Do I legally need an extractor fan in my bathroom?

Building Regulations Part F 2021 requires bathrooms with a bath or shower to have either 15 L/s intermittent extract OR 8 L/s continuous extract OR an openable window of at least 1/20th floor area (with trickle vents). Existing homes are not required to retrofit fans unless undergoing major renovation.

Do internal bathrooms need extractor fans?

Yes. Internal bathrooms (no external walls or windows) must have mechanical extract: 15 L/s intermittent or 8 L/s continuous. They cannot rely on natural ventilation. The fan must vent directly outside, not into voids or lofts.

Do I Need an Extractor Fan in Every Bathroom?

Q: Can I use a window instead of an extractor fan?

Yes, if the openable window area is at least 1/20th of the bathroom floor area AND the room has background ventilation (trickle vents providing 8,000 mm²). A 2m × 2m bathroom (4 m²) needs 0.2 m² (200,000 mm²) openable window. Most bathroom windows qualify.

UK Building Regulations requirements for bathroom ventilation explained, including exemptions and alternatives.

Last reviewed: 20 May 2026 | 3 min read | Verified against 2 sources

Building Regulations Part F 2021 requires bathrooms with a bath or shower to have 15 L/s intermittent extract OR 8 L/s continuous extract OR an openable window (minimum 1/20th floor area) with background ventilation (trickle vents providing 8,000 mm²). Internal bathrooms (no windows) must have mechanical extract. Separate WCs need 6 L/s extract or equivalent natural ventilation. Existing homes are exempt unless undergoing renovation work that requires Building Control approval.

Part F Requirements

Building Regulations Approved Document F 2021 sets minimum ventilation requirements for bathrooms:¹

Bathrooms with bath or shower: 15 L/s intermittent extract OR 8 L/s continuous extract
Separate WC: 6 L/s intermittent OR 6 L/s continuous extract
Alternative: Openable window (minimum 1/20th floor area) plus background ventilation (8,000 mm² trickle vents)

These requirements apply to new builds, extensions, and material alterations (work requiring Building Control approval). Existing bathrooms are not required to retrofit fans unless undergoing renovation.

When Is an Extractor Fan Mandatory?

An extractor fan (or mechanical extract system) is mandatory if:

The bathroom has no external window (internal bathroom).
The existing window is too small (less than 1/20th floor area).
The bathroom has no background ventilation (no trickle vents and cannot install them).
You're building new, extending, or doing major renovation requiring Building Control approval.

Internal bathrooms (no external walls) have no alternative to mechanical extract. The fan must vent directly outside through ductwork, not into lofts or voids.

Can I Use a Window Instead?

Yes, if the openable window meets the size requirement and you have background ventilation. A 2m × 2m bathroom (4 m² floor area) needs an openable window of at least 0.2 m² (200,000 mm²). Most standard bathroom windows qualify.

The window must be openable (not fixed glazing) and easily accessible. Roof windows or high-level windows that require a pole to open still count, but they're less practical for purge ventilation.

You also need background ventilation: trickle vents providing 8,000 mm² equivalent area. If your windows lack trickle vents, you can retrofit them or install wall vents.²

What About Internal Bathrooms?

Internal bathrooms (no external walls or windows) must have mechanical extract. There is no exemption. The fan must provide:

15 L/s intermittent (linked to light switch, with 15-minute overrun timer), OR
8 L/s continuous (running 24/7 at low speed, boosting when humid or when light is on)

Continuous-run fans are more effective for internal bathrooms because moisture is removed steadily. Intermittent fans rely on users remembering to leave them running, which often doesn't happen.

Do Existing Homes Need Retrofitting?

No, unless you're doing work that requires Building Control approval (loft conversion, extension, change of use). Existing homes without extractor fans are not required to retrofit them.

However, if you have persistent condensation or mould, installing an extractor fan is advisable even if not legally required. Cost is £100-£300 depending on ductwork complexity.

Intermittent Extract Rate: 15 L/s (bathrooms), 6 L/s (WCs)
Continuous Extract Rate: 8 L/s (bathrooms), 6 L/s (WCs)
Minimum Window Size: 1/20th floor area (openable)
Background Ventilation: 8,000 mm² (trickle vents or wall vents)

Sources

HM Government, Approved Document F: Ventilation (2021 edition), Table 1.1a, 2021. gov.uk (accessed 20 May 2026)
Chartered Institution of Building Services Engineers (CIBSE), Domestic Ventilation Compliance Guide, 2022. cibse.org (accessed 20 May 2026)

Last reviewed: 20 May 2026 | Word count: 589 | Reading time: 3 minutes