What Size Extractor Fan Do You Need? UK Bathroom and Kitchen Fan Regulations

Quick answer: building regulations guidance (Approved Document F) expects an intermittent extractor fan to remove at least 15 litres per second in a bathroom, 30 l/s from a cooker hood over the hob or 60 l/s elsewhere in a kitchen, 30 l/s in a utility room and 6 l/s in a separate WC. Those are rates once installed, not the number on the box.

An underpowered or badly installed extractor fan is one of the most common causes of condensation mould in UK homes, and one of the cheapest to fix. If you have black spots on a bathroom ceiling or streaming kitchen windows, the fan is the first thing to check. (See mould on the bathroom ceiling for the full diagnosis.) This guide sets out what the regulations expect, room by room, and how to tell whether your fan is actually doing its job.

The minimum extract rates at a glance

Approved Document F, Volume 1: Dwellings (2021 edition, in force in England from 15 June 2022) sets these minimums. The rates were unchanged from the previous edition.

• Kitchen, extract next to the hob (for example a ducted cooker hood): 30 l/s intermittent.
• Kitchen, fan elsewhere in the room: 60 l/s intermittent.
• Kitchen, continuous extract: 13 l/s minimum high rate.
• Utility room: 30 l/s intermittent, or 8 l/s continuous.
• Bathroom, en-suite or shower room: 15 l/s intermittent, or 8 l/s continuous.
• Separate WC (no bath or shower): 6 l/s.

To put 15 l/s in context: that's 54 cubic metres an hour, enough to change the air in a typical small bathroom several times an hour.

Intermittent or continuous?

An intermittent fan switches on when you need it, usually with the light or a pull cord, and runs at full speed. A continuous fan (often called dMEV, decentralised mechanical extract ventilation) runs quietly all the time at a low rate and boosts when humidity rises or the light goes on. Continuous fans suit airtight homes and new builds, and they can't be forgotten or switched off at the wrong moment, which is a large part of why they work.

Whole-house systems such as positive input ventilation (PIV) and mechanical ventilation with heat recovery (MVHR) are a different approach again. Our comparison of PIV vs MVHR vs extractor fans explains when each makes sense.

Kitchens

The kitchen rule rewards catching steam at source. A ducted hood directly above the hob captures it before it spreads, so 30 l/s is enough. A wall or ceiling fan elsewhere in the room has to clear moisture that has already mixed into the room air, so it needs 60 l/s.

Recirculating hoods don't count. They filter grease and smells and blow the air, moisture included, straight back into the kitchen. If your hood has a charcoal filter and no duct to outside, the kitchen needs a separate extract fan to meet the guidance.

Bathrooms, en-suites, shower rooms and WCs

What counts is whether the room has a bath or shower, not what it's called. An en-suite, shower room or wet room needs the same 15 l/s as a family bathroom. A cloakroom with only a WC and basin needs 6 l/s. A bathroom with no window has to rely entirely on mechanical extract, so the fan matters even more there.

Utility rooms and tumble dryers

Utility rooms get the same 30 l/s as a kitchen hood because laundry produces a lot of moisture: an average load of washing releases around two litres of water as it dries. A vented tumble dryer must exhaust through a wall or window vent to outside; a hose left loose in the room defeats the whole point. More in how to dry clothes indoors without causing mould.

Overrun timers and controls

Steam keeps coming off wet tiles, towels and the shower tray long after the water is off. A fan that stops the moment the light goes off leaves most of it behind.

• An intermittent fan in an internal room without an openable window should have a 15-minute overrun.
• Any automatic control, such as a humidistat or presence sensor, needs a manual override so occupants can switch the fan on themselves.
• In practice an overrun timer or humidistat is worth having in any bathroom, window or not.

The number on the box isn't the number that counts

A fan's rated flow is measured with no ducting attached. Every metre of duct, every bend and every flexible section that sags or kinks reduces what it actually moves. A fan sold as "15 l/s" connected to a long flexible duct through the loft can easily deliver well under half that. That's why the 2021 edition of Approved Document F puts weight on commissioning: measuring the installed airflow and recording it on a commissioning sheet.

Good installation practice:

• Choose a fan rated comfortably above the minimum for the duct run you have.
• Keep ducts as short and straight as possible; use rigid duct where you can.
• Insulate ducts that pass through a cold loft, so moisture doesn't condense inside and drip back.
• Terminate at a proper roof, soffit or wall vent. Never discharge into the loft.
• Measure the airflow once fitted, and keep the record.

Does this apply to existing homes?

Approved Document F is guidance on meeting the Building Regulations for new building work, so it doesn't force you to upgrade a fan that's already there. But the 2021 edition added sections specifically on existing dwellings, and replacement ventilation, a new bathroom, or an extension should meet its standards. The same applies to replacement windows, which shouldn't reduce background ventilation; see trickle vents and Approved Document F. These figures apply in England; Wales and Scotland have their own building standards.

Fan replacement in a bathroom is electrical work in a special location, so use a qualified electrician, ideally one registered with a competent person scheme who can certify the work.

How to check your fan in five minutes

1. Tissue test. Hold a single sheet of toilet paper against the grille with the fan running. If it isn't held firmly, the fan is probably underperforming.
2. Run-on. Turn the light off and see whether the fan keeps going.
3. Grille and blades. A clogged grille or greasy impeller cuts flow noticeably. Clean it (with the power off).
4. The loft. Follow the duct. It should be connected, unkinked, insulated and ending at an outside vent.
5. The room. If mirrors and windows are still fogged 20 minutes after a shower, the fan isn't keeping up.

For a definitive answer, a ventilation specialist can measure the flow with an anemometer and tell you whether it meets the rate for the room.

For landlords

A weak bathroom fan is a building issue, not tenant behaviour, and government guidance says it is unreasonable to blame damp and mould on tenants' lifestyle choices. In England, the Housing Health and Safety Rating System updated in June 2026 includes baseline indicators for ventilation and a 5% limit on observable damp or mould on any wall or ceiling surface in a room; see HHSRS 2026: how councils now assess damp and mould. Testing every fan before winter is the quickest win on our winter damp and mould checklist for landlords, and upgrading one is among the cheapest damp repairs there is.

Get it checked or upgraded

Find a verified ventilation specialist near you to measure your fans and recommend the right fix. For typical prices see how much condensation control costs, and for the bigger picture read how to prevent damp and mould, room by room.