Question 1

What is an unprotected area in Part B?

Accepted Answer

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).

Question 2

What is the "1 metre rule" for boundaries?

Accepted Answer

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).

Question 3

What are the four methods for calculating unprotected areas?

Accepted Answer

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.

Question 4

Does a conservatory count as an unprotected area?

Accepted Answer

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.

Question 5

Do Scotland and Northern Ireland use the same boundary rules?

Accepted Answer

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.

Min. Distance to Boundary	Max. Unprotected Area (m²)	Notes
Less than 1 m	None (fire-rated wall required)	Small openings up to 0.1 m² permitted at 1.5 m spacing
1 m	5.6 m²	Typical for terraced/semi-detached side walls
2 m	12 m²	Common for detached houses
5 m	40 m²	Most windows achievable
6 m	No limit	Wall can be fully unprotected
More than 6 m	No limit	No restriction on unprotected areas

Rule	Requirement
Relevant boundary	The boundary of the land, or the centre line of a road/railway/canal adjoining the site
Measurement	Perpendicular from the external wall face to the relevant boundary
Combustible cladding	Counts as unprotected area in most circumstances
Small openings	Openings ≤ 0.1 m² each (e.g. airbricks) may be disregarded if spaced ≥ 1.5 m apart
Portal frame buildings	Special rules apply — see AD-B Vol 2 Section 14

Method	Approach	Typical Use
Method 1	Simple table lookup (Diagram 40 / Table 14)	Houses and small residential up to 3 storeys
Method 2	Enclosing rectangle (BRE 187)	Larger buildings, non-residential
Method 3	Aggregate notional area with weighted zones	Complex facades with mixed materials
Method 4	Fire engineering (BS 9999 or equivalent)	Complex or unusual buildings

Part B Unprotected Areas & Boundary Distance

Method 1 — Permitted Unprotected Areas for Dwellings (AD-B Vol 1, Table 14 / Diagram 40)

Key Rules for Boundary Distance Assessment

Methods Summary (AD-B Section 13/14)

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