What Approved Document F requires
Volume 1 covers dwellings and gives two rates: a whole-dwelling ventilation rate, and extract rates for the wet rooms. It applies to England only. Wales, Scotland and Northern Ireland have their own rules and are not covered.
The whole-dwelling rate
The minimum is the higher of two values: 0.3 l/s per m² of internal floor area (all storeys together), and the value in Table 1.3 for the number of bedrooms. A dwelling with a single habitable room needs 13 l/s. Every bedroom above five adds 6 l/s.
| Bedrooms | Minimum rate (l/s) |
|---|---|
| 1 | 19 |
| 2 | 25 |
| 3 | 31 |
| 4 | 37 |
| 5 | 43 |
Extract rates for wet rooms
Extract can be intermittent (Table 1.1) or continuous (Table 1.2). With continuous extract, the total of all extract points must reach at least the whole-dwelling rate.
| Room | Intermittent (l/s) | Continuous, high rate (l/s) |
|---|---|---|
| Kitchen with a hood venting outside | 30 | 13 |
| Kitchen without a hood, or a hood that does not vent outside | 60 | 13 |
| Utility room | 30 | 8 |
| Bathroom | 15 | 8 |
| Toilet | 6 | 6 |
Doors and air paths
Internal doors need a gap equal to a 10 mm undercut below a 760 mm door, so that air can move from the habitable rooms to the wet rooms.
Worked example
A three-bedroom home of 100 m². The whole-dwelling rate is the higher of 0.3 × 100 = 30 l/s and the 31 l/s of Table 1.3, so 31 l/s. With continuous extract, a kitchen (13), a bathroom (8) and a toilet (6) add up to 27 l/s, so the extract points together must still be able to deliver 31 l/s.
How Aira applies it
Aira takes the higher of 0.3 l/s per m² and the Table 1.3 value for the number of bedrooms it finds on the plan. Supply is divided over the habitable rooms by floor area. Approved Document F divides by volume; a floor plan has no ceiling heights, so area is the approximation and is exact when ceiling heights are equal.
The total is the higher of the whole-dwelling rate and the sum of the high rates of the wet rooms. Supply and extract both get that total, so they are in balance, as the document asks (within 10 per cent). The extract is divided over the wet rooms in proportion to their high rates, so each wet room gets at least its Table 1.2 rate. You can adjust every figure in the table before you download the plan.
Source
The figures are taken from the official text, checked on 14 October 2026.