Start with the floor height and layout
Approved Document B distinguishes upper storeys no more than 4.5 metres above ground from those above that height. For the lower category, its house guidance allows arrangements involving suitable emergency escape windows or a protected stair, subject to the complete layout and relevant conditions. A bungalow conversion may fall within that category, but the actual floor level must be measured. [1]
For the typical conversion above an existing two-storey house, the new loft is higher. The guidance then addresses the protected route through the house or an appropriate alternative. Do not order an 'escape rooflight' on the assumption that its product name removes the need for downstairs fire precautions. [1] [2]
The opening dimensions
Where an emergency escape window is required under the house guidance, paragraph 2.10 provides for:
- An unobstructed openable area of at least 0.33 square metres.
- An opening at least 450mm high and at least 450mm wide.
- The bottom of the openable area no more than 1100mm above the floor. [1]
All these tests apply together. A 450mm by 450mm opening has an area of only 0.2025 square metres, so meeting both minimum side dimensions alone is not enough. That is a mathematical illustration, not a recommended window size.
Measure the actual clear opening available when the window is open. The frame's outside size, glass area and catalogue dimensions are not interchangeable with the usable escape opening. The guidance also requires the window to remain open without being held. [1]
Where does the opening lead?
The window must allow a person to reach a place free from danger from fire. A roof, enclosed court or obstructed area outside needs assessment; providing the correct opening dimensions does not resolve an unsuitable destination. Approved Document B includes conditions for enclosed courts and for any escape route over a flat roof. [1]
Ask the designer to show the inside approach, sill position and outside landing or onward route, where relevant. Furniture should be arranged so it does not defeat the access relied on in the fire strategy. [1]
Check the planning conditions too
A side-facing roof window proposed under householder permitted development must be obscure-glazed. It must also be non-opening unless the opening parts are more than 1.7 metres above the floor. Those planning restrictions can conflict with the position needed for an escape window. Do not assume one window will satisfy both jobs without a coordinated assessment. [3] [1]
Do not remove an existing escape function
When replacing a window that serves an escape purpose, the guidance protects its potential for escape, allowing reduction only to the specified minimum where the existing opening is larger. A replacement selected only for thermal performance or appearance may therefore be unsuitable. [1]
Flats have separate escape guidance. The house dimensions above should not be used as a complete fire strategy for a top-floor flat or a building with shared stairs. [1]