Building Regulations: Ventilation Requirements for Extensions

When you build an extension or add new rooms to your home, ensuring proper ventilation isn't just a recommendation – it's a legal requirement for your health and comfort. The exact rules depend on the type of room you're creating. Furthermore, if you're installing a new internal wall, you must make certain it doesn't negatively impact the airflow in existing areas or result in a new room without sufficient ventilation.

Your Essential Ventilation Rules

Purge Ventilation (Rapid Air Change)

This type of ventilation allows you to quickly replace the air in a room, usually by simply opening a window. For effective purge ventilation, the window's clear opening needs to meet specific criteria:

• If your window opens between 15 and 30 degrees, the clear opening must be at least 1/10th of the room's total floor area.

• Should your window open wider, exceeding 30 degrees, the required clear opening reduces to at least 1/20th of the room's floor area.

• Important: Windows that open less than 15 degrees do not qualify for purge ventilation.

Whole Building Ventilation (Continuous Airflow)

Often called 'trickle ventilation', this provides a constant, low level of fresh air throughout your home. You'll typically find these vents built into window frames, though other methods are also acceptable. The necessary 'equivalent area' — a way of measuring airflow capacity — differs depending on your room and building type:

• For a habitable room or kitchen within a building that has multiple floors, you'll need an equivalent area of 8,000 mm².

• If it's a habitable room or kitchen in a single-storey property, the requirement increases to 10,000 mm² equivalent area.

• For any bathroom, whether it includes a WC or not, you must provide an equivalent area of 4,000 mm².

Generally, your new rooms will require both purge and whole building ventilation methods. However, if you're considering alternative ventilation systems, you can propose them to your local Building Control team for their approval.

Specific Requirements: Extract Ventilation

To manage moisture and eliminate odours effectively, any new kitchen, bathroom (including shower rooms), utility room, or toilet you add needs 'extract ventilation'. This typically involves a mechanical fan that removes stale, humid air.

There's an important exception for toilets: instead of a mechanical extractor fan, you can use 'purge ventilation' (a window or door that opens directly to an external wall) to satisfy this requirement, provided that doing so doesn't create a security risk for your property.