Extension Fire Safety Calculator — Part B Compliance

Not necessarily. For a two-storey dwellinghouse the first floor is no more than 4.5m above ground, and AD B Vol 1 para 2.2 gives a CHOICE: each first-floor habitable room (kitchens excepted) needs EITHER an emergency escape window/external door (para 2.10) OR direct access to a protected stairway. Escape windows are the common solution — a protected stairway is an acceptable optional alternative, not a requirement. A protected stairway only becomes mandatory once a storey floor is MORE than 4.5m above ground (three storeys / a loft conversion), under para 2.5.

Where a protected stairway IS required (3 storeys / loft, para 2.5/2.21), AD B Vol 1 specifies fire doorsets of minimum E 20 — historically an FD20 door. Because certifiers (BWF, BM TRADA) no longer certify FD20 doors, FD30 is fitted in practice. Crucially, in a single-family dwellinghouse AD B Vol 1 does not require these fire doors to be self-closing (other than a door between the house and an integral garage), and there is no requirement for cold smoke seals (the 'S' designation) — the self-closer and smoke-seal rules apply to flats and HMOs, not houses. This interpretation was long set out in the (now-withdrawn) BCA Technical Guidance Note 9, but it follows directly from Approved Document B Volume 1 itself, so the position is unchanged. Always confirm the detail with your Building Control body.

AD B Vol 1 Section 1 (§1.1) requires at least a Grade D2 Category LD3 system to BS 5839-6; for new habitable rooms and loft conversions, Building Control practice is Grade D1 Category LD2. Smoke alarms should be mains-powered (BS EN 14604) with a battery standby supply, sited on the escape route (hall plus each landing), with a heat alarm (BS 5446-2) in the kitchen and a smoke alarm in the principal habitable room, all interlinked so one trigger sounds all. A system is required under §1.8 where a new habitable room is created above or below the ground storey. Battery-only alarms are not acceptable for new work.

On a two-storey dwellinghouse, yes — an emergency escape window is the primary escape strategy for first-floor habitable rooms (AD B para 2.2/2.10) unless you instead provide direct access to a protected stairway. The window must have an unobstructed openable area of at least 0.33 m², a minimum 450mm height AND 450mm width, with the bottom of the opening no more than 1100mm above the floor. Locks (with removable keys) and child-resistant opening stays ARE permitted (para 2.10c), and the window must be able to remain open without being held (para 2.10d).

A loft conversion creates a storey above 4.5m, so it carries the fuller requirements (AD B para 2.21): a protected stairway from the ground floor to the loft separated by fire-resisting construction (minimum REI 30), fire doorsets of minimum E 20 (FD20, in practice FD30) on rooms opening onto the stairway with existing doors upgraded where necessary, mains-powered interlinked smoke/heat alarms on every level, and escape provision for loft rooms. An open-plan alternative (para 2.23) allows sprinklers to BS 9251 plus a REI 30 partition / E 20 door separating ground from upper storeys and REI 30 around cooking, instead of fully enclosing the stair.