Part B Unprotected Areas & Boundary Distance

An unprotected area is any part of an external wall that does not achieve the required fire resistance. This includes windows, doors, combustible cladding, and any opening or non-fire-rated section. The permitted total unprotected area depends on the distance to the relevant boundary, calculated using methods in AD-B Section 13 (dwellings) and Section 14 (other buildings).

If a wall is 1 metre or more from the relevant boundary, some unprotected areas are permitted. At exactly 1 m from the boundary, dwellings up to 3 storeys may have up to 5.6 m² of unprotected area per side (per Diagram 40 / Table 14). At less than 1 m from the boundary, walls must generally be fire-rated with no unprotected areas unless they are very small (less than 1 m² in total).

Method 1 (Diagram 40) is the simplified method for small residential buildings. Method 2 uses the "enclosing rectangle" approach from BRE Report BR 187. Method 3 is the "aggregate notional area" method allowing trade-offs between different parts of the facade. Method 4 uses BS 9999 or other fire engineering approaches for complex buildings. Most domestic work uses Method 1.

Yes. If a conservatory or extension is within the relevant boundary distance, its glazing and any non-fire-rated walls count as unprotected areas. Where a conservatory is built against a boundary wall, the existing wall must maintain its fire resistance, and the conservatory walls facing the boundary must be fire-rated or set back sufficiently.

Scotland uses Technical Handbook Section 2.6 (Spread to neighbouring buildings) which follows similar principles but with different tables. Northern Ireland uses Technical Booklet E with boundary distance requirements broadly aligned to AD-B. Always refer to the relevant jurisdiction's documents for exact requirements.