When does glazing near a boundary need to be fire-rated?

External walls close to a site boundary may need to be fire-resisting to stop fire spreading to neighbouring buildings. Building guidance limits the amount of unprotected area, such as ordinary windows, based on distance to the boundary. Fixed glazing that meets the wall's full fire-resistance requirement, including insulation where that is required, is not counted as unprotected area, so it can let you keep glass in those walls.

Distance matters

The closer the wall is to the boundary, the smaller the unprotected area allowed. Very close walls may need to be almost entirely fire-resisting.

Which glazing counts

Only non-opening glazing that achieves the wall's full fire-resistance requirement stops counting as unprotected area. Where insulation is required, integrity-only (E) glazing generally still counts as unprotected, so EI glazing is usually needed. Check the calculation with your designer or building control.

Extensions and changes of use

Boundary issues often arise when extending towards a boundary or converting a building. Raise them early with your designer.

Updated 09/10/2026. General guidance only; your fire strategy, designer and building control body decide what applies to a specific building.