Part B Fire Safety: Escape Routes, Compartmentation & Surface Spread of Flame

Summary

Fire safety is one of the most legally consequential areas of Building Regulations. Failures in fire safety have caused deaths in UK residential buildings, and contractors who install non-compliant fire doors, inadequate detection, or compromised escape routes carry real legal exposure. Part B is not something to interpret loosely.

Approved Document B Volume 1 applies to dwellinghouses and flats. Volume 2 applies to buildings other than dwellings. The 2019 edition introduced changes following the Grenfell Tower fire, particularly around external wall cladding (important for buildings over 11m and irrelevant to most domestic work) but the fundamental domestic fire safety requirements are well-established.

The most common points of failure in domestic building work are: loft conversions without upgrading the escape route; conversions to flats without proper fire compartmentation; fire doors not installed to the correct specification; and smoke alarms that are not mains-wired or interconnected as required. Understanding these requirements saves callbacks and avoids liability.

Key Facts

• Smoke alarms — new dwellings: Mains-powered (230V), battery back-up, interconnected; minimum one per storey, plus one in every principal habitable room
• Smoke alarms — material alteration (loft conversion, extension): Mains-wired interlinked alarms required throughout existing dwelling as part of the upgrade
• Smoke alarm type: Optical (for living spaces and bedrooms), heat (for kitchens — smoke alarms are not suitable in kitchens), CO alarm (for rooms with combustion appliances)
• Escape window: In loft conversion bedrooms, an openable escape window is required; minimum 0.33m² area, minimum 450mm height and width, sill not more than 1,100mm above floor; OR a protected escape route (fire doors and corridor)
• Fire door specification — FD30S: 30-minute fire-resisting door with smoke seal; required to protect the staircase escape route in a 2-storey house with a loft conversion
• Fire door specification — FD60: 60-minute door for certain flat conversions and HMO situations
• FD30 hinges: Minimum 3 × CE-marked fire-rated steel hinges; self-closing device required
• Intumescent strips: Fitted in the door or frame rebate — expand in fire to seal the gap between door and frame
• Compartmentation (flats): Each flat must be a self-contained fire-resisting unit — walls, floors, and ceilings forming the flat boundary must achieve 60-minute fire resistance (REI 60)
• Protected escape route: A corridor or staircase from which escape to safety is possible; must be enclosed with 30-minute fire-resisting walls, ceiling, and doors
• Surface spread of flame: Ceilings and walls in escape routes must be minimum Class 1 (BS 476 Part 7) or Class B (BS EN 13501-1); no Class 3 or D materials
• Cavity barriers: Required at junctions between compartments and in cavities — prevents fire spreading horizontally through concealed spaces
• Party walls (flats): Full-height fire and acoustic compartmentation — no penetrations without fire stopping; pipes and cables must be fire-stopped with intumescent collars or mastic
• Sprinklers: Required in all new dwellings in Wales (since 2016); strongly recommended in high-risk residential in England

Quick Reference Table

Detailed Guidance

Loft Conversion Fire Safety

A loft conversion adding a new habitable room (bedroom, study, etc.) creates a three-storey dwelling. The third floor is the highest risk — occupants furthest from the exit. Part B requires one of two solutions:

Option 1 — Escape window:

• An openable window in the new loft room suitable for escape and fire brigade access
• Minimum opening: 0.33m² area, minimum 450mm height × 450mm width
• Sill height: maximum 1,100mm above floor level (so occupants can easily reach it)
• The window must open fully — not restricted by a vent limiter

Option 2 — Protected escape route:

• The staircase from the loft room to the ground floor is enclosed with 30-minute fire-resisting construction (walls, ceiling, doors)
• All doors opening onto the staircase from ground floor to loft are FD30S fire doors with self-closers
• Smoke detection throughout: mains-wired interconnected smoke alarms in the loft room, on the landing of each floor, and on the ground floor

In practice, Option 2 (protected stair) is more common because it provides better protection and many loft bedrooms do not have suitable windows for escape.

Upgrading the existing dwelling: When a loft conversion is added to an existing 2-storey house, the existing dwelling must also be upgraded. This typically means:

• Installing mains-wired interlinked smoke alarms throughout (if not already present)
• Installing FD30S fire doors to rooms opening onto the existing staircase
• Ensuring the staircase corridor meets 30-minute fire resistance

Building Control will inspect and require this upgrade before signing off the loft conversion.

Conversions to Flats

Converting a house to flats is one of the highest-risk domestic building operations from a fire safety perspective. Each flat must be a separate fire compartment.

Compartment floors and walls:

• The floor/ceiling between flats must achieve REI 60 (60-minute fire resistance for structure, integrity, and insulation)
• This typically requires a 125mm+ concrete slab or a timber floor with additional fire protection
• Standard 150mm timber floor joists with 12.5mm plasterboard ceiling: typically 30 minutes — NOT adequate for flat compartmentation
• For timber floors, achieve REI 60 by using 2 × 12.5mm plasterboard ceiling or 15mm + 12.5mm, combined with acoustic/fire quilt between joists

Flat entrance doors:

• Each flat must have an FD30S (or FD60 in certain HMO situations) entrance door with a self-closer
• The corridor/staircase outside the flat is the common escape route — it must be protected (30-minute fire resistance walls, FD30 doors to any risers/service cupboards)

Fire stopping:

• All penetrations through the compartment floor and walls (pipes, cables, ducts) must be fire-stopped
• Soil pipes passing through compartment floors must have intumescent pipe collars
• Electrical cables through compartment walls must be sealed with intumescent mastic or blocks

Smoke Alarms

New dwellings (Building Regulations Part B):

• Grade D, Category LD2 as a minimum: mains-powered (230V) with battery back-up, interconnected
• Minimum positions: one on each storey of the dwelling; one in each principal habitable room (bedroom or living room above ground floor)
• Kitchen: heat alarm (not smoke alarm); smoke alarms in kitchens cause constant false alarms
• Interconnected: all alarms activate when one triggers — critical so that occupants in any part of the building are alerted

Existing dwellings (material alteration — e.g. loft conversion):

• Upgrade to mains-wired interconnected system as part of the building work
• Battery alarms are not acceptable for Building Regulations compliance on notifiable work

Carbon monoxide (CO) alarms: Required under Building Regulations Approved Document J when a new combustion appliance is installed; strongly recommended in all rooms with a gas or solid fuel appliance. From October 2022, CO alarms are also required in all rooms with a fixed combustion appliance in rental properties (Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022).

Surface Spread of Flame

The classification of interior surfaces affects how fire spreads. In circulation spaces (corridors, stairwells, lobbies), high surface spread of flame would allow fire to travel rapidly and block escape.

Requirements for escape routes:

• Walls and ceilings must be minimum Class 1 (BS 476 Part 7) or Class B (BS EN 13501-1, Euroclass)
• Standard painted plasterboard is typically Class 1 — acceptable
• Most standard paint finishes are acceptable
• Not acceptable in escape routes: Exposed timber panelling (unless treated), most vinyls, Class 3 wall coverings, certain insulation products if exposed

Testing and certificates: Products used in escape routes should have fire test certificates. If specifying unusual finishes in a staircase or common area, confirm the fire classification with the manufacturer.

Frequently Asked Questions

Does a smoke alarm need to be hardwired in an extension?

If the extension adds a new habitable room and constitutes notifiable building work, the smoke detection throughout the dwelling must meet Building Regulations standards — which means mains-wired, interconnected alarms. Battery alarms are not compliant for notifiable building work. The electrical work for the alarm installation is itself notifiable under Part P.

What type of fire door do I need for an internal door in a loft conversion?

FD30S — 30 minutes fire resistance with cold smoke seal. The door must also have a self-closing device (typically a surface-mounted or concealed overhead closer). The door must be installed in a solid hardwood or engineered timber frame — not a standard softwood lining. The combination of door, frame, hinges, intumescent strips, smoke seals, and closer must match a tested and certified specification.

Can a customer refuse to have fire doors installed in their loft conversion?

No — fire doors on the escape route are a Building Regulations requirement, not a preference. Building Control will not sign off the loft conversion without them. The customer can choose the aesthetics (there are attractive fire doors in period styles) but cannot opt out of the fire safety requirement.

What is the difference between an FD30 and FD30S fire door?

FD30 provides 30 minutes fire resistance but has no smoke seal — it will pass fire resistance tests but allows smoke to pass around the door. FD30S has an additional cold smoke seal (a brush or intumescent seal in the door rebate or frame) that limits smoke passage under normal temperature conditions. On escape routes, FD30S is required — smoke is more dangerous than flame in most domestic fire fatalities.

Do I need Building Regulations sign-off for changing fire doors in an HMO?

HMOs are subject to the Housing Act 2004 (licensing and HMO Management Regulations) as well as Building Regulations. A fire risk assessment is required for HMOs with 5+ occupants. Replacing fire doors is generally maintenance, but if the building is being re-fitted or altered, Building Control may be involved. In all cases, the fire door specification must meet the requirements of the HMO's fire risk assessment.

Regulations & Standards

• Building Regulations Approved Document B (Fire safety), Volume 1 (Dwellings), 2019 edition — primary domestic fire safety standard

• BS 476 Part 20–22 (Methods for determination of the fire resistance of elements of construction) — fire resistance test standards (old series; still widely referenced)

• BS EN 13501-1 (Fire classification of construction products) — Euroclass system, replacing BS 476 for new products

• BS 5839 Parts 1 and 6 (Fire detection and alarm systems) — alarm system design and installation standards

• Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 — CO alarm requirements in rental properties

• Building Regulations Approved Document B — GOV.UK — Official AD B document

• FDIS — Fire Door Inspection Scheme — Fire door standards and inspection guidance

• Chief Fire Officers Association (CFOA) — Fire safety guidance for residential buildings

• BS 5839-6 — BSI — Domestic fire alarm system standard

• door frames — Fire door frame installation requirements

• staircase regs — Stair requirements in escape routes

• building regs overview — Overview of all Building Regulations parts

• building control — When building control sign-off is needed

• landlord certificates — HMO and landlord fire safety obligations