Fire Safety and Means of Escape

Fire is the Part of the Building Regulations where the answer changes character completely depending on how high the work goes. A single storey extension on the back of a house carries a short and manageable list. Put a room in the roof and the requirement reaches back down through the whole building, onto floors you were not planning to touch, and it is the reason a loft conversion quotation looks so different from an extension quotation of the same size.

The trigger is height, not the number of storeys. That distinction matters more than it sounds, and it is where most of the confusion on this subject comes from.

The two questions Part B asks

Can everybody get out, and does it spread.

Everything in the fire requirements resolves into one of those. Means of escape covers early warning, a route out of every room and a route out of the building that stays usable while people are using it. Spread covers how long the structure holds up, whether fire moves through concealed spaces in the construction, and whether it can jump to the house next door.

On a domestic extension the escape half generates almost all the design decisions and the spread half generates almost all the detailing. The two get tangled together on site because the same fire door is doing both jobs.

Escape from a ground floor extension

Modest requirements, firmly applied, and a window that is not what it looks like.

New habitable rooms need a way out. In a house where the stairway is not enclosed, that is normally an emergency escape window from each habitable room other than a kitchen, opening into a position from which somebody can reach a place free from danger. A window opening onto a sealed lightwell or into an enclosed side passage with a locked gate at the end is not that position.

An escape window is a specification rather than a category. The clear openable area has to be at least 0.33 square metres, with a minimum of 450mm in each direction, and the bottom of that openable area no more than 1100mm above the floor. Take those apart and two things follow. A wide window with a narrow top opening light does not qualify, no matter how large the frame is. And a window whose opening starts above chest height does not qualify either, which rules out a good many designs that look right on an elevation drawing.

The obstruction people meet late is furniture and fittings. A window that satisfies the rule with an empty room can be unusable once a fitted wardrobe or a worktop is under it, and the surveyor will be operating it at completion rather than reading it off a drawing.

Inner rooms, which is the problem extensions create

A room you can only leave by walking through another room is a specific risk with a specific set of answers.

The room you have to pass through is the access room, and if the fire starts in there the inner room is cut off. The guidance limits what an inner room is allowed to be. A kitchen, a utility or laundry, a dressing room, a bathroom, WC or shower room, or a gallery, are the recognised cases. A habitable inner room, a bedroom or a study, needs its own escape window.

Extensions produce inner rooms almost by accident. A new kitchen goes on the back and the old kitchen becomes a snug reached only through it. A garage conversion creates a room off the hall that then gets partitioned. A wide rear extension swallows the back door and leaves an existing room with only one way out, through the new space. That last version is the awkward one, because the room in question is in the existing house and the work that created the problem is next door to it.

None of this is expensive to solve at drawing stage. It is expensive to solve after the studwork is up.

Open plan, and the stair that no longer lands in a hall

The commonest fire problem in a knock through, and the one clients least expect.

The traditional house works because the staircase sits in an enclosed hall with doors off it. Anyone upstairs has a route down that is separated from the rooms where fires start. Open the ground floor into one space, which is the whole point of a great many extensions, and that separation goes. The stair now discharges into the kitchen.

Building control will not simply refuse it, but they will want one of a limited number of answers. Keeping a fire resisting enclosure around the stair and its route to the front door, even a partial one formed by a glazed screen. Fitting a domestic sprinkler system so the open plan space is protected. Providing an alternative escape route from the first floor. Or, in more modest cases, enclosing the kitchen itself so the highest risk is separated from the escape route.

Which of those works depends on the plan, and it is a decision worth making before the layout is agreed rather than after, because a sprinkler system and a glazed fire screen are not similar amounts of money and neither is invisible in the finished room.

Going up: where the requirement steps

The line is drawn at a floor more than 4.5 metres above ground level, and it is a height rather than a count.

Where the top storey of a house sits more than 4.5 metres above the ground outside, an escape window is no longer accepted as the means of escape from it. The reasoning is straightforward: above that height, dropping from a window is not a survivable option and the fire service cannot reach it with a ladder off a garden path.

This is why loft conversions are treated as a different order of work from extensions. It is also why the storey count is a poor guide. A bungalow with a room built into the roof creates a two storey house whose upper floor may sit comfortably under the line, and a tall Victorian house can have a first floor already above it. Measure it rather than count it, because it decides the entire fire strategy for the job.

What a loft conversion actually requires

A protected route from the new storey to the front door, and it is work on all three floors.

Once the new floor is above the line, the standard answer is a protected stairway: a fire resisting enclosure running from the top storey all the way to a final exit, fire doors to the habitable rooms that open onto it, and a fire resisting floor to the new storey. That means the existing first floor landing has to be enclosed as well, existing doors on the first floor are usually replaced, and the ceiling below the new room has to perform.

People are prepared for the loft work and are not prepared for that. It is the honest reason a conversion touches rooms nobody intended to redecorate, and it is worth understanding before the budget is fixed rather than at the point somebody explains why the bedroom doors are going in a skip. What that means for a conversion in practice is set out on the service page.

There are relaxations for a modest conversion. Where only one storey is being added, and the new storey is small and contains only a limited number of habitable rooms, the guidance allows the existing floor construction to be brought up to a modified standard rather than a full one, which can save taking down and rebuilding an entire first floor ceiling. The qualifying conditions are specific, and they are exactly the sort of thing that changes between editions, so they are worth checking against the current Approved Document B for your job rather than taken from any general description.

Where a protected stairway genuinely cannot be formed, the recognised alternatives are a sprinkler system or a second escape route. Both are real options and both cost more than doing it the ordinary way, which is why the stair enclosure gets designed first and the alternatives get looked at only when it will not work.

Alarms, and the word that gets left out

Interlinked is the requirement. Fitted is not the same thing.

A house being extended needs detection covering the new work, and where a storey is added the system generally has to be extended and upgraded rather than simply added to. The baseline is alarms in the circulation space serving each storey, mains powered with a backup supply, and interlinked so that one detecting sounds all of them. A heat alarm rather than a smoke alarm goes in a kitchen that opens onto the escape route, because a smoke alarm over a hob is an alarm that gets taken down.

The grade and category of system required depend on the size and the height of the house, and those are set out in Approved Document B rather than being a single national answer. What does not vary is the interlink. It is the item most often missed on an otherwise complete job, and it is checked at the completion inspection by triggering one unit and listening for the rest.

Two separate things get confused here. The alarm requirements under the Building Regulations attach to the building work. There is a separate regime requiring alarms in rented homes, which applies to landlords regardless of whether anything is being built. They are different rules with different triggers, and satisfying one says nothing about the other.

Fire doors

A fire door is a tested assembly, not a heavy door.

Where a door is required to be fire resisting it is normally a thirty minute door, hung on the right hinges, with the intumescent strips and smoke seals it was tested with, and a frame and stop that match. The certification belongs to the assembly. An old panelled door that somebody has stripped and waxed is not a fire door and cannot be turned into one by painting it, however solid it feels.

Self closing devices are not generally required on fire doors inside a house. The door between a house and an integral garage is the exception, and it needs one.

Garages, integral and converted

A garage attached to a house is treated as a hazard next to it, which cuts both ways.

Where a garage is attached to or built into a house, the two are separated by fire resisting construction, the connecting door is a fire door with a self closer, and either the garage floor falls away from that door or the doorway sits at least 100mm above the garage floor, so that spilt fuel cannot run under it. That last detail catches people out because it looks like a step somebody forgot to remove.

Converting the garage into a room removes the hazard and the separation requirement with it. What arrives in their place is everything that applies to a habitable room: escape, whether it becomes an inner room, and detection extended to cover it.

The boundary, and fire spreading outwards

How much of an extension can be window and door depends on how far it stands from the boundary.

The external walls of a building have to resist fire spreading to and from neighbouring property, and the amount of unprotected area, meaning glazing, doors and any combustible surface, is limited by the distance to the relevant boundary. Close to that boundary the wall has to resist fire from both sides and the permitted area of opening is severely limited. As the distance increases the permitted area increases with it, and past a reasonable separation it stops constraining the design at all.

Two things are worth knowing. The relevant boundary is not always the fence: where a road, a railway, a watercourse or a strip of open land runs alongside, the notional boundary can sit out in the middle of it, which is often more generous than owners expect. And the roof covering itself carries a designation requirement close to a boundary, which is why a rooflight or a particular covering may be restricted on a flank that sits tight against next door.

The restrictions on combustible materials in external walls that followed Grenfell are a separate regime, and they attach to tall residential buildings rather than to houses. Timber cladding on a house extension is a Part B question about distance to the boundary, not one caught by that ban.

Basements

A habitable room in a basement has its own escape problem, since it cannot rely on the stair alone. The usual answer is an external door or a suitably sized window opening into a lightwell with a way out of it, or a protected stairway to a final exit. It is the first thing to establish on a basement job, before anything is drawn, because it frequently determines whether the room can be a bedroom.

Where this leaves you

Fire is the Part where a decision made casually at sketch stage becomes very expensive at first fix. Whether the stair stays enclosed, whether a room ends up inner, and whether the top floor is above or below the height line are all settled on a drawing, in an afternoon, for nothing. We work these through before anything is submitted, and the answer regularly changes the plan. Which Parts apply to your job in the first place is covered in our guide to the Building Regulations for extensions.

Approved Document B is revised between editions and the provisions summarised here depend on the height, layout and construction of your particular house. Treat this as an explanation of how the fire requirements work rather than confirmation of what your job needs.

Last reviewed August 2026. Planning and Building Regulations change, and your property may differ.