Smoke Alarm Spacing Calculator — BS 5839-6 Compliance

These are categories of fire detection in dwellings defined by BS 5839-6:2019. LD1 provides the highest level of protection — detection in every room and area (except bathrooms, shower rooms and cupboards under 1m²), with a heat alarm in the kitchen. LD2 is the standard for new builds and rewires — smoke alarms in all circulation spaces (one per storey) plus the principal habitable room (the main living room), with a heat alarm in the kitchen; the principal habitable room is a single detector for the dwelling, not one per storey. LD3 is the minimum — detection in escape routes only (hallways, landings, stairways). The Smoke and Carbon Monoxide Alarm (England) Regulations 2022 require at least one smoke alarm on each storey with living accommodation (broadly an LD3 layout) plus a CO alarm where there is a fixed combustion appliance; they do not specify an LD category or mandate a kitchen heat alarm.

For new builds, extensions, and full rewires, yes — smoke alarms must be mains-powered (Grade D1) with integral battery backup and interlinked (when one triggers, they all sound). This is required by Approved Document B and BS 5839-6. For existing rented or social-housing dwellings in England, the Smoke and Carbon Monoxide Alarm Regulations 2022 require at least one smoke alarm on every storey with living accommodation but do not mandate mains power or interlinking, so battery alarms are acceptable — sealed long-life lithium units (10-year battery) are strongly recommended. Those regulations do not apply to owner-occupiers. In Scotland (since February 2022) every home, regardless of tenure, must have interlinked alarms (smoke in the main living room, smoke in each circulation space, heat in the kitchen). For commercial fire alarm systems, see our Fire Alarm Cable Calculator.

Smoke alarms should be mounted on the ceiling, at least 300mm from any wall or light fitting. They should be positioned in the centre of the circulation space (hallway or landing). Do not place smoke alarms in kitchens or bathrooms — use a heat alarm in the kitchen instead. Avoid locations near air vents, extractor fans or areas where steam or cooking fumes could cause false alarms. On a pitched or sloped ceiling, BS 5839-6 requires a smoke alarm to be sited within a maximum of 600mm vertically below the apex; a heat alarm must be within a maximum of 150mm of the apex.

Under BS 5839-6:2019 (Categories LD1 and LD2) and Approved Document B for new builds and rewires, yes — a heat alarm (not a smoke alarm) is placed in the kitchen. Heat alarms respond to temperature rise rather than smoke particles, so they are not triggered by normal cooking. They are also recommended in integral garages. A heat alarm rated to about 58°C (BS EN 54-5 Class A1) is standard for kitchens. Note that the Smoke and Carbon Monoxide Alarm (England) Regulations 2022 do NOT require a kitchen heat alarm — they require at least one smoke alarm per storey with living accommodation and a CO alarm where there is a fixed combustion appliance. A kitchen heat alarm is a BS 5839-6 / Building Regulations recommendation, not a statutory requirement of the 2022 Regulations (in Scotland, however, a kitchen heat alarm is a legal requirement in all homes).

Houses in Multiple Occupation (HMOs) require the highest level of fire detection — LD1, Grade D1. This means a mains-powered, interlinked alarm in every habitable room (bedrooms, living rooms, dining rooms) and every circulation space (hallways, landings, stairways), plus heat alarms in all kitchens. The system must have battery backup. Some local authorities have additional requirements, so always check with your council's housing team. HMOs also require fire doors, emergency lighting and signage.