Do I need mains smoke alarms or will battery ones do?

For owner-occupied homes in England, the Building Regulations (Part B) require at least one smoke alarm on every storey. Battery alarms meet the minimum requirement. For rented properties, landlords in England must fit smoke alarms on every floor, and mains-powered interlinked alarms are strongly recommended. In Scotland, all homes must have interlinked alarms since February 2022.

Is fitting a mains smoke alarm notifiable work under Part P?

Connecting a new spur from a lighting circuit to feed smoke alarms is notifiable work under Part P. Replacing an existing alarm with one of the same type and wiring it to the same connections is not. If in doubt, use a registered electrician or check with your local building control office.

Where exactly should smoke alarms go?

BS 5839-6 Grade D recommends: hallway at ground floor level, landing at each upper floor level, top of the stairs. Position them on the ceiling, at least 300 mm from any wall or light fitting, and no more than 7.5 metres from any point in the area covered.

What is a heat alarm and do I need one?

A heat alarm responds to a rise in temperature rather than smoke particles. Kitchens and garages produce enough steam or fumes to false-trigger a smoke alarm, so heat alarms are fitted there instead. A heat alarm interlinked with the smoke alarms means a kitchen fire will still set off every alarm in the house.

How to Fit Mains-Powered Interlinked Smoke Alarms (UK)

Helpful video reference. This video by John Ward (jwflame), a UK electrician based in Dorset, covers the wiring of mains-powered interlinked smoke alarms in full, including cable sizing, the interlink wire, and a working test at the end. John's channel explains UK-specific electrical work clearly and he addresses the Part P question directly. Watch on YouTube.

Before you start. Replacing an existing mains alarm with an identical model and reconnecting the same wiring is not notifiable work under Part P. However, adding a new spur from a lighting or power circuit to supply smoke alarms is a new circuit addition, and that is notifiable. If your project involves running new cable from the consumer unit or splicing into an existing circuit, you should either use a registered electrician or notify your local building control before starting. In Scotland, all homes have been required to have interlinked alarms since February 2022, and the rules cover rental and owner-occupied properties alike.

1. Work out what the regulations require for your property

The standard for domestic smoke alarm systems in the UK is BS 5839-6. For most homes this means Grade D (mains-powered with battery backup) and Category LD3 as a minimum (alarms in all circulation spaces: hallways and landings). Many electricians now recommend Category LD2, which adds the main living room, or LD1 for all rooms.

Rented properties in England must have a working smoke alarm on each storey. Scotland requires interlinked alarms in all homes. Consult your local authority or a registered electrician if you are a landlord in Wales, as the requirements differ and have changed recently.

2. Plan positions

Ceiling-mount alarms at least 300 mm from any wall or light fitting. On a landing or hallway, position the alarm so it can detect smoke from any room opening onto that space. Do not place alarms directly above a cooker, shower, or boiler flue — the steam and fumes trigger false alarms constantly.

The circulation spaces come first: ground-floor hallway, first-floor landing, top of the stairs. Add a heat alarm in the kitchen and garage. If you want full LD2 coverage, add a smoke alarm in the main living room as well.

3. Choose your wiring method

Two options exist for domestic mains alarms:

Hardwired interlink. Three-core cable runs between alarms. Permanent live and neutral supply the alarm. The third (switched) conductor is the interlink wire: when one alarm detects smoke, it raises the interlink wire to trigger all others. This gives the most reliable interconnection.
Radio-interlinked mains alarms. Each alarm needs only a permanent live and neutral feed. The interlink signal is transmitted wirelessly. This is easier to retrofit in a house where routing three-core cable through ceilings would mean significant disruption.

For new builds or properties being rewired, hardwired interlink is standard. For an existing property where you want to upgrade without major plastering work, radio-interlinked alarms are the practical choice.

4. Fix the base plates and run cables

Mark the fixing positions on the ceiling. Drill and plug if fixing into a plasterboard ceiling with no joist behind the position; use joist fixings where the alarm sits directly over a joist. Fix the base plate before the cabling is in place, not after.

Cable runs should be on-axis: parallel or perpendicular to the room edges, never diagonal. Clip cables at 400 mm centres in accessible areas, 600 mm maximum. If you are chasing cables into a solid ceiling, mark their position carefully in photos before plastering over them.

5. Connect to the mains

This is the step that requires electrical competence and may be notifiable. Typical supply options are:

A spur from an existing lighting circuit. Switch off the circuit at the consumer unit, confirm dead with a voltage tester, then connect at a convenient ceiling rose or junction box.
A dedicated smoke alarm circuit from the consumer unit. More involved but means a tripped lighting circuit does not take out the alarms.

At each alarm base plate you will have: permanent live (brown), neutral (blue), earth (green/yellow), and the interlink wire if using hardwired interconnection. Follow the manufacturer's wiring diagram exactly. Terminal labels vary between makes.

6. Wire the interlink

On a hardwired system, the interlink terminals on each alarm are connected in a chain using the third core of the cable. This is typically marked "IN" or "INT" on the alarm base. Check the manufacturer's data sheet, as polarity and connection method varies. Some alarms also support a heat alarm in the chain using the same interlink wire; others need a separate terminal or resistor.

On a radio-interlinked system, the pairing procedure varies by brand. Carry out the pairing sequence after restoring power, with all alarms fitted to their base plates.

7. Test every alarm on the circuit

Restore power at the consumer unit. Press the test button on the first alarm. Every other alarm in the chain should sound within two seconds. Go to each alarm and test it individually. If any alarm fails to trigger the others, re-check the interlink connections at both ends of the affected section.

Once all alarms trigger correctly, press the hush button to silence, wait for them to reset, and test one more time. A second clean test confirms the system is working.

Stop and call an electrician if: any alarm shows a constant fault light after installation; alarms will not silence after a test; the interlink fails to trigger all alarms even after re-checking connections; you find unexpected wiring in the ceiling void (older cable colours, no earth, or cloth sheathing) that does not match what you expected; or you are not sure whether your proposed supply connection is notifiable work. A smoke alarm system is worth getting right. The cost of a one-hour visit from a qualified electrician is small relative to what interlinked alarms are there to protect.

When to call us

Richard fits and tests interlinked mains smoke alarm systems across east Kent. If you are a landlord in Sandwich, Deal, Dover or the surrounding area and need a compliant installation with a certificate, get in touch. Most single-property installations take under a day.

Need smoke alarms fitted properly?

Richard can advise on the right grade and category for your property, supply the alarms and provide a completion certificate for landlord records.

Contact Richard

How to fit mains-powered interlinked smoke alarms