Glossary
Part B: fire safety rules for London extensions and lofts
Fire safety. Means of escape, fire-resisting construction, fire spread.
Fire safety. Means of escape, fire-resisting construction, fire spread.
Approved Document B covers means of warning and escape, internal and external fire spread, and access for the fire service. In domestic work it is the document that drives loft conversion design: a third storey triggers a 30-minute protected stair enclosure with fire doors to every habitable room off it.
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 each room opening onto it.
Mains-powered interlinked smoke alarms are required on every storey, with a heat alarm in the kitchen. An open-plan ground floor complicates this considerably, because the protected route runs through the open space — the usual solutions are a sprinkler system or an alternative escape route from the first floor.
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 height and width, with the bottom of the opening between 800 and 1,100 mm above floor level.
External fire spread is controlled by distance to the boundary. Unprotected areas — windows and combustible cladding — are limited on an elevation within 1 m of a boundary, which is why a flank wall built close to the line is normally blank masonry, and why a timber-clad garden room within 1 m of a boundary is a problem.
A worked Waltham Forest example of approved document b (fire safety): with 17 conservation areas — the Walthamstow Village Conservation Area and the Lloyd Park Conservation Area among the best known — London Borough of Waltham Forest (Waltham Forest Council) sees approved document b (fire safety) constantly, and an Article 4 direction is in force there, so permitted development cannot be assumed. Where approved document b (fire safety) pushes a Waltham Forest scheme out of permitted development, the route becomes a £548 application decided in 8 weeks rather than a £249 prior approval in 6 weeks.
| Protected stair trigger | Floor over 4.5 m above ground |
|---|---|
| Fire resistance | 30 minutes |
| Door specification | FD30 with intumescent strips |
| Escape window clear opening | 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 from the first floor.
Within 1 m of a boundary, yes — combustible cladding is restricted.
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