Extension Ventilation Calculator — Part F Requirements

Part F 2021 requires three types of ventilation in extension rooms: background ventilation via trickle vents in windows (8,000mm² per habitable room/kitchen), extract ventilation in wet rooms (kitchen 30 l/s with a hood ducted outside or 60 l/s without, utility room 30 l/s, bathroom 15 l/s, WC 6 l/s — ADF Table 1.1), and purge ventilation via openable windows (1/20th of floor area for windows opening 30° or more). All three must be provided.

Yes. All new windows in extensions must have trickle vents (background ventilators) to comply with Part F 2021. Per ADF 2021 Vol 1 Table 1.7 the equivalent area is 8,000mm² per habitable room and per kitchen (the kitchen is treated the same as a habitable room), 4,000mm² for a bathroom, and no minimum for a utility room or WC. Single-storey dwellings use 10,000mm² per habitable room/kitchen, and at least 5 background ventilators are required across the habitable rooms and kitchens (4 for a one-bedroom dwelling). Trickle vents are slotted ventilators built into the window frame, controllable by the occupant.

A kitchen needs 30 litres per second (l/s) intermittent extract or 13 l/s continuous extract. This can be an extractor fan, cooker hood ducted to outside, or connection to a centralised MEV system. The extract must discharge to outside — recirculating cooker hoods do not satisfy Part F.

Purge ventilation is rapid ventilation for diluting pollutants, provided by openable windows. Per ADF 2021 Vol 1 Table 1.4 the openable area must be at least 1/20th of the room floor area for windows opening 30° or more, sash windows or external doors; 1/10th for hinged or pivot windows opening 15°–30°; windows opening less than 15° are not suitable. This calculator reports the 1/20 figure — double it for windows opening 15°–30°. Purge ventilation is separate from background (trickle) and extract ventilation — all three are required.

For a simple extension adding 1–2 rooms, individual extract fans with trickle vents is the easiest solution. For larger extensions or whole-house renovations, centralised MEV (mechanical extract ventilation) or MVHR (mechanical ventilation with heat recovery) is more efficient. MVHR recovers up to 90% of heat from extracted air.