Smoke Alarms for an Extension: What Building Control Actually Wants

The build is finished. The kitchen is in, the plaster is painted, and the building control officer is doing the final walk-round to sign it off. He looks up at the kitchen ceiling, sees nothing, and says he cannot issue the completion certificate: no heat alarm, not interlinked with the rest of the house. Now you are getting the electrician back for a half-day call-out, chasing cable through a finished ceiling. The sign-off has slipped two weeks over one detail almost every homeowner gets wrong: the kitchen needs a heat alarm, not a smoke alarm.

What the system actually is

It is a small network of interlinked detectors wired into the mains, so when one alarm senses a fire, every alarm in the house sounds at once. That last part, the interlinking, is what separates a compliant extension system from the battery-powered units you buy in a supermarket. If a fire starts in the kitchen during the night, an alarm going off in the kitchen is no use to someone asleep upstairs. An interlinked system means the landing alarm screams at the same instant.

The rules come from two places. Building regulations set the duty to protect escape routes. The technical detail of how alarms are graded and where they go sits in a British Standard, BS 5839-6, the code of practice for fire detection in homes.

When you extend a house, building control treats the work as a "material alteration". That triggers the requirement to bring fire detection up to the current standard, at least in the new and affected areas, and often on the existing escape route too. You do not get to keep an old battery smoke alarm and call it done.

Grade and category: what the jargon means

BS 5839-6 describes every domestic alarm system with two labels: Grade and Category. They sound technical, but they answer two simple questions. Grade tells you how the alarms are powered. Category tells you which rooms they cover.

Grade and category together define exactly what an inspector expects to see.

The grade you care about is Grade D. A Grade D system runs off the mains with battery backup, so it keeps working in a power cut. Grade D splits into two:

• Grade D2 uses a user-replaceable battery backup. It is cheaper, but you will be changing batteries over the life of the alarm.
• Grade D1 uses a sealed, tamper-proof backup battery that lasts the full ten-year life of the unit. There are no battery changes and nothing to forget.

Category is about coverage. Two matter for an extension:

• Category LD3 covers escape routes only: hallways, landings and stairs. This is the bare legal minimum.
• Category LD2 covers escape routes plus high-risk rooms where fires actually start, principally the kitchen and any room with an open fire or wood burner.

D1 / LD2

In practice, building control officers expect Grade D1, Category LD2 for an extension: mains-powered alarms with sealed ten-year backup batteries, covering escape routes and the kitchen. The bare legal floor is D2/LD3, but specify to D1/LD2 and you remove the most common sign-off arguments before they start.

Here is the honest position. The absolute minimum the regulations allow for minor extensions is Grade D2, Category LD3. But most BCOs treat an extension as material alteration work that warrants Category LD2, and that pulls the kitchen heat alarm into scope. If you specify the system to D1/LD2 from the start, you are covered either way. The extra cost over the minimum is small. It is a far better position than arguing with an inspector at sign-off while the completion certificate is delayed.

Three products you actually need

This is where homeowners get caught out. The extension fire detection system is not one type of alarm repeated around the house. It is typically three different products, each doing a different job.

Optical smoke alarms go in the circulation spaces: hallway, landing and bottom of stairs. "Optical" means the alarm detects the larger smoke particles given off by smouldering fires (sofas, bedding, electrics) using a light beam inside the chamber. These are the front-line escape-route detectors. You need one on every storey escape route at a minimum.

A heat alarm goes in the kitchen, and this is the part people get wrong. You must not fit a smoke alarm in the kitchen. Toast, frying and steam set off optical and ionisation smoke alarms constantly, so a homeowner who gets nuisance-tripped every time they cook will end up pulling the battery out, which defeats the entire system. A heat alarm solves this. It ignores smoke entirely and triggers on a rapid rise in temperature instead, so it only fires on a genuinely dangerous fire, not when you are browning onions.

A carbon monoxide alarm is required wherever you have a new or relocated fixed combustion appliance: a boiler moved into the new utility room, a relocated gas hob's flue, or a wood burner in a new room. Under Approved Document J, a CO alarm is legally required, in all homes, whenever an appliance is installed or replaced. It is not optional and not just a rental requirement. If the build moves a boiler, budget for one.

How interlinking works

For alarms to sound together, they must talk to each other. There are two ways to do that, and both are accepted by building control as long as the system meets BS 5839-6.

Hard-wired interconnection runs a third "interconnect" wire to every alarm alongside the mains supply. It is the traditional method, marginally more reliable because there is nothing to lose signal, and the obvious choice when ceilings are open at first fix anyway. The Aico 3000 Series, for example, allows up to twelve alarms on one hard-wired interconnect circuit.

Radio interlinking uses a wireless signal between alarms instead of a wire. Aico's version is called RadioLINK. Each alarm still needs a mains supply, but they communicate over radio rather than a physical interconnect wire. This is the practical choice on a loft conversion, where running interconnect through existing ceilings is disruptive. The piece of advice that comes up again and again on this: ask the BCO which they are happy with before you buy. Most accept radio. A small number prefer hard-wired. A two-minute email avoids buying the wrong system.

Where each alarm goes

Positioning is governed by BS 5839-6 in some detail, but knowing the rules lets you spot a bad install.

Alarms are ceiling-mounted, near the centre of the room or circulation space. They need at least 300 clearance from any wall, because the corner where ceiling meets wall is a dead air pocket where smoke is slow to reach. Keep them at least 300mm from any light fitting, so the fitting does not disturb airflow into the sensor. Each alarm should sit within 7.5 of the door to any habitable room it is protecting.

Sloped or vaulted ceilings change the rules, because heat and smoke collect at the highest point. On a slope, the heat alarm goes within 150 of the apex, measured down the slope from the peak. Get this wrong on a vaulted kitchen-diner ceiling and the alarm sits in cooler air and takes longer to trigger.

How the electrician works it, and when the BCO looks

You will not be fitting these yourself. Mains alarm work is notifiable electrical work, so a registered electrician installs and certifies it. But you are the one coordinating the build, so the sequencing problem is yours to get right.

The cable goes in at first fix, before plasterboard. The electrician runs the mains supply and, if you are going hard-wired, the interconnect wire to each alarm position, and fits back boxes marking where every head will sit. This is the cheap, easy moment to get it right. Once ceilings are boarded and skimmed, adding or moving an alarm means chasing through finished work.

Building control inspects at two points. At first fix, the BCO checks that the cable routing and alarm positions are correctly marked and meet the spacing rules, before anything is covered. At final inspection, the alarm heads are fitted and the inspector function-tests the system: press the test button on one alarm and every alarm in the house must sound. If the kitchen heat alarm is not there, or is not interlinked, that is where the certificate stalls.

What it costs

Two things drive the bill: the alarm units themselves, and the electrician's labour to fit and certify them.

The UK market standard is the Aico 3000 Series. Most electricians quote on it because it is reliable, widely stocked, and inspectors recognise it. A mains-interlinked optical smoke alarm (the Ei3016) runs £54 – £60 per unit. The matching kitchen heat alarm (the Ei3014) is £55 – £60. If the build needs carbon monoxide detection, an interlinked CO alarm (the Ei3018) is £65 – £70.

There are cheaper compliant units. The FireAngel SW1-R is a mains-interlinked smoke alarm at £18 – £20 that meets BS 5839-6 perfectly well. So if money is tight, you are not forced onto Aico pricing. But the 3000 Series is what the trade defaults to, and the price gap on a two or three alarm system is too small to be worth fighting over.

For a typical single-storey rear extension, you are looking at one optical smoke alarm, one kitchen heat alarm, and a CO alarm if there is a new gas or solid-fuel appliance. Supplied and fitted by the electrician as part of first fix, the whole system lands at £200 – £400.

Where to buy

Aico 3000 Series is stocked at every major trade outlet: Screwfix, Toolstation, and electrical wholesalers like CEF and Edmundson. You can walk in off the street. It is completely normal for alarms to appear on the electrician's invoice as supplied-and-fitted rather than you buying them separately, and that is generally the simplest route because it keeps responsibility for compatibility with them. If you do want to source the kit yourself, buy direct from Aico or from a fire-safety specialist like Safelincs, who carry the full range plus accessories and can advise on the right grade. Buy every head from one supplier and one product family so the interconnect, whether hard-wired or RadioLINK, is guaranteed to talk to itself. Watch the grade carefully when ordering: Grade D1 and Grade D2 versions of the same alarm sit on the shelf side by side, look almost identical, and only differ in whether the backup battery is sealed for ten years (D1) or user-replaceable (D2). For an extension you want D1, and it is easy to grab the wrong box if you are only reading the headline product name.

Common mistakes

The first trap is buying a multipack of smoke alarms and assuming that covers everything. People dot a set of optical units around the house and think they are done. They are not: supermarket multipacks are usually battery-only Grade F devices, whereas an extension needs alarms wired into the mains and interlinked so they all sound together. Fitting standalone units is a wasted purchase that will not pass.

The second trap is open-plan layouts. If the extension creates an L-shaped kitchen-diner that flows into the hallway with no door, the cooking zone counts as part of the escape route, and the heat alarm requirement becomes non-negotiable. These layouts create grey areas that different inspectors read differently. Draw a clear plan, send it to the BCO, and ask the direct question: "Does the kitchen need an interlinked heat alarm?" Get the answer in writing.

The third trap is assuming you only need to cover the new room. If the extension alters escape routes upstairs, or you are adding a storey, building control can require the whole house alarm system to be brought up to standard, not just the extension. Budget for that possibility on bigger jobs rather than being surprised at first fix.

The fourth trap is misunderstanding why a smoke alarm cannot live in the kitchen at all. An optical or ionisation smoke alarm sited above a hob triggers constantly, because it cannot tell the difference between cooking steam, toast fumes and a real fire. After a fortnight of false alarms in the middle of dinner, the homeowner does the predictable thing and removes the battery or tapes the unit shut to stop the noise. That is an operability failure: detection in the highest-risk room in the house is now switched off by the person living there. A heat alarm exists precisely to break that cycle, because it senses temperature rise rather than smoke and so stays quiet through everyday cooking and only fires on a genuine blaze. The lesson is that the kitchen problem is not just about ticking a box for the inspector, it is about fitting a detector the household will actually leave switched on.

Where you will need this

• First fix electrics - alarm cabling and back boxes go in at this stage, before ceilings are boarded
• Building control final inspection - BCO function-tests the interlinked system before issuing the completion certificate

Fire detection sits across the electrical and building-control threads of any extension or renovation project, not just kitchen builds. Any material alteration that touches an escape route or adds a combustion appliance pulls these requirements into scope.