What BR18 requires
The regulations ask for outdoor air supply in living rooms and in the dwelling as a whole, and, in a mechanical system, supply in living rooms and extract in bathroom, toilet, kitchen and utility room, with heat recovery. Houses with natural ventilation only need the air supply and the cooker hood requirements (§ 446).
Air supply
At least 0.30 l/s per m² of heated floor area. With demand-controlled ventilation that measures air quality and humidity, this may drop to 0.15 l/s per m² in longer periods when the home is not used.
Extract rates
Minimum extract rates per room, which can be raised:
| Room | Minimum extract (l/s) |
|---|---|
| Bathroom and toilet | 15 |
| Toilet without bathroom | 10 |
| Utility room | 10 |
| Kitchen | 20 |
The cooker hood
Above the hob there must be controllable mechanical extract to the outside. The air flow must be at least 120 l/s, which can be reduced with correction factors or by showing a capture efficiency of at least 75 per cent. This is a separate requirement from the 20 l/s kitchen extract, so both figures are correct. Earlier versions of BR18 named 20 l/s here, so the requirement depends on the version.
Worked example
A home of 120 m² heated floor area. Air supply: 0.30 × 120 = 36 l/s, or 18 l/s with demand control in periods when the home is not used. Extract: 15 l/s for a bathroom with toilet, 10 l/s for a utility room and 20 l/s for the kitchen, plus the cooker hood of at least 120 l/s.
How Aira applies it
Aira counts all rooms inside the dwelling as heated floor area for the air supply: at least 0.30 l/s per m² for the dwelling, and for each habitable room at least 0.30 l/s per m² of its own area. Supply and extract are equal. The extract is divided over the bathrooms, toilets, utility rooms and the kitchen in proportion to their minimum extract rates (bathroom and toilet 15, toilet without bathroom and utility room 10, kitchen 20 l/s), which are also shown as capacity; the cooker hood of 120 l/s is an installation requirement and is not part of the balance. You can adjust every figure in the table before you download the plan.
Source
The figures are taken from the official text of BR18 (current version), checked on 16 October 2026.