Glossary
Part F: the ventilation London building work must provide
Ventilation. Background and purge ventilation requirements.
Ventilation. Background and purge ventilation requirements.
Approved Document F sets ventilation requirements for dwellings — background ventilation, extract from wet rooms and purge ventilation. The 2021 edition raised the standards significantly, requiring larger trickle ventilators and, where windows are replaced, ventilation at least equivalent to what was there before.
Intermittent extract of 30 litres per second in a kitchen with a cooker hood, 60 without one, 15 in a bathroom and 6 in a WC. Purge ventilation needs an openable area of at least one twentieth of the floor it serves.
| Room | Intermittent extract rate | Notes |
|---|---|---|
| Kitchen, hood over the hob | 30 l/s | — |
| Kitchen, elsewhere in the room | 60 l/s | — |
| Utility room | 30 l/s | — |
| Bathroom | 15 l/s | with or without a WC |
| Sanitary accommodation | 6 l/s | or 4 air changes per hour |
| Purge ventilation, any room | openable area ≥ 1/20 of floor area | hinged or pivot windows ≥ 30° |
Source: Approved Document F, Volume 1: Dwellings (2021 edition), Tables 1.1 and 1.3, read at 1 September 2026. Part F bites on a London extension whenever the work covers over an existing window or removes a room’s ventilation route — a rear extension across a kitchen window is the commonest trigger, and it is checked by building control rather than by planning.
Intermittent extract: 30 l/s in a kitchen if above a hob, or 60 l/s elsewhere in the kitchen; 15 l/s in a utility room; 6 l/s in a bathroom; 6 l/s in a WC. Continuous extract runs lower — 13 l/s kitchen, 8 l/s bathroom, 6 l/s WC — with a boost rate above.
Purge ventilation must be provided in every habitable room, normally by an openable window with an opening area of at least one twentieth of the floor area.
The 2021 edition requires that replacement windows provide background ventilation no worse than the windows they replace, and where the existing windows had trickle ventilators, the new ones must too. The recommended equivalent area for a habitable room in an existing dwelling is 8,000 mm² per room.
This is the single most commonly missed requirement in London window replacement projects, and it is a building regulations breach that surfaces on sale.
Tightening the fabric without matching ventilation causes condensation and mould. Approved Document F is designed to be read with Part L: as air permeability falls, the ventilation system has to take over from adventitious leakage.
A worked Ealing example of approved document f (ventilation): with 30 conservation areas — the Ealing Broadway Conservation Area and the Pitshanger Conservation Area among the best known — London Borough of Ealing (Ealing Council) sees approved document f (ventilation) constantly, and an Article 4 direction is in force there, so permitted development cannot be assumed. Where approved document f (ventilation) pushes a Ealing scheme out of permitted development, the route becomes a £548 application decided in 8 weeks rather than a £249 prior approval in 6 weeks.
| Kitchen extract, over hob | 30 l/s intermittent |
|---|---|
| Kitchen extract, elsewhere | 60 l/s intermittent |
| Bathroom extract | 15 l/s intermittent, 8 l/s continuous |
| WC extract | 6 l/s |
| Purge ventilation | 1/20 of floor area openable |
| Trickle vent, habitable room | 8,000 mm² equivalent area |
Yes, where the existing windows had them, and ventilation must not be reduced.
Only if it extracts to outside at the required rate — recirculating hoods do not count.
Each new habitable room needs background and purge ventilation.
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