What is the difference between LD1, LD2 and LD3?

These are categories of fire detection in dwellings defined by BS 5839-6. LD1 provides the highest level of protection — alarms in every room (except bathrooms and cupboards under 1m²). LD2 is the standard for new builds and rewires — alarms in all circulation spaces, the kitchen, and principal habitable rooms like the living room and bedrooms. LD3 is the minimum — alarms only in escape routes (hallways, landings, stairways). LD3 is the minimum legal requirement for existing dwellings under the Smoke and Carbon Monoxide Alarm Regulations 2022.

Do smoke alarms need to be mains-powered?

For new builds, extensions, and full rewires, yes — smoke alarms must be mains-powered (Grade D1) with battery backup and interlinked (when one triggers, they all sound). This is required by Approved Document B and BS 5839-6. For existing dwellings (rental or owner-occupied), battery-powered alarms are acceptable, but sealed long-life lithium units (10-year battery) are strongly recommended. The 2022 regulations require at least one alarm on every storey.

Where should I position smoke alarms?

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. In rooms with sloped ceilings, mount the alarm between 600mm and 1500mm below the apex.

Do I need a heat alarm in the kitchen?

Yes, under both BS 5839-6 and the Smoke and Carbon Monoxide Alarm Regulations 2022, a heat alarm (not a smoke alarm) should be installed 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 garages and boiler rooms. A heat alarm rated to 58°C (Grade A1) is standard for kitchens.

What are the HMO smoke alarm requirements?

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.

Smoke Alarm Spacing Calculator — BS 5839-6 Compliance Guide

Smoke Alarm Spacing Calculator — BS 5839-6 Compliance

Calculate the number and type of smoke and heat alarms needed for your property to comply with BS 5839-6 and Approved Document B.

Number of Floors

Include all storeys (ground = 1)

Rooms per Floor

Average habitable rooms per floor (exc. bathrooms)

Property Type

Determines the detection level and grade required

Kitchen present

A heat alarm is required in the kitchen

Integral garage

Garages require a heat alarm

How We Calculate This

This calculator determines the number of smoke alarms and heat alarms needed based on your property type, number of floors, and room count. It follows the requirements of BS 5839-6 (Fire detection and fire alarm systems for buildings — Code of practice for the design, installation, commissioning and maintenance of fire detection and fire alarm systems in domestic premises).

Detection levels

LD1: Alarms in all rooms (except bathrooms/small cupboards) — required for HMOs
LD2: Alarms in circulation spaces + kitchen + principal habitable rooms — standard for new builds/rewires
LD3: Alarms in escape routes only — minimum for existing dwellings

System grades

Grade D1: Mains-powered with integral battery backup, interlinked — required for new builds
Grade D2: Mains-powered with integral battery backup, not interlinked
Grade F1: Battery-powered, interlinked (radio-linked sealed units)
Grade F2: Battery-powered, standalone — minimum for existing dwellings

Regulations

This calculator references BS 5839-6:2019, Approved Document B (Fire safety), and the Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022. For HMOs, additional requirements may apply under the Housing Act 2004 and local licensing conditions. Always verify requirements with your local Building Control or Fire and Rescue Service.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Smoke Alarm Spacing Calculator — BS 5839-6 Compliance

How We Calculate This

Detection levels

System grades

Regulations

Frequently Asked Questions

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