Part B and what it covers
Approved Document B sets out fire safety requirements under Schedule 1 of the Building Regulations. It is split into Volume 1 (dwellinghouses) and Volume 2 (buildings other than dwellinghouses, including flats). Part B covers means of warning and escape (B1), internal fire spread (linings B2 and structure B3), external fire spread (B4), and access for the fire service (B5). For a single-family dwellinghouse, the most relevant requirements are escape routes from upper floors, fire-protected stairs once you reach three storeys, fire doors to certain rooms, mains-wired interlinked smoke alarms throughout, and structural fire resistance to floors and partitions. The detail in Part B is technical — your architect and building control inspector apply it; you need to understand the consequences.
Escape routes for loft conversions — the critical change at three storeys
A loft conversion turns a two-storey house into a three-storey house, and three-storey escape rules are significantly more demanding. The new staircase must be fire-protected — meaning the stair enclosure walls must achieve 30 minutes fire resistance, and every door opening onto the stair from a habitable room (bedrooms, living rooms) must be a 30-minute fire door (FD30) with intumescent seals and smoke seals. The escape window in the loft alone is no longer sufficient at three storeys; the stair itself must be the protected escape route. This is the single most common compliance failure on London loft conversions — homeowners install lightweight internal doors that don't meet FD30 standards, and building control rejects the final inspection.
Mains-wired interlinked smoke alarms
Part B requires Grade D Category LD2 mains-wired smoke alarms on every storey of a dwelling, interlinked so that if one alarm sounds they all sound. For loft conversions, that means one alarm on each landing and one in the loft itself. Battery-only alarms do not meet the requirement; smoke alarms must be wired to the mains with battery backup. In kitchens and bathrooms where steam triggers false alarms, heat alarms replace smoke alarms but still must be interlinked. Installation must be by a Part P registered electrician (see Building Control Inspections guide) and the system commissioning certificate is part of the Final Certificate paperwork.
Sprinklers — when London requires them
England-wide, sprinklers are mandatory in new blocks of flats above 11 metres, in care homes, and in school blocks above a certain size. In single-family dwelling houses, sprinklers are NOT mandatory under English Building Regulations regardless of height — even a four-storey London townhouse with a loft conversion does not require sprinklers. Wales is different: all new homes in Wales need sprinklers. Some London boroughs (notably Wandsworth and Croydon) have explored introducing local sprinkler requirements through their local plans, but as of 2026 there is no statutory requirement for sprinklers in single-family dwellings in any London borough. For flats, the 11-metre rule applies — a new four-storey block of flats triggers sprinklers.
Post-Grenfell external wall requirements
Since 2018 the rules on external walls have tightened sharply. Under Regulation 7(2), external wall constructions on residential buildings above 11 metres must be of materials achieving European Class A2-s1, d0 or better — effectively limiting external cladding to non-combustible materials. This affects loft conversions on flats above 11 metres and new external wall insulation (EWI) systems. For single-family houses below 11 metres, the rules are less restrictive but still require careful specification of any combustible insulation. The Building Safety Act 2022 introduced the Building Safety Regulator and a new gateway regime for higher-risk buildings (above 18 metres or 7 storeys) — most single-family work is unaffected, but flat conversions above the threshold trigger significant additional scrutiny.
Fire doors — specification and installation
Fire doors are the single most common point of failure on London residential fire safety. A compliant FD30 door has a 44mm thick solid core, intumescent strip around the leaf or frame that expands under heat to seal the gap, smoke seals (brush or flexible blade), self-closing hinges, and certified door furniture (handles, latches) rated for fire performance. Tolerances are tight: the gap between leaf and frame must be 2–4mm; gaps larger than 4mm fail because the intumescent cannot bridge them. Installation must be by a competent installer — the BWF Certifire scheme and FDIS schemes both register competent installers. Cheap fire doors from generic suppliers regularly fail building control because they lack certified hinges or seals.
Practical fire safety in a London loft conversion
A typical compliance package for a London loft conversion in 2026 looks like this: new staircase enclosed in 30-minute fire-resistant partition walls, FD30 doors with smoke seals to every bedroom and habitable room on the escape route, mains-wired interlinked smoke alarms on every storey plus a heat alarm in the kitchen, an escape window in the loft (clear opening 0.33 sqm minimum, lowest point of opening within 1.1m of floor), and a fire-resistant ceiling between loft and second floor achieving 30 minutes fire resistance. Budget £2,500–£4,500 within the build cost for fire compliance — doors alone run £350–£600 each fully fitted. This is non-negotiable: building control will withhold the Final Certificate until every element passes.
