BS 5839-1 defines eight categories of fire alarm system for non-domestic buildings: M (manual), five life-protection categories L1 to L5, and two property-protection categories P1 and P2. Category M uses manual call points only; L1 and P1 cover the whole building with automatic detection; L2, L3 and L4 cover progressively smaller areas focused on escape routes; L5 and P2 protect specific, defined areas. The right category for a building is set by the fire risk assessment or fire safety specialist – not chosen by the installer.
This guide explains each category, what changed in the 2025 edition, and how the categories are selected.
What is BS 5839-1?
BS 5839-1 is the British Standard code of practice for the design, installation, commissioning and maintenance of fire detection and fire alarm systems in non-domestic premises. The current edition, BS 5839-1:2025, was published by BSI on 30 April 2025 and supersedes BS 5839-1:2017. Fire alarms in homes are covered by a separate part, BS 5839-6, which uses its own grades and categories (see our guide to BS 5839-6 grades and categories).
According to the Fire Industry Association (FIA), the category definitions themselves are unchanged in the 2025 edition, although some of the recommendations within them have changed (see below).
The BS 5839-1 fire alarm categories
| Category | Purpose | Where automatic detection is installed |
|---|---|---|
| M | Manual system | None – manual call points only |
| L1 | Life protection | Throughout all areas of the building |
| L2 | Life protection | Everything in L3, plus defined high-risk areas |
| L3 | Life protection | Escape routes plus rooms opening onto them |
| L4 | Life protection | Escape route circulation areas, such as corridors and stairways |
| L5 | Life protection | Areas designed to meet a specific fire safety objective |
| P1 | Property protection | Throughout all areas of the building |
| P2 | Property protection | Defined parts of the building only |
Category M: manual systems
A Category M system has no automatic fire detectors. It relies on people discovering a fire and operating a manual call point to raise the alarm. The FIA notes that Category M is generally sufficient to meet fire safety legislation in workplaces where no one sleeps.
Category L1: whole-building life protection
An L1 fire alarm system has automatic detection installed throughout all areas of the building. It provides detection wherever a fire starts; the FIA notes that premises where people sleep generally need an L2 or L1 system.
Category L2: L3 plus high-risk areas
An L2 fire alarm system covers everything required for an L3 system, plus additional defined areas where there is either a high likelihood of fire starting or a high risk to life if a fire does start – under the 2025 edition, this includes rooms where people sleep. Whoever specifies an L2 system needs to state which extra rooms are to be protected and the type of detector to use.
Category L3: escape routes and rooms opening onto them
An L3 fire alarm system is designed to warn occupants early enough for everyone – other than possibly those in the room where the fire starts – to escape before escape routes become impassable because of fire, smoke or toxic gases. To achieve this, detectors are installed in escape routes and, other than in very short corridors, in all rooms or areas that open onto those escape routes.
Category L4: escape routes only
An L4 system has detection only in the circulation areas and spaces that form escape routes, such as corridors and stairways. The FIA describes L4 as probably the least likely L category to be specified; it is sometimes used to enhance safety in workplaces beyond the legal minimum, for example where staff may work alone after hours.
Category L5: engineered for a specific objective
In an L5 system, the protected areas or detector locations are designed to meet a specific fire safety objective other than that of L1 to L4. It can be very simple – for example, a single smoke detector in an access room to warn someone in an inner room, instead of a vision panel. Combined with a manual system, this would be described as Category M/L5. An L5 specification should always state the areas to be protected and the detector types.
Categories P1 and P2: property protection
Category P systems are intended to protect property and business continuity rather than life. A P1 system has detection throughout all areas of the building and gives the highest level of property protection. A P2 system covers only defined parts, such as areas of high fire hazard or where the risk to property or business continuity is high. P2 areas may be set by a property or business interruption risk assessment, or by insurers.
What changed in BS 5839-1:2025?
The FIA's guide to the 2025 edition highlights several changes relevant to categories:
- Sleeping rooms in L2 systems – rooms where people sleep are now treated as high-risk rooms needing protection in an L2 system.
- No heat detectors in sleeping rooms – heat detectors should no longer be used in rooms where people sleep in L2 or L3 systems. Smoke, multi-sensor or carbon monoxide fire detectors are preferred. This is not retrospective: existing systems need not be changed unless they are being substantially replaced, upgraded or extended.
- Stairway lobbies – these are now areas requiring automatic detection, rather than being treated as low-risk exclusions.
- L4 and flue-like structures – a detector is now recommended at the top of flue-like structures in L4 systems.
Who chooses the category?
The FIA is clear that responsibility for choosing the category does not rest with the fire alarm designer or installer. It rests with the fire safety specialist – such as the fire risk assessor, fire engineer or enforcing authority. A specification that simply says "BS 5839-1" without a category is virtually meaningless. For L2, L3, L5 and P2 systems, the specification should also state which areas are protected and the type of detectors required. If no category is given, BS 5839-1 recommends that the designer confirms the proposed category with the purchaser before an order is placed.
Products for BS 5839-1 systems
Whatever the category, a system is built around control and indicating equipment and the right mix of detectors and call points. Browse our fire alarm panels, smoke detectors, heat detectors, multi-sensor detectors and manual call points. All products are genuine, brand new and factory sealed, with fast, tracked UK delivery.
Frequently asked questions
What is the difference between L1, L2 and L3 fire alarm systems?
L1 has automatic detection throughout the building. L3 covers escape routes and rooms opening onto them. L2 covers everything in L3 plus specified high-risk rooms.
What is a P1 fire alarm system?
A P1 system is a property-protection system with automatic detection throughout all areas of the building, designed to give early warning of fire so it can be dealt with early, protecting property and business continuity.
Which fire alarm category does my building need?
That is decided by your fire risk assessment or a fire safety specialist. As a general guide, the FIA notes that Category M is often sufficient where no one sleeps, while premises where people sleep normally need L2 or L1.
Is BS 5839-1 a legal requirement?
BS 5839-1 is a code of practice rather than law, but it is widely used to show that a fire alarm system meets the requirements of fire safety legislation and building regulations.
This guide is general information; follow your fire risk assessment and a competent person's advice.