Could the Wrong Fire Alarm Design Leave People at Risk?

Two buildings might look almost identical, but that doesn’t mean they need the same fire alarm system. Choosing the wrong detection or fire alarm category, overlooking a particular hazard or installing unsuitable detectors could leave people without the warning they need when a fire starts.

Imagine you’re opening a new office and ask a fire alarm contractor for a quotation.

They visit the premises, look around and tell you that you’ll need a Category L3 fire alarm system because that’s what they normally install in offices.

It sounds reasonable. After all, they’re the fire alarm specialists, and they’ve probably installed hundreds of similar systems.

But there’s something missing from that conversation.

Nobody has asked how many people will work in the building, whether anyone works alone, what happens outside normal working hours or whether a fire could develop unnoticed in an unoccupied area.

Nobody has asked about the building’s evacuation strategy, the fire risks associated with its activities or whether the alarm needs to operate other fire safety equipment.

And nobody has looked at the fire risk assessment.

The problem is that choosing a fire alarm system based purely on the type of building can result in the wrong protection being installed.

Two offices can have very different fire risks.

The same applies to restaurants, warehouses, workshops, hotels and other commercial premises.

So how should the correct fire alarm system be selected, who decides what category is needed and what should you ask before spending thousands of pounds on an installation?

Does Every Office or Commercial Building Need the Same Fire Alarm System?

No. And the legislation doesn’t approach fire detection that way either.

Under Article 13 of the Regulatory Reform (Fire Safety) Order 2005, the Responsible Person must ensure that appropriate fire detection and warning arrangements are provided where necessary to protect people.

What is appropriate depends on factors such as the size and use of the premises, the activities undertaken, the equipment and substances present, and the maximum number of people who may be inside.

The legislation doesn’t say that every office needs Category L3, every warehouse needs Category L2 or every restaurant needs a particular number of smoke detectors.

Those decisions need to reflect the actual circumstances.

Consider two small office buildings.

The first is a single-storey, open-plan office occupied by eight people during normal working hours. Everyone can see much of the building, escape routes are straightforward and a developing fire is likely to be discovered quickly.

The second is a three-storey office with individual rooms, a basement storeroom, an enclosed staircase and employees occasionally working alone in the evening.

Although both are described as offices, their fire risks and warning requirements are quite different.

The Home Office’s fire safety guidance for offices and shops recognises this distinction. It explains that a small, uncomplicated workplace may sometimes need no more than a simple means of raising the alarm, while automatic fire detection may be necessary where a fire could develop unnoticed and compromise escape routes.

The starting point should therefore be the building’s fire risks and the protection needed, rather than simply copying the system installed in a similar property.

What Does BS 5839-1:2025 Say About Fire Alarm Design?

BS 5839-1:2025 is the current British Standard code of practice covering the design, installation, commissioning and maintenance of fire detection and alarm systems in non-domestic premises.

Published on 30 April 2025, it replaced the 2017 edition.

The standard provides recognised recommendations for selecting equipment, positioning detectors and manual call points, designing alarm warning arrangements and documenting the completed installation.

It also establishes different categories of fire alarm system according to their purpose and the extent of automatic fire detection provided.

These categories are important because they describe what the system is intended to achieve.

What Are the Fire Alarm Categories?

BS 5839-1 identifies eight categories, divided into manual systems, life protection systems and property protection systems.

Category M systems rely on people discovering a fire and manually raising the alarm. They don’t incorporate automatic fire detectors.

Category L systems provide automatic fire detection intended to protect life. These are divided into Categories L1 to L5.

Category L1 provides automatic detection throughout the building, subject to the detailed recommendations of the standard.

Category L2 provides detection in the areas required for Category L3, together with additional areas where there is a greater likelihood of fire starting or a higher risk to life.

Category L3 is intended to provide sufficiently early warning to protect escape routes. This generally involves detectors in escape routes and rooms opening onto them, subject to the standard’s detailed provisions.

Category L4 provides automatic detection within the circulation areas forming escape routes, such as corridors and stairways.

Category L5 addresses a specific fire safety objective that isn’t covered by the other life protection categories.

Category P systems are intended primarily to protect property or business continuity. Category P1 provides automatic detection throughout the building, while Category P2 protects specified areas where the risk to property or business operations justifies detection.

A building can also have combined life and property protection objectives.

Our guide to different fire alarm system types explains these categories in more detail.

But knowing what the categories mean is only part of the process. The important question is how the appropriate category is selected.

Who Should Decide Which Fire Alarm Category You Need?

This is one of the most important distinctions in fire alarm design.

In December 2025, the Fire Industry Association published guidance specifically addressing the selection and specification of fire alarm categories.

The FIA explains that selecting the appropriate category is fundamentally a fire safety decision, rather than simply an equipment design decision.

That responsibility should sit with somebody who understands the building’s fire risks and fire safety objectives, such as a competent fire risk assessor, fire safety consultant or fire engineer.

The fire alarm designer then develops the detailed technical design needed to meet that specification.

Of course, a fire alarm contractor may also have the qualifications and experience needed to undertake fire safety assessment and category selection. The important issue is competence for both roles, rather than the job title.

But a contractor shouldn’t simply assume the required category because they’ve installed similar systems elsewhere.

The FIA also makes another important point: specifying that a system must comply with BS 5839-1, without identifying the required category, provides insufficient information about what is actually needed.

For some categories, the specification must also identify particular areas requiring protection and the types of detection needed.

Otherwise, the designer may be left making assumptions that should have been resolved by the fire safety assessment.

What Should Be Considered Before Designing a Fire Alarm?

At ESI: Fire Safety, we believe the sensible approach is to establish what the system needs to achieve before deciding where detectors should be fitted.

That involves looking at the building, its occupants, activities and existing fire precautions.

What Is the Fire Alarm Intended to Achieve?

The first question is whether the system is primarily intended to protect life, protect property, support business continuity or achieve several objectives together.

A system might also need to operate other fire protection measures.

For example, the fire alarm may release electromagnetic door holders, shut down ventilation equipment, operate smoke control systems or interface with access control arrangements.

Those functions need to be considered during the design.

The building’s evacuation strategy is particularly important.

A hotel operating simultaneous evacuation has different requirements from a care facility using progressive horizontal evacuation.

Similarly, a purpose-built block of flats operating a stay-put strategy may have very different fire detection arrangements from a commercial office building.

The FIA’s guidance on BS 5839-1:2025 emphasises that the fire alarm design should support the building’s required evacuation procedures, rather than forcing the evacuation strategy to fit a predetermined alarm system.

Where the building has an existing fire strategy, that should be considered at the start of the design process.

Who Is in the Building, and When?

The people using a building are just as important as its layout.

A small office occupied during normal working hours presents different circumstances from premises where people sleep, work alone or may require assistance to evacuate.

The Home Office’s guidance for Responsible Persons recognises that premises where people sleep generally require automatic detection, primarily using smoke detectors.

In many daytime workplaces where nobody sleeps, a suitable manual alarm system may be sufficient.

However, that doesn’t mean automatic detection is never necessary in an office or warehouse.

Consider a lone employee working late in a building where a fire could develop in an unoccupied area.

Or an inner room where the occupant cannot see a fire starting in the room through which they must escape.

In these circumstances, automatic detection may be needed to provide adequate warning.

The correct solution depends on the particular layout and risks.

What Fire Hazards Are Present?

A building’s activities can have a significant effect on detector selection.

Consider a commercial kitchen producing steam, cooking fumes and occasional smoke during normal operations.

An ordinary point smoke detector installed in an unsuitable position may generate repeated unwanted alarms.

In a woodworking workshop, dust could affect detector performance. A warehouse with high ceilings may require a different detection approach from an ordinary office.

Strong airflow, temperature extremes, ventilation arrangements and the nature of the expected fire can also influence detector selection.

Depending on the circumstances, the design might use point smoke detectors, heat detectors, multisensor devices, optical beam detectors, aspirating smoke detection, flame detectors or linear heat detection.

Each technology has particular characteristics and limitations.

The correct choice involves considering how quickly the detector should respond to a credible fire and whether it can do so reliably in the normal working environment.

The cheapest detector isn’t necessarily the right detector, and the most sophisticated equipment isn’t automatically the best solution either.

Why Is Using Heat Detectors to Stop False Alarms Sometimes a Mistake?

This is a particularly important issue.

Imagine a hotel where smoke detectors have been causing unwanted alarms.

Someone suggests replacing them with heat detectors because heat detectors are less likely to respond to ordinary smoke or aerosols.

That might reduce unwanted activations, but it could also significantly delay warning during a real fire.

Heat detectors generally need a substantial temperature rise before operating.

In a bedroom where somebody is asleep, that delay could be critical.

BS 5839-1:2025 strengthened the recommendations concerning detector selection in sleeping rooms.

The updated standard favours smoke, multisensor or suitable carbon monoxide fire detection rather than heat detection in sleeping accommodation, unless the particular circumstances and fire risk assessment justify a different approach.

A carbon monoxide fire detector in this context is specialist fire detection equipment, not a substitute for an ordinary domestic carbon monoxide alarm intended to warn of carbon monoxide poisoning.

The change doesn’t automatically mean every existing hotel with heat detectors in bedrooms must immediately replace them. Existing systems need to be considered in the context of their design, age, condition, applicable requirements and fire risk assessment.

However, new installations and relevant alterations should take account of the current recommendations.

More generally, repeated false alarms should be investigated rather than simply fitting slower-responding detectors.

The cause might be contamination, incorrect detector placement, unsuitable equipment or changes in how the premises are used.

The Home Office’s guidance on reducing false alarms warns that frequent unwanted activations can cause occupants to become complacent and respond less effectively to a genuine alarm.

The objective should be reliable early detection without unnecessary activations.

Can the Wrong Fire Alarm Design Cost More in the Long Run?

Absolutely.

An inadequate design may require additional detectors, new wiring, revised programming or changes to the control equipment once the deficiencies are identified.

Those changes are often considerably more disruptive after a building has been occupied and decorated.

But over-design can also create unnecessary expenditure.

A specification that calls for detection throughout every room without a clear justification may result in more equipment, additional maintenance requirements and higher long-term costs than are reasonably necessary.

The appropriate system needs to meet the fire safety objectives, taking account of statutory requirements, the fire strategy, the fire risk assessment and any relevant insurance conditions.

For a business owner, this is why an independent assessment of the required category and scope can be valuable before obtaining installation quotations.

It gives contractors a consistent specification to price and reduces the risk of comparing quotations for systems that provide different levels of protection.

Why Is Simply Specifying Category L2 Not Enough?

Imagine asking three fire alarm contractors to quote for a Category L2 system.

You might expect them to provide broadly comparable quotations.

But Category L2 includes the detection necessary for Category L3, together with additional protection in areas of higher fire risk.

Which additional areas need detecting?

Does the basement storeroom require protection? What about the electrical intake, plant room or high-risk workshop?

And what types of detectors should be installed in those areas?

Unless the specification identifies these requirements, different contractors may make different assumptions.

The FIA’s category selection guidance explains that an L2 specification should identify the additional areas requiring detection beyond those protected by L3, together with the appropriate detector types.

Similar additional information is needed for certain other categories, including L3, L5 and P2.

A properly developed specification should therefore establish not just the category, but the intended extent of coverage and the relevant fire safety objectives.

This avoids unnecessary ambiguity and helps ensure that the system installed is the system the building actually needs.

What About Fire Alarm Design in HMOs and Residential Buildings?

It’s important not to confuse commercial fire alarm system categories with domestic fire alarm requirements.

BS 5839-1 principally addresses non-domestic premises, while BS 5839-6 covers fire detection and alarm systems in domestic premises.

Houses in Multiple Occupation, commonly called HMOs, may require particular grades and categories of domestic fire detection, sometimes incorporating a Grade A system designed with reference to BS 5839-1.

The appropriate arrangements depend on the accommodation, layout, occupancy, fire risk assessment and any relevant licensing or regulatory requirements.

Our guide to HMO fire alarm grades and categories explains these distinctions in more detail.

A purpose-built block of flats may also have a fire strategy that doesn’t call for a communal fire alarm throughout the building.

Installing a commercial fire alarm simply because the property contains several flats could therefore be inappropriate if it conflicts with the intended evacuation arrangements.

The building’s fire strategy and applicable guidance should be established before deciding what detection is necessary.

What Happens When You Extend or Modify an Existing Fire Alarm?

Fire alarm systems often need changing as buildings evolve.

An office might be divided into smaller rooms, a warehouse may gain a mezzanine, or a shop could be converted into a restaurant.

These changes can affect the detection coverage, escape arrangements and fire safety objectives of the original installation.

A new partition could leave a detector on the wrong side of a wall. A mezzanine might create new areas where smoke could collect, while a change of use might introduce fire hazards that weren’t considered previously.

The 2025 edition of BS 5839-1 includes a dedicated section dealing with extensions and modifications.

New work should follow the current applicable recommendations, even where the original installation was designed to an earlier edition.

That doesn’t automatically require the entire existing system to be replaced, but the effect of the alterations needs to be assessed.

The design should also establish whether the original category and extent of protection remain appropriate.

This is why building alterations should be considered through the fire risk assessment and fire safety management process before contractors start moving detectors or adding devices.

What About Variations from BS 5839-1:2025?

BS 5839-1 is a code of practice, and some variations from its recommendations can be appropriate where they are properly justified and agreed.

However, the 2025 edition places greater emphasis on documenting those decisions.

The FIA’s guidance on the 2025 changes explains that agreed variations should be recorded in the fire alarm system logbook.

The standard also identifies certain departures that should not be regarded as acceptable variations.

One example is the absence of an appropriate fire alarm zone plan where a building has more than one fire detection zone on a storey.

Another concerns the absence of necessary alarm transmission to an Alarm Receiving Centre in specified residential care and supported housing circumstances.

These aren’t matters that should simply be dismissed as an acceptable difference of opinion between installers.

Where variations are proposed, they should be technically justified, appropriately agreed and documented.

The Responsible Person should understand what has been changed and why.

What Should You Ask Before Approving a Fire Alarm Design?

Before accepting a fire alarm quotation, ask the designer or consultant to explain how the required category was established.

Does it follow the fire risk assessment or fire strategy? Has the building’s occupancy, layout and use been considered? Are there specific risks that require additional detection?

The specification should identify the category, the areas to be protected and the detector types where necessary.

It should also establish any relevant interfaces, alarm zoning, warning arrangements, cause-and-effect requirements and monitoring arrangements.

For more complex systems, the documentation should explain how the alarm supports the building’s evacuation strategy.

It’s also worth asking what happens if the building changes.

Can the system be extended? Will the proposed equipment remain supportable? Are suitable replacement parts available, and will future maintenance require specialist manufacturer programming tools?

A competent fire alarm designer should be able to explain the reasoning behind the installation rather than simply provide a drawing showing where devices will be fitted.

And if the category has been selected purely because it’s what the contractor normally installs in similar buildings, it’s reasonable to ask whether the actual fire risks have been properly considered.

The Right Fire Alarm Starts With the Right Questions

Fire alarm design isn’t simply about deciding how many smoke detectors to install or which control panel to use.

It’s about understanding what the system needs to achieve.

Who is in the building? What are they doing? How might a fire start, and how quickly could it develop? What warning do people need, and what other systems must respond?

Only when those questions have been considered can the appropriate category, extent of detection and equipment be properly specified.

A suitable fire alarm system should support the building’s fire strategy and provide the required protection without unnecessary complication or expenditure.

Because the real risk isn’t simply buying the wrong fire alarm panel. It’s installing a system that looks complete but doesn’t provide the warning or protection your building actually needs.

Fire Alarm Design and Consultancy from ESI: Fire Safety

At ESI: Fire Safety, we help business owners, landlords, managing agents and developers establish the appropriate fire detection and warning requirements for their buildings.

Our services include fire risk assessments, fire alarm design, commissioning and validation, together with independent consultancy on existing installations, system alterations and compliance with BS 5839-1:2025.

We work with offices, hotels, restaurants, workshops, warehouses and mixed-use developments across Newbury, Thatcham, Maidenhead, Marlow, Henley-on-Thames and High Wycombe, together with surrounding areas.

Whether you’re planning a new fire alarm installation, converting a building to a different use or unsure whether your existing system provides the right level of protection, we can help establish the appropriate requirements before expensive installation decisions are made.

ESI: Fire Safety — No Shortcuts in Safety.

Contact ESI: Fire Safety or call 01276 300 351 to discuss your fire alarm design or fire safety consultancy requirements.

This article focuses on non-domestic fire detection and alarm systems in England and Wales. The appropriate design depends on the building, its fire safety strategy, the risks present and any relevant statutory, insurance or contractual requirements.