Extractor Fan Regulations UK: The Part F Rates That Pass

Five numbers decide every fan specification in a home, and a recirculating cooker hood doubles one of them. What Approved Document F actually requires.

Extractor fan regulations in the UK sit in Part F of the Building Regulations, and the practical guidance is in Approved Document F Volume 1, 2021 edition, which covers dwellings. Most people meet it through a builder or an electrician and never see the numbers. That is fine until a building control officer asks for the extract rate on a new en suite, or an air flow test comes back short, or a beautiful downlit bathroom turns out to have a fan the size of a coaster in it.

This is what the document actually requires, in the situations that come up in a London renovation.

The rates

Extract ventilation to the outside has to be provided in four kinds of space: kitchens, utility rooms, bathrooms and sanitary accommodation. It can be intermittent, meaning a fan that comes on when you need it, or continuous, meaning a fan that runs all the time at a low rate and boosts when required.

Intermittent fans (Table 1.1)

The kitchen figure is the one that catches people out. A recirculating cooker hood, which filters through charcoal and blows the air back into the room, does not count as extracting to the outside. If that is what you are fitting, the kitchen's separate extract has to hit 60 l/s, not 30. This is a common and expensive discovery in flats where there is no external wall behind the hob.

Continuous systems (Table 1.2)

If you go continuous instead, each room needs a minimum high rate: 13 l/s for a kitchen, 8 l/s for a utility room, 8 l/s for a bathroom and 6 l/s for sanitary accommodation. The continuous rates of every extract point added together have to be at least the whole-dwelling rate, which is set by bedroom count: 19 l/s for one bedroom, 25 for two, 31 for three, 37 for four, 43 for five, and 6 l/s more for each bedroom beyond that. There is also a floor: at least 0.3 litres per second per square metre of internal floor area, counting every storey.

Where the fan goes

Two rules, and both are checkable by eye:

• The fan or extract terminal must be as high as practicable in the room, and no more than 400mm below the ceiling. Cooker extract hoods are exempt from this, for obvious reasons.
• Where a cooker hood extracts to the outside, it should sit at the height the manufacturer specifies, or between 650mm and 750mm above the hob surface if no figure is given.

The 400mm rule is the one that gets broken in bathrooms with sloping ceilings or bulkheads, where the easy duct run is halfway down the wall. It is worth resolving at the design stage rather than at inspection.

Controls, and the 15 minute overrun

Ventilation has to be controllable, manually or automatically. Three specifics matter:

• An internal room with no openable window needs an overrun. An intermittent extract fan in such a room must be provided with controls that keep it running for at least 15 minutes after the room is vacated. Almost every internal en suite and internal WC in a London flat falls into this.
• Humidity sensors are not allowed for WCs. They can be used in moisture-generating rooms such as kitchens and bathrooms, but not for sanitary accommodation, where the pollutant is odour and there is no humidity spike to detect.
• Automatic controls need a manual override. If an intermittent fan runs on a humidity sensor, there has to be a way for the occupant to switch it on or off.

Adding a new bathroom or en suite

This is the most common trigger in a renovation. When a wet room is added to an existing dwelling, the whole-dwelling ventilation has to be extended and extract provided in one of three ways:

1. An intermittent fan at the Table 1.1 rate, plus a background ventilator of at least 5000mm² equivalent area in the same room.
2. A continuous extract system at the Table 1.2 rate.
3. A single-room heat recovery ventilator, running continuously at the Table 1.2 high rate with 50% of that as its continuous rate. No background ventilator is needed in the same room if you use one of these.

Option one is where the trickle vent requirement comes from, and it surprises people who have just specified frameless windows or a room with no window at all. If there is nowhere to put a 5000mm² ventilator, options two and three exist precisely for that reason.

Internal doors also need to let air move through the dwelling: a free area equivalent to a 10mm undercut in a 760mm wide door, measured 10mm above a fitted floor finish or 20mm above the bare floor if the finish is not down yet. Solid oak doors fitted tight to a thick carpet are a recurring failure.

Refurbishing an existing kitchen or bathroom

Here the rules are more relaxed than most people assume.

Where building work is carried out in an existing kitchen or bathroom, any existing fans, including cooker hoods that extract outside, should be retained or replaced, and any retained device should be checked to confirm it still works. But if there was no ventilation system in the original room, you do not have to add one. The caveat is that additional ventilation may be needed if the work makes the building less compliant than it was, which in practice means airtightness work, insulation or window replacement.

One useful piece of small print: if an extractor fan or cooker hood is replaced and it uses the existing cabling, that work does not need to be notified to a building control body. It falls under Schedule 4 of the Building Regulations. New cabling is a different matter, and that is where Part P notification comes in.

Purge ventilation, which is not the same thing

Every habitable room needs purge ventilation as well: a means of extracting at least four air changes per hour directly to the outside, normally an openable window or door, or a mechanical system. This is the rule that stops a bedroom being designed with a sealed window. Background ventilators are separate again, should be at least 1700mm above floor level to reduce draughts, and are intended to be left open.

These three layers, extract, purge and background, are independent requirements. Meeting one does not excuse another, which is why "there is a window" is not an answer to a missing extractor in a bathroom.

Testing and sign-off

Air flow rates have to be measured and commissioned, and the results recorded. A fan rated at 15 l/s on the box will not necessarily deliver 15 l/s through four metres of flexible duct with two bends and a louvred grille. Rigid duct, the shortest sensible run and the fewest bends is the difference between a pass and an expensive retrofit. Specify the fan on the flow it achieves in the installed configuration, not the free-air figure on the packaging.

The full document is worth having: Approved Document F Volume 1, 2021 edition is free from GOV.UK, and the tables quoted here are Tables 1.1, 1.2 and 1.3.

Historic and traditional buildings get some latitude. Listed buildings, properties in conservation areas and vapour-permeable traditional constructions should comply where reasonably practicable, without harming the significance of the building or increasing the risk of long-term fabric damage. Consult the conservation officer. Our guides to listed building interior alterations and conservation area renovations cover that ground.

For the other regulations that bite on a London renovation, see the building regulations overview or start at the Vertigo Interiors home page.