Part B Unprotected Areas & Boundary Distance
Boundary distance calculations and permitted unprotected areas for external walls, referencing Approved Document B: Volume 1 - Dwellings and Volume 2 - Buildings other than dwellings (2019 edition incorporating 2020 amendments), Diagram 40 and Table 14.
Method 1 — Permitted Unprotected Areas for Dwellings (AD-B Vol 1, Table 14 / Diagram 40)
For residential buildings not more than 3 storeys, the maximum total unprotected area on any side depends on the minimum distance from the side of the building to the relevant boundary.
| 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 |
Key Rules for Boundary Distance Assessment
| 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 |
Methods Summary (AD-B Section 13/14)
| 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 |
Related Calculators
Use the Loadbearing Wall Calculator for structural design. See the Part B Fire Resistance Ratings page for fire rating requirements by element.
Always check the latest edition of Approved Documents on the Planning Portal (planningportal.co.uk) as regulations are updated periodically.
How We Calculate This
Frequently Asked Questions
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Last updated: April 2026
All calculations are estimates. Verify with your supplier.