Part B
Approved Document B: Fire Safety
The part of the Building Regulations covering fire safety: means of escape, fire spread within and between buildings, and access for the fire service. For extensions it governs matters such as escape windows, protected stairways and how close to a boundary certain roof coverings can sit.
Also known as
Part BApproved Document BAD BFire SafetyApproved Document B Volume 1AD B Vol 1
What Part B requires for a home project
The Part B requirements and terms the buildwiz guides rely on, with paragraph references where the document gives one. Check the current edition on gov.uk before you build to it.
Limits and figures
- Cavity barrier maximum horizontal spacing
- Maximum horizontal spacing between vertical cavity barriers in an external cavity wall, to limit fire spread through the cavity. Additional barriers are required around openings and at the junction with compartment walls. This 20m limit is in Approved Document B Volume 2 (Table 9.1, buildings other than dwellings) and does not apply to dwellinghouses: Volume 1 sets no such spacing, and masonry cavity walls meeting Diagram 5.3 need only the cavity closed at the top and around openings.
- Cavity barrier: minimum fire resistance
- Minimum fire performance of a cavity barrier: 30 minutes' integrity (E 30) and 15 minutes' insulation (I 15), tested from each side (Approved Document B Volume 1 para 5.20), at the para 5.18 locations such as the edges of cavities and around openings. Distinct from the structural steel fire-protection minimum (same 30-minute value, different element and requirement).
- Escape window maximum sill height above floor
- Maximum height of an escape window sill above the finished floor level of the room, under Approved Document B. Above 1,100mm the opening cannot practicably be used as a fire escape.
- Escape window minimum unobstructed opening area
- Minimum unobstructed openable area of an escape window in a habitable room, under Approved Document B para 2.10. The opening must also be at least 450mm high and 450mm wide, with the sill no more than 1100mm above the floor. A bare 450mm x 450mm opening is only 0.2025 m2 and does NOT meet this area test; at 450mm wide, the opening needs about 733mm of height to reach 0.33 m2.
- Escape window minimum unobstructed opening height
- Minimum unobstructed openable height of an escape window in a habitable room, under Approved Document B. Combined with a 450mm minimum width and an openable area of at least 0.33m², the sill must be no more than 1,100mm above the finished floor.
- Escape window minimum unobstructed opening width
- Minimum unobstructed openable width of an escape window in a habitable room, under Approved Document B. Works with the matching 450mm minimum height and 0.33m² minimum area rules.
- Existing first-storey construction fire resistance
- Approved Document B Volume 1 (para 5.4): when a storey is added to a two-storey dwellinghouse, the existing first-storey construction should have a minimum rating of R 30. Table B1 item 3b also gives R 30 and EI 15 for floors in the upper storey of a two-storey dwellinghouse, and its note 4 points to BRE Digest 208 for increasing the fire resistance of existing timber floors.
- FD30 fire door lining: minimum thickness
- Minimum lining thickness for an FD30 fire door set under BS 476 Part 22 / BS EN 1634-1. The deeper section accepts a routed rebate that holds an intumescent strip to seal the door-to-frame gap when heated. Approved Document B Volume 1 sets a minimum of E 20 (FD 20) for doors onto the protected stairway in a loft conversion or three-storey house (para 2.21b; Table C1 item 11b) and E 30 Sa (FD 30S) between an integral garage and the house (Table C1 item 11a). FD30 exceeds the stair-door minimum and is the rating usually fitted, so this lining applies wherever an FD30 or FD30S doorset goes in.
- Fire door minimum on the escape route of a new storey
- Where a loft conversion creates a new storey above 4.5m, Approved Document B Volume 1 (para 2.21b) sets the minimum for fire-resisting doors on the escape route at E 20, including upgrading existing doors where necessary. FD30 doorsets exceed this and are what suppliers commonly sell, but FD30 is not the stated minimum.
- Fire resistance of steel carrying the new loft floor
- A steel that carries the new loft floor is an element of structure (AD B para 5.1) and must have at least the fire resistance of the floor it supports (para 5.2). With the new floor at REI 30 (para 5.4), that is 30 minutes for the steel; R is the relevant letter because a beam only carries load. This is a reading of 5.2 applied to 5.4, not a sentence in the Approved Document. A steel that supports only the roof is excluded by para 5.3a unless the roof acts as a floor or the member is essential to the building's stability; the engineer and building control decide which steels fall where.
- Fire resistance of the ceiling above a protected stairway
- Approved Document B Volume 1 (para 2.5, Diagram 2.3) asks for cavity barriers or a fire-resisting ceiling of at least EI 30 above a protected stairway enclosure, so fire cannot spread over the top of the enclosure through the void.
- Fire resistance period for a three-storey dwellinghouse
- Approved Document B Volume 1 Table B2 note 5 gives a minimum of 30 minutes in the case of three-storey dwellinghouses, increased to 60 minutes only for compartment walls separating buildings. A loft conversion that makes a two-storey house three-storey therefore does not raise the stair enclosure to 60 minutes.
- Fire-resisting partition minimum on the escape route of a new storey
- Approved Document B Volume 1 (para 2.21b) sets the minimum for fire-resisting partitions on the escape route of a loft conversion that creates a new storey above 4.5m at REI 30, and para 2.5a confirms REI 30 for the protected stairway enclosure. Para 5.4 also gives new floors a minimum of REI 30 when a storey is added to a two-storey dwelling.
- Height above which a loft storey needs an alternative escape route or sprinklers
- Approved Document B Volume 1 (para 2.6, Diagram 2.1d): in addition to a protected stairway, a dwellinghouse should provide either an alternative escape route from each storey more than 7.5m above ground level, or a sprinkler system throughout designed and installed to BS 9251. Height is measured from the upper floor surface of the top storey to ground level on the lowest side of the building (Appendix A 'Height', Diagram D6).
- Height above which a new loft storey triggers escape route upgrades
- Approved Document B Volume 1 para 2.21 applies its requirement to address the full escape route and provide fire-resisting doors and partitions where a conversion creates a storey above 4.5m. Measure the height as the Approved Document directs.
- Loft conversion floor relaxation: maximum habitable rooms
- Approved Document B Volume 1 para 5.4 allows reduced integrity and insulation performance for the floors of a loft conversion only where one storey is added containing a maximum of two habitable rooms, and the new storey's area is within the stated limit.
- Loft floor fire resistance relaxation: maximum new storey area
- Approved Document B Volume 1 (para 5.4) allows the fire performance to be reduced for integrity and insulation when both conditions are met: only one storey is added, containing at most two habitable rooms, and the new storey has a total area of at most 50m². Redbridge Council uses the same 50m² and two-room limits for its local alternative to replacing doors, which is not in Approved Document B.
- Minimum fire alarm grade for a dwelling
- Approved Document B Volume 1 (para 1.1) sets Grade D2 Category LD3 to BS 5839-6 as the minimum fire detection and alarm system for any dwelling, and para 1.8 requires a system where a new habitable room is provided above or below the ground storey, as in a loft conversion. The gov.uk Approved Document B FAQ (Q1) says Grade D2 Category LD3 is considered adequate for Part B. Para 1.1 also says a higher standard should be considered where occupants would be at special risk from fire.
- New loft floor fire resistance
- Approved Document B Volume 1 (para 5.4): where an additional storey is added to a two-storey single family dwellinghouse, new floors should have a minimum REI 30 fire resistance, and any floor forming part of the enclosure to the circulation space between the loft conversion and the final exit should also reach REI 30.
- Residential external-wall insulation guidance height (AD B paragraph 10.7)
- Approved Document B Volume 1 paragraph 10.7 recommends A2-s1,d0 or better for external-wall insulation and filler in residential buildings in purpose groups 1 and 2 with a storey at least 11m above ground, subject to its masonry-cavity exception. This differs from regulation 7(2), whose relevant-building definition uses 18m and has statutory exemptions. Do not apply a blanket prohibition to all insulated plasterboard: assess its position in the complete wall build-up and the project's applicable provisions.
- Steel elements deemed to satisfy a 15-minute fire resistance requirement
- Approved Document B Table B2 note 8 deems three types of steel element, under full design load and within stated section-factor limits, to satisfy a minimum fire resistance of 15 minutes: beams supporting concrete floors (Hp/A up to 230 m-1), free-standing columns (up to 180 m-1), and wind bracing and struts (up to 210 m-1). It covers 15-minute requirements only, so it does not mean steel under a 30-minute timber loft floor can go unprotected.
- Storey area that makes a dwellinghouse large for alarm purposes
- Approved Document B Volume 1 (paras 1.5 to 1.7) treats a dwellinghouse as large when it has more than one storey and at least one storey exceeds 200m². A large two-storey house needs Grade A Category LD3 and one of three or more storeys needs Grade A Category LD2, so a very large house can reach the higher standard after a loft conversion adds a storey.
- Structural steel minimum fire resistance period
- Fire resistance for structural steel in a house where the steel supports a floor or other element of structure that itself needs fire resistance: Approved Document B Volume 1 paras 5.1 and 5.2 make the supporting steel need at least the same rating, and para 5.4 sets 30 minutes for loft conversion floors. Para 5.3a excludes structure that supports only a roof, unless it is essential to stability. The protection method (fire-rated board, intumescent coating) and its specification come from the engineer, the product's tested data and building control.
- Three-storey escape route fire resistance
- Minimum fire resistance period for the walls enclosing the protected stairway in a house with a storey more than 4.5 m above ground, which includes most loft conversions on a two-storey house. Approved Document B Volume 1 asks for REI 30 partitions at every storey, running to a final exit (para 2.5a; para 2.21b for loft conversions), with a fire resisting ceiling (minimum EI 30) or cavity barriers above the enclosure. Table B2 note 5 also keeps a three-storey house's elements of structure at a minimum of 30 minutes. That is the same period as a two-storey house, so a single layer of 12.5 mm fire-rated board in a tested system usually achieves it. The stair enclosure does not step up to 60 minutes; that figure applies to compartment walls separating buildings, such as party walls.
- Two-storey top-floor height threshold
- Maximum height of the topmost storey floor above external ground level for a building to be treated as a standard two-storey dwelling under Approved Document B. Above this trigger, Table B2 lists 60 minutes for elements of structure, but note 5 to the table sets a minimum of 30 minutes for three-storey houses (60 minutes only for compartment walls separating buildings). The 60-minute period applies in full to houses of four or more storeys; a house with a top storey more than 18 m above ground is outside the scope of the dwellinghouse guidance (note 4).
- Window distance to boundary triggering fire-rated glazing
- Distance from a site boundary within which a new external wall must be fire resisting (Approved Document B Volume 1 Table B1 item 5a: REI 30 for a dwellinghouse, insulation included), so a window in it needs fire-resisting glazing unless it counts as a small unprotected area under Diagram 11.5. Applies to new external walls, not to like-for-like replacement windows. Outside this zone the fire-rating requirement relaxes, so positioning glazing away from the boundary is cheaper than specifying expensive fire-rated units.
Terms explained
- Alarm grade and category (BS 5839-6)
- BS 5839-6 divides domestic fire alarm systems into Grades (A to F2, indicating power source and backup type) and Categories (LD1 to LD3, indicating coverage level). Approved Document B Volume 1 para 1.1 sets Grade D2 Category LD3 as the minimum for all dwellings, and para 1.8 requires a system where a new habitable room is provided above or below the ground storey, as in a loft conversion. Larger dwellinghouses (a storey over 200m²) need Grade A (paras 1.5 to 1.7). Some building control bodies ask for more coverage than the minimum, so check what yours expects.
- Bespoke large-format aluminium rooflight
- Made-to-order large-format fixed aluminium rooflights supplied by specialist glazing companies. Substantially different from standard Velux pivot windows: installed by the specialist supplier rather than the roofer, cost £10,000-15,000 for large units, 8-16 week lead time. May require 30-minute fire resistance glazing if over escape routes. A documented 55m² kitchen extension in Oxfordshire used two large units from a specialist supplier.
- BROOF(t4)
- The top reaction-to-fire classification for roof coverings exposed to an external fire, tested under BS EN 13501-5 using method t4. A BROOF(t4) covering can usually be built close to a boundary without extra fire restrictions under Building Regulations Part B.
- BS EN 13501
- Fire classification of construction products and building elements. Part 1 (reaction to fire) defines the Euroclass system used in Approved Document B; Part 5 covers fire on roofs.
- BS EN 14604
- Standard for smoke alarm devices. Approved Document B Volume 1 (para 1.2) says smoke alarms should be mains operated and conform to BS EN 14604.
- C24 Structural Timber
- Higher-grade structural softwood timber graded to C24 strength class. Stronger and stiffer than C16, allowing longer spans or smaller section sizes. Sometimes specified by structural engineers for specific load-bearing applications.
- Cavity barrier
- An element that closes or divides a concealed cavity to stop fire spreading through it. Approved Document B Volume 1 para 5.18 requires one at the edges of cavities (including around openings) and where a cavity wall meets a compartment wall or floor; a masonry cavity wall meeting Diagram 5.3 needs only its cavity closed at the top and around openings. The BCO checks cavity barriers at the structural stage.
- Cavity closer
- Prefabricated insulated frames installed at window and door openings to close the cavity and prevent thermal bridging. They help meet Parts C (moisture) and L (thermal) of the Building Regulations; the 0.45 m²K/W minimum thermal resistance comes from the Accredited Construction Details, not Part L itself, and in a masonry house wall Approved Document B (Diagram 5.3) asks only that the cavity is closed around openings, with no fire rating for the closer. First-fix installation during wall construction is required by NHBC/LABC standards.
- Circulation space (fire escape)
- The hall, landing and stair areas people pass through to leave a dwelling, as the term is used in Approved Document B Volume 1 (for example para 1.9: smoke alarms should be provided in the circulation spaces of the dwelling). Approved Document B does not say 'every level' or 'interlinked' in that paragraph; those words come from Planning Portal guidance and BS 5839-6 practice. Not the same as the NHBC kitchen working clearance in gr8:concept/circulation-space.
- Door closer
- A spring or hydraulic device fitted between door and frame that closes the door from any open angle. Approved Document B defines a self-closing device as one that closes a door, when open at any angle, against a door frame. Under Appendix C para C5 closers are not required on fire doorsets within a dwellinghouse, but they are required on the door between a dwellinghouse and an integral garage. Some building control bodies or designers ask for them anyway, and the gov.uk FAQ says they may still be needed between private areas in an HMO.
- Door Lining Kit
- Pre-cut timber frame kit that lines the door opening in a stud or masonry wall. Includes head and two jambs. Available in different widths to suit wall thickness. Must be installed plumb and square or the door will not hang properly.
- Door self-closer
- A device that returns a fire door to the closed, latched position after use. Approved Document B does not require self-closers on fire doors inside a single-family dwellinghouse (Appendix C exempts them), with the exception of the door between the house and an integral garage. They are commonly specified by building control, insurers or lenders on a loft stair, are required on flat entrance doors, and are required on HMO fire doors where local licensing conditions call for them. Overhead hydraulic closers certified to BS EN 1154 (size 3 minimum for a domestic fire door) latch reliably; weak concealed closers often fail to pull the door fully shut.
- Door Stop
- Thin timber strip (typically 12x32mm) nailed to the inside of a door lining that the door closes against. Positioned after the door is hung to ensure correct alignment.
- Egress window
- Openable window meeting Part B minimum dimensions (450mm clear opening, 0.33m² area, bottom of the opening max 1100mm from floor), needed as a means of fire escape in habitable rooms (not kitchens) that do not open onto a hall leading to a final exit (ground storey) or onto a protected stairway (upper storeys up to 4.5m): Approved Document B Volume 1 paras 2.1, 2.2 and 2.10. Locks, with or without removable keys, are allowed.
- Element of structure
- The Approved Document B term for the parts of a building whose collapse in a fire would endanger it: frames, beams, columns, loadbearing walls and floor structures. Each must meet the fire resistance in Appendix B Table B1 (para 5.1), and an element that holds up another needs at least the same fire resistance as the one it supports (para 5.2). A structure that supports only a roof is excluded (para 5.3a) unless the roof acts as a floor or the member is essential to the stability of the building. So a steel carrying the new loft floor needs the floor's fire resistance, while whether a roof-only steel does depends on the engineer and building control.
- EPS Insulation (Expanded Polystyrene)
- Lightweight, rigid expanded polystyrene insulation boards. Lower thermal performance per mm than PIR but significantly cheaper. Commonly used below floor slabs and in less thermally demanding applications.
- Escape route
- The path occupants must be able to use to exit a building in a fire: halls, landings, and staircases. BS 5839-6 Category LD3 (the minimum) requires smoke alarms to cover escape routes. Approved Document B Volume 1 para 1.8 requires an alarm system where a new habitable room is provided above or below the ground storey, or at the ground storey without a final exit.
- FD20 fire rating
- The BS 476-22 label for a doorset that holds back fire for 20 minutes. Approved Document B Table C1 (item 11b) gives E 20 (FD 20) as the minimum for doors forming part of the enclosure to a protected stairway in a single family dwellinghouse. FD20 doors are no longer commonly tested or certified, so suppliers sell FD30 doorsets, which exceed the minimum (FIS, May 2022). From 2 September 2029 the Approved Document drops the FD labels and states the same minimum as 20 minutes.
- FD30 fire door
- A fire-resisting internal doorset: leaf, certified frame, intumescent strips, cold smoke seals and marked ironmongery. Approved Document B's minimum for a dwellinghouse stair-enclosure door is FD20/E20; FD30 is the grade usually bought and the one many councils specify, while FD30S (with smoke seals) is required for a house-to-integral-garage door. FD30 refers to the whole installed assembly, not just the leaf.
- FD30 Fire Rating
- A 30-minute fire-resistance rating for an internal doorset (leaf, frame, intumescent strips and ironmongery), tested to BS 476-22 or BS EN 1634-1. In a dwellinghouse, Approved Document B's minimum for a door in the enclosure to a protected stairway is FD20/E20, not FD30; FD30 is the grade most commonly bought and the one many councils specify. FD30S, with cold smoke seals, is required for a house-to-integral-garage door. Fire doors in flats and HMOs are a separate, stricter regime.
- Final exit
- The end of an escape route that gives direct access to a street, passageway, walkway or open space, sited so people move away from the building and are no longer in danger from fire or smoke. Approved Document B Appendix A notes that windows are not acceptable as final exits. In a house it is normally the front or back door, and a loft conversion's protected stairway must lead to one.
- Fire alarm design, installation and commissioning certificate
- The certificate Approved Document B Volume 1 (para 1.14) says should be provided for a fire detection and alarm system, because systems must be properly designed, installed and maintained. A homeowner should ask the loft company or electrician for it at handover and keep it with the building control completion certificate.
- Fire door
- A complete doorset (leaf, frame, seals, hinges and ironmongery) tested to resist fire for a stated period. On a loft conversion that creates a new storey above 4.5m, Approved Document B Volume 1 (para 2.21) calls for fire-resisting doors and partitions on the full escape route, including upgrading existing doors where necessary. The stated minimum for the door is E 20; FD30 doorsets are what suppliers commonly sell.
- Fire resistance rating (R, E, I)
- How long an element keeps performing in a standard fire test, in minutes. R is loadbearing capacity (it keeps holding weight), E is integrity (it stops flames and hot gas passing through) and I is insulation (the far face stays cool enough). A floor needs all three, so REI 30 means 30 minutes on all three; a steel beam only carries load, so the letter that matters for it is R. Approved Document B states the performance required, not the method.
- Fire-resisting glazing
- Glass that is part of a tested fire door or screen and holds back flame for the stated period, unlike ordinary or toughened glass. Approved Document B Appendix A defines a fire doorset as the frame, leaf, essential hardware, edge seals and glazing, so a glazed door onto a protected stair needs glazing that is part of the tested assembly. Table B3 also limits uninsulated glazing in the enclosure to a protected stairway.
- Grey Graphite EPS
- Graphite-enhanced expanded polystyrene achieving lambda 0.030 W/mK vs 0.038 W/mK for standard white EPS. Provides 20-25% thickness saving at a small cost premium.
- GRP Fibreglass Flat Roof System
- Glass-reinforced polyester flat roof system applied as a liquid resin and glass fibre matting that cures to form a seamless, rigid, waterproof surface. Very durable (25+ years) but more skill-intensive to apply than EPDM. Best installed by a specialist.
- GRP topcoat
- The pigmented resin coat applied over a cured GRP laminate to give the finished, weatherproof, UV-resistant surface of a fibreglass roof.
- Inner room
- A room whose only way out is through another room. Approved Document B Volume 1 (para 2.11) permits inner rooms only in listed cases: a kitchen, laundry or utility room, dressing room, bathroom, WC or shower room, a gallery, or any room on a storey a maximum of 4.5m above ground that has an emergency escape window. Diagram 2.1c marks the escape route for a top storey above 4.5m as 'No inner rooms allowed', so a loft bedroom cannot be reached only through another room.
- Insulated Plasterboard (Thermal Laminate)
- Plasterboard factory-bonded to a layer of PIR insulation. Provides insulation and a plaster-ready surface in one product. Used for internal wall insulation, particularly on solid masonry walls where cavity insulation is not possible.
- Intumescent Coating
- A paint-like coating applied to structural steel that swells into an insulating char when heated, delaying the point at which the steel loses strength in a fire. Used to give an exposed beam or column a 30 or 60 minute fire rating instead of boxing it in plasterboard. Applied to a specified dry-film thickness for the required rating. Distinct from the intumescent strip used around fire doors.
- Intumescent Strip
- Intumescent (graphite or sodium silicate) seal in a plastic carrier, fitted into a groove in fire door frames. Expands when exposed to heat, sealing the gap between door and frame to prevent fire and smoke spread. Required for FD30 fire door assemblies.
- LED downlight
- The dominant ceiling lighting fitting for kitchen extensions, recessed into plasterboard. Fire-rated versions required where the ceiling is beneath a bedroom or bathroom. Typical extensions specify 20-29 downlights.
- Loft staircase
- A fixed staircase serving a loft conversion, designed to Approved Document K: minimum 2m headroom (reducible to 1.8m at the edge of a stair in a loft), maximum 42 degree pitch, maximum 220mm rise and minimum 220mm going. Its position and run length frequently decide whether a loft conversion is feasible at all.
- MICC Cable
- Mineral insulated copper clad cable comprising copper conductors packed in magnesium oxide powder inside a copper sheath. Used for fire-survival circuits (emergency lighting, fire alarm systems, smoke extract) because it maintains circuit integrity at temperatures above 1000°C. Distinct from standard twin-and-earth or SWA cable.
- Open-plan ground floor
- A ground floor where the stair lands in, or opens straight onto, living space, so the protected route to the front door is not enclosed. For a loft conversion, Approved Document B Volume 1 gives two routes: enclose the route with new fire-resisting partitions (para 2.21, Diagram 2.2), or the alternative in para 2.23, which needs all three of sprinklers in the open-plan areas, a fire-resisting partition (REI 30) and door (E 20) separating the ground storey from the upper storeys with access for the loft occupants to a first-storey escape window, and cooking separated from the open-plan area by REI 30 construction. The choice changes the downstairs layout, so it belongs at design stage, not on site.
- PIR Insulation Board 50mm
- 50mm polyisocyanurate rigid insulation board with foil facing. High thermal performance for its thickness. Used in walls, roofs and floors. Common brands include Celotex and Kingspan. Standard board size 2400x1200mm.
- PIR Insulation Board 75mm
- 75mm polyisocyanurate rigid insulation board. Often used in combination with other thicknesses to achieve the required U-value specified by building control.
- Pitched roof window
- Standard pivot-type roof windows (Velux GGL, Fakro FT series, RoofLITE equivalent) for pitched roofs. Require a specific flashing kit matched to tile type, structural trimmer rafters, and a manufacturer-specified underfelt collar. Available in standard size codes from builders merchants with 1-2 week lead times for stock sizes.
- Plasterboard Fire Rated (Pink Board)
- Fire-resistant plasterboard with added glass fibre and other additives, identified by its pink paper facing. Required where Building Regulations demand fire-rated partitions, such as walls and ceilings around a garage conversion or walls protecting escape routes.
- Plasterboard Standard (12.5mm)
- Standard 12.5mm gypsum plasterboard for internal wall and ceiling linings. The workhorse of internal finishing. Fixed to timber or metal studs and either skim-plastered or dry-jointed. Standard sheet size 2400x1200mm. Tapered edge boards allow flush jointing.
- Protected stairway
- A stair, together with the hall or passage from its foot to the final exit, enclosed in fire-resisting construction so occupants can leave while a fire burns elsewhere in the house. In a dwellinghouse with one stair, Approved Document B Volume 1 (para 2.5a) asks for the enclosure to be at least REI 30 at every storey and to reach a final exit, or two separated ground-level final exits, with cavity barriers or an EI 30 ceiling above it. In a loft conversion it is the escape route for the new top storey, because escape windows stop being the main route above 4.5m.
- Residential sprinkler system
- A fixed installation of water sprinklers designed for homes. In a loft conversion, Approved Document B Volume 1 uses sprinklers in two places: as one part of the para 2.23 alternative for an open-plan ground floor, and as an alternative to a second escape route where a storey is more than 7.5m above ground level (para 2.6b, systems designed and installed to BS 9251). Public cost guides quote a loft-only zone system at a fraction of the cost of a whole-house retrofit.
- Rigid ducting 150mm
- The standard-diameter rigid duct pipe used for cooker hood extraction runs. 150mm is the industry-standard diameter for performance; substituting smaller diameter (e.g. 120mm) degrades airflow significantly.
- Sheet Materials
- Plasterboard and specialist board products used for wall and ceiling linings, fire protection, moisture resistance and tile backing.
- Smoke alarm system
- The fire detection and alarm system Approved Document B requires where an extension creates a new habitable room above or below the ground storey, or at the ground storey without a final exit (Volume 1 para 1.8). The minimum is Grade D2 Category LD3 (para 1.1): mains-powered smoke alarms with battery backup covering the escape route on each storey, to BS 5839-6, which also recommends interlinking.
- Soil Pipe 110mm
- 110mm diameter PVC pipe for connecting toilets and carrying foul waste to the main drain. Also acts as the soil vent pipe (SVP), which must be ventilated, either by an open vent above the roofline or by an air admittance valve inside the building where Approved Document H (paragraph 1.33) allows. Push-fit ring-seal joints. Building control will inspect the drainage layout.
- Structural steel
- Steel beams used to support loads in building construction, including lintels, ridge beams, and support frames for openings in walls.
- Utility room
- A room or alcove for the washing machine, tumble dryer, sink and often the boiler, separate from the kitchen. Approved Document B para 2.11 allows it as an inner room; Approved Document F Table 1.1 sets a 30 l/s intermittent extract rate (8 l/s continuous high rate). Its size is a design choice: no Approved Document sets a minimum for an extension.
- Water mist fire suppression system
- A fire suppression system that releases a fine water mist rather than a sprinkler spray, often discussed by homeowners as a cheaper or smaller alternative to sprinklers. Approved Document B Volume 1 names only sprinklers to BS 9251 in paras 2.6 and 2.23, so whether building control accepts mist in place of an enclosed escape route is a decision to agree in writing before ordering.
- Weep vent
- Small plastic insert placed in the perpend (vertical mortar joint) below a cavity tray to allow water that has entered the cavity to drain back outside. Required at maximum 900mm centres per Approved Document C.
Pages where Part B comes up
21 guides reference Part B, grouped by build phase.
Planning
Pre-construction
Structure
Second fix
Completion
Want the full picture?
The buildwiz build guides walk through a UK extension in sequence and link to Part B (and every other Approved Document) at the exact stage it bites. That is usually a more productive way to use this material than reading the document cover to cover.