Fire alarm categories explained: BS 5839-1 L1 to L5, M, P1 and P2

Approved Document B Volume 2, Section 1, starts simply: every building should have a way of detecting fire and raising the alarm. In a small building, a shouted warning may be enough, as long as it can be heard throughout. Most others need an electrically operated fire alarm.

Automatic detection is expected in hotels, hostels, care homes and similar residential buildings. It's also expected where a fire could start unseen in a part of the building nobody visits, and cut off the escape, and where doors or smoke control must operate automatically. Paragraph 1.8: where a system is fitted, it should comply with BS 5839-1.

BS 5839-1 is the British Standard code of practice for fire detection and fire alarm systems in non-domestic premises. It covers design, installation, commissioning and maintenance. The current edition came into effect on 30 April 2025, replacing the 2017 edition.

BSI publishes it, and it's written for the designers, installers and maintainers who do the work. This guide describes its main recommendations in our own words, so always check the standard itself for the detail. Homes are covered by a different part, BS 5839-6, which has its own guide on this channel.

The 2025 edition brought five main changes. The standard was restructured, and a new section covers extensions and modifications to existing systems. Variations from the recommendations are still possible by agreement, but after a number of fatal fires, some are no longer acceptable, and clause 6 now lists them.

The rules for siting manual call points were simplified. And in rooms where people sleep, the standard now prefers forms of automatic detection other than heat detectors, such as smoke detection.

Paragraph 1.9 names three families of system. Category M is manual: call points and sounders, with no automatic detection. It relies on someone seeing the fire and raising the alarm.

Call points should meet BS 5839-2, or BS EN 54-11 Type A, which operates directly. Type B, which needs a second action, should only be used with building control's approval. Where everyone leaves at once, every sounder should operate, so the alarm is heard throughout.

Where should call points go? BS 5839-1 puts them on escape routes, at every storey exit, and at every exit to open air, so nobody can leave a floor without passing one. Nobody should have to travel more than 45 metres to reach one.

If the final layout isn't known at the design stage, the straight-line distance should be no more than 30 metres. Mount them at about 1.4 metres above the floor, where they're easy to see and reach. Every call point should be Type A, behind a clear protective cover, and it should sound the evacuate signal within 3 seconds.

How loud should it be? With the doors shut, the alarm should reach at least 65 decibels throughout the building. Stairways, small rooms of up to about 60 square metres, and specific points of limited extent can be 60 decibels.

Where the system has to wake people, it should reach 75 decibels at the bedhead, which normally means a sounder in the room itself. And it shouldn't exceed 120 decibels anywhere people normally go. For people with impaired hearing, visual alarm devices add a warning they can see.

Category L systems protect life, and there are five of them. L1 covers the whole building, with detection throughout. L2 covers defined parts of the building.

It normally includes everything an L3 system would, plus areas such as high-risk rooms and rooms where people sleep. L3 is designed to warn early enough for everyone to escape before the escape routes become impassable, except possibly those in the room where the fire started. BS 5839-1 does this with detection on the escape routes and in the rooms that open onto them.

L4 puts detection only in the circulation parts of the escape routes, such as corridors and stairs. It protects the way out, not the rooms. L5 is the bespoke category.

The protected areas, or the detector positions, are designed to meet one specific fire safety objective that L1 to L4 don't cover. For example, detection in a storage area where a fire could otherwise grow unseen and block the escape.

Category P systems protect property, not people. P1 covers the whole building. P2 covers only defined parts, such as a store of valuable stock or a server room.

The Building Regulations are about life safety, so a P category is a choice for the owner, often prompted by their insurer. One system can serve both aims, combining an L category with a P category.

So which category? Paragraph 1.7 says every building design should be assessed on its own, and points to the guidance in BS 5839-1. The answer comes from the fire strategy, and for an existing building, the fire risk assessment.

Think about who is inside. Where people might not respond quickly, or don't know the building, a voice alarm to BS 5839-8 may be better. For people with impaired hearing, visual alarms or vibrating pagers may be needed.

And where the alarm releases doors or starts smoke control, follow BS 7273 for that link.

False alarms disrupt businesses and tie up the fire and rescue service. BS 5839-1 asks everyone who designs or maintains a system to know the main causes, and every false alarm should be recorded with its cause, or as unknown if nobody is sure. If a service visit finds more than four false alarms per 100 detectors over the past 12 months, the service company should investigate and recommend ways to reduce them.

Common factors include where detectors are and what they're exposed to, what goes on in the building, and contractors working nearby.

A system is only as good as its upkeep. Approved Document B asks for a design, installation and commissioning certificate, and recommends third party certification. Once it's in use, the Home Office guide for small premises says: test the alarm every week, from a different call point each time, so you know it's heard throughout.

Have it inspected and tested by a competent contractor every six months. And keep a record of every test and visit, in a logbook or electronically.