Fire alarm categories explained: L1 to L5, M, P1 and P2

Every fire alarm system designed to BS 5839-1 is given a category. The category tells you what the system is designed to do: protect people, protect property, or simply give a manual warning. If your fire risk assessment says "the building requires an L2 system", this guide explains what that means and what you should expect to see on the ceiling.

The three families

There are three families of category in BS 5839-1:2025.

• M: manual systems. Break-glass call points only, with no automatic detection. People raise the alarm themselves.
• L: life protection. Automatic detection designed to give people time to escape. There are five levels, L1 to L5.
• P: property protection. Automatic detection designed to call the fire and rescue service early enough to limit damage. There are two levels, P1 and P2.

A building can have a combined category, such as L2/P2, where one system does both jobs.

The L categories

• L1: detection throughout the building, in every room and space, with very few exceptions. Used where the risk to life is high, such as care homes and some hotels.
• L2: detection in escape routes, plus rooms that open onto escape routes and any defined high-risk rooms such as kitchens, plant rooms and sleeping areas.
• L3: detection in escape routes and in rooms that open onto escape routes. The aim is to warn people before smoke blocks their way out.
• L4: detection in escape routes only, such as corridors and stairways.
• L5: a bespoke system designed to protect a specific risk identified in the fire risk assessment, such as a single room with no other protection.

The P categories

• P1: detection throughout the building, to give the earliest possible warning to the fire service.
• P2: detection in defined high-risk areas only.

Property categories are often asked for by insurers, especially for warehouses, plant rooms and buildings with valuable contents.

Which category does my building need?

Your fire risk assessment should state the category. As a rough guide:

• Small shops and offices with good visibility and simple escape routes often need an M system, sometimes with L5 or L4 detection added.
• Larger offices and multi-storey commercial buildings usually need L3 or L2.
• Hotels, hostels and buildings where people sleep usually need L2 or L1.
• Care homes and hospitals normally need L1.
• The common parts of most purpose-built blocks of flats with a stay-put strategy do not need a communal alarm at all. Where one is needed, for example in converted buildings, it is usually designed with BS 5839-6 for the flats and BS 5839-1 for the common parts.

Domestic premises use different grades

Houses, flats and HMOs are covered by BS 5839-6, which uses grades (A, C, D1, D2 and F) to describe the equipment and power supply, and categories (LD1, LD2, LD3) to describe coverage. Our guide to landlord fire safety covers these in more detail.

What the 2025 edition changed

BS 5839-1 was revised in 2025. The categories themselves are the same, but the new edition puts more emphasis on how a system is extended or modified, on the competence of the people designing and servicing it, and on recording variations. If your system is being altered, make sure the work is designed to the 2025 edition.

Getting it right

The most common problem we see is a system installed to one category while the building is now used in a way that needs another. An office converted to sleeping accommodation, a warehouse with a new mezzanine, or a shop with flats added above all change the picture. If your building has changed since the alarm was designed, ask for a review before you rely on it.