Question 1

What ventilation do extension rooms need under Part F?

Accepted Answer

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 adjacent to hob, bathroom 15 l/s), and purge ventilation via openable windows (1/20th of floor area). All three must be provided.

Question 2

Do all new windows need trickle vents?

Accepted Answer

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. Trickle vents are slotted ventilators built into the window frame, controllable by the occupant.

Question 3

What extract rate is needed for a kitchen extractor?

Accepted Answer

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.

Question 4

What is purge ventilation?

Accepted Answer

Purge ventilation is rapid ventilation for diluting pollutants, provided by openable windows. Each habitable room needs an openable window area of at least 1/20th of the floor area. Purge ventilation is separate from background (trickle) and extract ventilation — all three are required.

Question 5

Should I use MEV or MVHR for an extension?

Accepted Answer

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.

Extension Ventilation Calculator — Part F Requirements

How We Calculate This

Extract rates

Background ventilation

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