What ventilation do extension rooms need under Part F?

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.

Do all new windows need trickle vents?

Yes. All new windows in extensions must have trickle vents (background ventilators) to comply with Part F 2021. The total equivalent area should be 8,000mm² per habitable room/kitchen and 4,000mm² per wet room (Table 5.2b). Trickle vents are slotted ventilators built into the window frame, controllable by the occupant.

What extract rate is needed for a kitchen extractor?

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.

What is purge ventilation?

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.

Should I use MEV or MVHR for an extension?

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.

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Extension Ventilation Calculator — Part F Requirements

Calculate Part F ventilation requirements for extension rooms including trickle vents, extract rates for wet rooms and purge ventilation specifications.

Total extension floor area

Kitchens

Bathrooms

En-suites

Living rooms / bedrooms

Openable windows

How We Calculate This

This calculator determines Part F ventilation requirements based on room types and floor area.

Extract rates

Kitchen: 30 l/s, bathroom: 15 l/s, utility: 15 l/s, WC: 6 l/s. These are intermittent rates (when in use).

Background ventilation

Trickle vents (Part F 2021 Table 5.2b): 8,000mm² per habitable room/kitchen, 4,000mm² per wet room. Divided among windows in each room.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.