Trades master copy
Last reviewed: March 2026
What Part B is
Part B is the fire safety part of Building Regulations. For houses and flats it covers the basics you deal with all the time: fire doors, escape routes, compartmentation, smoke/heat alarms, and access for the fire service.
The aim is simple: if there's a fire, people must be warned in time, have a safe way out, the structure must hold up long enough, and the fire must not rip through the building or into the neighbours too quickly.
This guide is a summary to make Part B easier to use on site. It does NOT replace Approved Document B: Fire Safety - Volume 1: Dwellings (latest edition) or BS 9999 / relevant British Standards.
You must read and follow the full Approved Document B for detailed requirements.
This guide is written for England. Wales, Scotland and Northern Ireland have their own versions of building regulations - the principles are similar but the documents and approval routes differ, so check local requirements if you're working outside England.
Where it applies on your jobs
For you as a builder/installer, Part B bites hardest on:
- Refurbs and layout changes - knocking through rooms, opening up stairs, removing doors or walls on escape routes.
- Loft conversions - turning a two-storey house into three storeys, adding a new habitable floor.
- HMOs and flat conversions - subdividing houses into bedsits/flats, adding extra units.
- Extensions and porch/garage infills - anything that affects escape routes, door positions or access for fire crews.
Volume 1 covers dwellings: houses, flats, maisonettes and their common areas. HMOs and more complex residential stuff often pull in extra guidance (for example the LACORS fire safety guide and HMO licence conditions) on top of Part B - and most councils now treat that LACORS standard as the expected baseline for HMOs, even though it's technically guidance not law.
If the building is tall or complex (e.g. a mid- or high-rise block), you may also be caught by extra rules on external walls and structure - see note under "External walls" below.
Key "trigger points" for fire safety
These are the bits of work that quietly turn into Part B problems if you don't plan them.
Escape routes
- Removing or opening up walls around the stair.
- Turning doors off the hall/landing into open archways.
- Adding inner rooms (you must pass through another room to escape) without providing proper early warning or alternative escape arrangements.
Fire doors and compartmentation
- Loft conversions: doors onto the stairway often need to be upgraded to FD20/FD30, with self-closers in some cases.
- HMOs: doors to bedsits and some common rooms usually need full fire doorsets, intumescent strips, cold smoke seals and closers, in line with both Part B and local HMO/LACORS expectations.
Ceilings/structure
- Cutting joists or drilling/boxing around beams without keeping required fire resistance to the floor/ceiling.
- Removing lath & plaster ceilings and replacing with thinner boards without checking fire rating.
Smoke and heat alarms
- Refurbs and lofts: you're expected to bring alarms up to current standard - usually mains-powered, interlinked, correct grades/categories for the dwelling or HMO.
- Kitchens: often need a heat detector, not a smoke alarm, especially if open to the escape route.
External walls and proximity to boundaries
- New dormers and extensions near the boundary: materials and window sizes may be limited to reduce external fire spread.
- Re-cladding or over-cladding: materials need to meet the fire performance rules for that height/use, and cavity barriers must be correctly detailed.
- If the building is over 18 m (roughly 7 storeys), external wall materials face much stricter rules under Regulation 7(2) and related guidance - if you're working on cladding or external wall build-ups on anything that tall or close to it, stop and get specialist advice before you price or start.
Access for fire service
- Hardstandings, parking layouts, gates and landscaping that might block a fire engine from getting close enough.
- On larger schemes, hose distances and turning circles matter - follow the access diagrams in AD B or get confirmation from Building Control.
Quick reference table - common jobs
Loft conversion in a two-storey house
- Protected stair from loft down to final exit (fire-resisting walls/doors around the stair).
- Fire doors to rooms opening onto escape route.
- Mains interlinked smoke alarms on each storey, heat alarm in kitchen.
Knock through kitchen and dining room to open plan
- If the stairs discharge through this space, you might be removing protection to the escape route.
- May trigger need for extra alarms, possibly sprinklers/mist in some layouts, or an alternative escape strategy agreed with Building Control.
Convert house into 4-bed HMO with shared kitchen
- Fire doorsets (with closers and seals) to each bedroom and high-risk rooms.
- Protected escape route to final exit, emergency lighting if required.
- Appropriate alarm system (often Grade A or C, LD2/LD3 depending on design, licence and LACORS guidance).
- Possible compartmentation between units and between units and common areas.
Single-storey rear extension
- Don't create an inner-room arrangement with no adequate escape route/window.
- Check separation to boundary and external wall/fire spread rules for cladding, eaves, etc.
- Smoke alarm coverage in the new area and connection to existing system.
New flat over a shop
- Proper fire separation between shop and flat (floors, walls, penetrations).
- Protected escape route from flat, separate from commercial fire risk where required.
- Appropriate detection in both the commercial and residential parts, as per the agreed fire strategy.
Routes to compliance for trades
There isn't a "Part B scheme" like Part P - you're usually going through Building Control or an Approved Inspector. For most small domestic jobs, compliance boils down to:
Use Approved Document B Vol 1 as your design baseline
Follow the diagrams and tables for escape routes, door ratings, distances, alarm types etc. If you stick to that, Building Control will normally accept it as "reasonable provision".
Get the right people to design the fire strategy when it's not standard
Larger HMOs, blocks of flats, or awkward layouts might need a fire engineer or experienced designer to set the strategy. Don't invent your own work-around because "it looks fine" - run it by Building Control or a fire engineer.
Work with Building Control early
Flag fire-critical changes at plan stage: open-plan layouts, stair positions, door removals, lofts, HMOs. Agree door ratings, compartment lines, alarm grades/categories, and any sprinklers/mist early so you're not ripping stuff out at final inspection.
Install products as tested
Fire doors as complete doorsets or matched door + compatible frame, hinges, latches, seals and closers - no mixing random ironmongery from the back of the van.
Penetration seals, downlighters, insulation etc. fitted to manufacturer's fire-test details - not "near enough".
If you depart from the standard guidance in Approved Document B, you need a clear, defensible reason - usually a fire engineer's design or specific written agreement with Building Control.
Who is responsible for what
On a typical domestic project:
- The designer (architect, technician, or whoever did the drawings) is responsible for designing a compliant fire strategy on paper - escape routes, door ratings, compartmentation, alarm spec etc.
- The builder/main contractor is responsible for actually building that fire strategy correctly: installing the right doors, not cutting away protection, and not changing layouts without checking the fire knock-on.
- The specialist installers (door fitters, alarm installers, sprinkler contractors, dryliners) are responsible for installing their kit to the tested details and manufacturer instructions.
- The client/owner is the one who ends up with enforcement notices, licence conditions, and possible prosecution if the finished building doesn't comply.
One line worth spelling out for small builders:
If you're the builder managing the job and you bring in cheap doors, bodge the frames, or let the electrician punch holes through fire-stopping without fixing it properly, that's your problem as much as theirs. Building Control and the fire authority will look at the whole job, not just one trade.
Simple rule to drum into your team
If you're about to remove a door or wall on or near the stairs, open up a layout, add a storey, or carve a house into smaller units, treat it as a fire job first and a "nice layout" job second. Check escape, doors and alarms with the designer/Building Control before you start knocking through.
On-site checklist (Part B)
Before you start
- Mark the escape route(s) from every habitable room to final exit - on a quick sketch if nothing else.
- Highlight any doors and walls that currently "protect" that route (especially around the stairs).
- Check the plans and Building Control comments for: fire doors, upgraded ceilings/partitions, alarms, and any notes on inner rooms or open-plan layouts.
- If it's a loft, HMO, or flat conversion, confirm the agreed fire strategy in writing (door ratings, alarm grade, any sprinklers/mist).
While you're working
- Don't remove doors or walls on escape routes without checking what's replacing their fire protection.
- Don't oversize notches, chase out fire-resisting walls, or butcher ceilings that are providing fire resistance, unless you know how they're being made good.
- Keep records of where you've run services through fire-resisting elements so they can be properly fire-stopped - no random mouse-holes left open.
When you finish
- Check fire doors: correct leaf, frame, seals, hinges, latch, closer and gaps (normally 3-4 mm on edges, sealed at frame/wall junction).
- Check alarms: right type (smoke vs heat), right locations, interlinked where required, with test certificates if installed as a system.
- Walk the escape routes: make sure they're continuous, protected where they need to be, and not blocked by last-minute cupboards, storage or meter boxes.
- Make sure your as-built details match what Building Control signed off - if you deviated, get it agreed and recorded before final inspection.
Sources
Based on:
- Approved Document B: Fire Safety - Volume 1: Dwellings (2019 edition incorporating 2020 and 2022 amendments) and related gov.uk FAQs.
- Planning Portal summary of Part B for dwellings.
- Industry and local authority guidance on loft conversions, HMOs (including LACORS expectations) and residential fire safety, plus post-Grenfell external wall/Regulation 7(2) requirements for taller buildings.
This guide was last reviewed March 2026. SiteKiln does not provide legal, financial or tax advice. All content is for general information purposes only. Always seek professional advice for your specific situation.
Know someone who needs this?
Working in Wales? The building rules are different. See our Working in Wales guides.
Working in Scotland? Building standards work differently. See our Working in Scotland guides.
Working in Northern Ireland? The system uses Technical Booklets. See our Working in Northern Ireland guides.
Was this guide useful?
Didn't find what you were looking for?
Spotted something wrong or out of date? Email us at hello@kilnguides.co.uk.
In crisis? Samaritans 116 123 ·