechanical extraction — typically an extractor fan providing a minimum extract rate, commonly cited as 15 litres per second for intermittent extraction in a standard bathroom (check current Part F guidance or with your installer, as exact figures and methods can be updated). Even bathrooms with a window often still benefit from a fan, since opening a window doesn't reliably clear steam and humidity the way mechanical extraction does.

When you legally need one

• Any bathroom or shower room without an openable window needs mechanical extraction — this is a hard requirement, not a recommendation.
• New bathrooms created during a renovation or extension (e.g. converting a bedroom to an ensuite) need to comply with current Part F requirements as part of the building regulations sign-off for that work.
• Replacing an existing fan like-for-like generally doesn't trigger a fresh building regulations application, but if you're doing notifiable electrical work as part of a wider renovation, it may need to be captured under that.

What "compliant" actually means in practice

Compliance isn't just "has a fan" — it covers extract rate, whether the fan runs on a timer/humidity sensor or simple pull-cord/light-switch link, and how it's ducted (ideally to outside air, not into a loft space where moisture can cause problems). A competent electrician or bathroom fitter installing to current regulations will size and specify this correctly; if in doubt, ask them to confirm the fan meets current Part F requirements for your bathroom's use.

Typical costs

A standard extractor fan supplied and fitted typically costs £100-£300, including the electrical connection. More sophisticated units (humidity-sensing, timer-delay, in-line fans ducted over a longer run) cost more, often £200-£500 installed. Since this is electrical work in a bathroom — a zoned, higher-risk area under UK wiring regulations — it should be installed or signed off by a qualified electrician, and notifiable work needs building control notification (via a registered competent person scheme in most cases).

Common mistakes

• Assuming an openable window is enough. In practice, most people don't leave bathroom windows open in cold weather, so a room without mechanical extraction still tends to suffer condensation and mould even where it's technically compliant.
• Under-specifying the extract rate for the room size, particularly in bathrooms with a shower cubicle producing a lot of steam.
• Ducting into an unventilated loft space instead of to outside air, which just moves the moisture problem rather than solving it.

Choosing the right fan for your bathroom

Fans are generally sized and specified based on room volume and how the bathroom is used — a small ensuite with just a basin and toilet needs less extraction than a family bathroom with a shower producing a lot of steam. A humidity-sensing fan, which switches on automatically when it detects rising moisture rather than relying on a light-switch link or manual pull-cord, is a genuinely useful upgrade in bathrooms where people don't reliably remember to switch extraction on, and it tends to do a better job of controlling condensation over time.

Retrofitting extraction into an older bathroom

Many older UK bathrooms, especially in properties renovated decades ago, either have no extraction at all or an underpowered, poorly ducted fan. Retrofitting compliant extraction as part of a wider bathroom renovation is one of the most cost-effective improvements you can make — it's relatively inexpensive compared to the rest of a renovation budget, but has an outsized effect on preventing condensation, mould and the redecoration costs that come with recurring damp problems.

If you're already seeing mould or damp signs, see our guide to why your bathroom is growing mould — poor or failed extraction is one of the most common causes. For a wider view of when building regulations apply to bathroom work generally, see our guide to building regs for bathroom renovations.

Noise levels matter too

A fan that's noisy enough to be annoying often ends up switched off manually by the household, which defeats the purpose entirely. Fan noise is usually specified in decibels (dB) on the packaging or spec sheet — as a rough guide, anything under about 30dB is genuinely quiet, while older or budget units can run considerably louder. If noise has been a problem with a previous fan, it's worth specifically asking your electrician to recommend a quieter model rather than just a like-for-like replacement.

Maintaining an existing fan

Extractor fans lose effectiveness over time as dust builds up on the blades and grille, which can happen gradually enough that it's not obvious until the bathroom starts steaming up more than it used to. Wiping the external grille every few months and, where the fan design allows, removing and cleaning the internal blades once or twice a year keeps it working at its rated extract rate. A fan that's become noticeably noisier or less effective than when it was new is often simply due a clean rather than a full replacement.