Regulations
Bathroom extractor fan rules in Scotland
Scotland's building standards expect a bathroom or shower room to have mechanical extraction capable of at least 15 litres per second, or a passive stack ventilation system, according to Standard 3.14 of the Domestic Technical Handbook. A continuously running fan is another option. This guide explains the rules, why ventilation matters in Scottish homes and how to choose a fan.
What does Standard 3.14 say?
| Option | What it needs |
|---|---|
| Intermittent extractor fan | At least 15 litres per second |
| Continuous extract fan (dMEV) | 4 litres per second continuous, with an 8 litres per second boost |
| Passive stack ventilation | A system meeting the standard, in place of a fan |
| Trickle ventilation | 10,000 mm² recommended, doubled if the vent is ducted |
This is a summary. Check the current handbook, or ask your council's building standards team, for your own project.
Why ventilation matters
Showers and baths release a lot of moisture. Without good extraction, it condenses on cold walls and windows, which leads to mould, peeling paint and damage over time. Scottish homes, especially older tenement flats with cold external walls, are particularly prone to condensation.
Bathrooms with no window
An internal bathroom or en-suite with no window relies entirely on the extractor fan, so the fan needs to be the right size and properly ducted to the outside. A fan linked to the light, or one with a humidity sensor, makes sure it runs whenever it is needed. See our guide to adding an en-suite.
Choosing a fan
- Check the extract rate meets the standard for your room.
- Keep duct runs short and straight, because long or kinked ducts reduce the airflow.
- Choose a quiet model, as noisy fans tend to get switched off.
- Consider a humidity sensor or a continuous low-speed fan for rooms that steam up badly.
- Have the wiring done by a registered electrician, following the bathroom zone rules.
Does a new fan need a building warrant?
Providing an extractor fan is listed as work that does not need a building warrant, although it must still meet the building standards. See our building warrant guide. We include the extractor fan when we plan a new bathroom or en-suite.
Sources
- Scottish Government: Domestic Technical Handbook, 3.14 Ventilation
- Scottish Government: Domestic Technical Handbook, 0.5 Work not requiring a warrant
Figures from other organisations are theirs, not our prices. Check each source for the latest version.
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Is an extractor fan a legal requirement in a Scottish bathroom?
For new work, Scotland's building standards expect a bathroom or shower room to have mechanical extraction of at least 15 litres per second, a continuous extract fan, or a passive stack system. Standard 3.14 of the Domestic Technical Handbook sets this out. If you are renovating, it makes sense to bring ventilation up to this standard while the walls are open.
What size extractor fan do I need for a bathroom?
Scotland's Standard 3.14 asks for mechanical extraction capable of at least 15 litres per second for an intermittent bathroom fan, or 4 litres per second continuous with an 8 litres per second boost for a continuous fan. Long duct runs reduce airflow, so the fan should be chosen with the duct route in mind. An installer can check this for your room.
Does a bathroom with a window need a fan?
Under Standard 3.14, a bathroom or shower room is expected to have mechanical extraction or a passive stack system, not just a window. In practice, people rarely open the window in winter, so a fan is the reliable way to clear steam. A fan with a humidity sensor runs automatically when the room is damp. Opening a window still helps in summer.
Why does my bathroom still get mould with a fan?
The fan may be too weak for the room, the duct may be long, kinked or blocked, or the fan may not run long enough after a shower. Cold walls also attract condensation. Check the fan's extract rate and the duct, and consider a humidity-controlled fan. Clearing existing mould and improving heating can help too. We check fans and ducts at the survey.