Loft Fire Safety Calculator

Where a loft conversion adds a storey more than 4.5m above ground, Approved Document B Vol 1 (§2.21) requires the stairway to be protected: fire-resisting doors to a minimum of E20 (FD20) and partitions to a minimum REI 30 (30-minute) construction, upgrading the existing doors on the escape route where necessary. It also requires interlinked mains-powered smoke alarms to Grade D LD2 (§1.8–1.9, BS 5839-6). With an enclosed protected stairway, escape windows are NOT required in the loft rooms — the protected stair is the means of escape.

The Approved Document B minimum for a door forming part of the enclosure to a protected stairway in a single-family dwellinghouse is FD20 (E20) — see Table C1, item 11(b) of AD B Vol 1 (2019). FD30 is a common voluntary over-spec that some building control bodies prefer, but it is not the regulatory minimum. (FD30/FD30s is reserved for cavity barriers and the door between a house and an integral garage.) Existing doors opening onto the stairway are upgraded to FD20 where necessary.

Adding a habitable room above the ground storey triggers fire detection in the circulation spaces (AD B Vol 1 §1.8–1.9). For a loft conversion to a three-storey house the established standard is Grade D Category LD2 to BS 5839-6:2019 — interlinked, mains-powered smoke detectors with battery standby in all circulation spaces (hallways, landings) plus the principal habitable rooms, with a heat alarm in the kitchen. LD2 is more comprehensive than the LD3 baseline for an ordinary new dwelling.

The default solution is an enclosed protected stairway, which does not need sprinklers. Sprinklers (or a mist system) come in only as the §2.23 alternative where the route cannot be enclosed — most commonly an open-plan ground floor with no hallway between the living area and the front door. That alternative sprinklers the open-plan area and provides a REI 30 partition plus an E20 door so loft occupants can reach a first-storey escape window.

Not where an enclosed protected stairway is provided (the standard solution) — the protected stair is the means of escape, and escape via windows above first-floor level is treated as too hazardous. An emergency egress window only appears in the §2.23 open-plan/sprinkler alternative, where ONE first-storey escape window is reached via the E20 door. Such a window needs at least 0.33m² clear openable area, at least 450mm high and wide, with the sill no more than 1100mm above the floor (§2.10).

Fire safety for a typical 2-storey to 3-storey loft conversion is broadly £3,000–£6,000. This includes the FD20 fire doors (around £300–£450 each fitted), a Grade D LD2 alarm system (£400–£800) and fire-resistant stairway lining (£500–£1,500). If the §2.23 sprinkler alternative is used (open-plan ground floor), add roughly £2,000–£4,000 for suppression plus a first-storey escape window. All figures are indicative — get quotes and confirm scope with building control.