Building-Regs FAQ
What is Part B?
Part B covers fire safety. Loft conversions require fire-protected escape route, mains-wired smoke alarms, 30-minute fire doors on protected corridor.
Part B covers fire safety. Loft conversions require fire-protected escape route, mains-wired smoke alarms, 30-minute fire doors on protected corridor.
Part B is the fire safety section of the Building Regulations: means of warning and escape, internal and external fire spread, and access for the fire service. In domestic work it is what drives loft conversion design — a third storey requires a 30-minute protected stair with fire doors to habitable rooms.
Once a floor is more than 4.5 m above ground level, escape can no longer rely on windows. The stair becomes the protected escape route, enclosed in 30-minute fire-resisting construction from the top storey to a final exit, with FD30 doors fitted with intumescent strips to every habitable room off it.
Mains-powered interlinked smoke alarms are required on every storey, with a heat alarm in the kitchen.
This is the most common conflict in a London loft conversion. The protected route would run through the open space, which Part B does not accept. The usual solutions are a domestic sprinkler system to BS 9251, or an alternative escape route from the first floor, or reinstating a hall enclosure.
In a one- or two-storey house, habitable rooms other than kitchens need an openable escape window with a clear opening of at least 0.33 m², at least 450 mm in both directions, with the cill 800 to 1,100 mm above floor level. A rooflight can serve this purpose in a loft if it meets the same figures.
External fire spread limits unprotected areas — windows and combustible cladding — on an elevation within 1 m of a boundary. That is why a flank wall built close to the line is normally blank masonry, and why timber cladding on a garden room within 1 m of a boundary is a problem.
What is Part B? in a Westminster context: City of Westminster (Westminster City Council) runs 56 conservation areas including the Mayfair Conservation Area and the Belgravia Conservation Area, and an Article 4 direction is in force there, so permitted development cannot be assumed. That decides how what is part b? is handled: £548 and 8 weeks for a Westminster householder application, or £274 for the Lawful Development Certificate that records what is part b? as lawful. The law behind what is part b? sits in the Town and Country Planning Act 1990, the GPDO 2015 and the Building Regulations 2010.
| Protected stair trigger | Floor over 4.5 m above ground |
|---|---|
| Fire resistance | 30 minutes |
| Doors | FD30 with intumescent strips |
| Escape window | 0.33 m², min 450 mm each way |
| Cill height | 800–1,100 mm |
| Boundary rule | Unprotected areas limited within 1 m |
Yes — FD30 doors to habitable rooms opening onto the protected stair.
Only with a sprinkler system or an alternative escape route.
Yes, if the clear opening and cill height comply.
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