BS 9991: Fire Safety in Residential Buildings

BS 9991 is the British Standard for fire safety in the design, management and use of residential buildings. It applies to all types of residential accommodation, purpose-built flats, houses in multiple occupation (HMOs), student accommodation, care homes, and sheltered housing, and sets out how fire safety should be approached from design through to ongoing management.

The standard was revised in November 2024. The 2024 edition introduced significant changes: the smoke control annex became normative (mandatory), the height threshold for natural AOVs was reduced to 18 metres, and the reference test pressure for smoke control dampers was raised to 1000 Pa. In March 2025, the Building Safety Regulator (BSR) published a clarifying letter setting out its approach to BS 9991:2024 for higher-risk building applications, confirming that compliance remains voluntary and that the baseline for demonstrating compliance is Approved Document B.

This page explains what BS 9991 covers, how it interacts with BS EN 12101, and what the BSR’s position means in practice.

Scope

BS 9991 covers:

• The design of means of escape in residential buildings
• Smoke ventilation strategies for common areas, corridors, lobbies, and stairwells
• Compartmentation and fire separation requirements
• Detection and alarm systems
• Fire service access and facilities
• Ongoing management and maintenance of fire safety measures

It is a guidance standard - it provides a methodology for demonstrating compliance with the Building Regulations rather than imposing absolute prescriptive requirements. Buildings designed in accordance with BS 9991 can demonstrate compliance with the functional requirements of the Building Regulations.

Regulatory Status and the BSR’s Position

BS 9991:2024 is a voluntary standard. For the purposes of the Building Regulations, compliance with British Standards is not mandatory, the legal obligation is to meet the functional requirements of the Building Regulations 2010, and compliance with a British Standard is one route to demonstrating that, not the only route.

In March 2025, the Building Safety Regulator (BSR), acting as the building control authority for higher-risk buildings (HRBs) in England, published an official letter setting out its position following the publication of BS 9991:2024. The key points are:

• The baseline is Approved Document B, not BS 9991:2024. For HRBs, designers must demonstrate a level of safety at least equivalent to that provided by the version of Approved Document B applicable when the building regulation application is deposited.
• ADB references BS 9991:2015 in specific sections only. References to BS 9991 in Approved Document B are to particular provisions of the 2015 edition, and should not be read as a blanket endorsement of the whole standard or the 2024 revision.
• Progressed schemes are not expected to redesign. The BSR does not expect schemes sufficiently progressed in design, currently under construction, or sufficiently progressed on site to undertake a fundamental redesign on the basis of the 2024 publication.
• The BSR will be pragmatic. It will take into account the timing of the application and any changes to guidance or standards during the design development period.

For new applications and designs not yet progressed, BS 9991:2024 is the current edition and the appropriate reference. For schemes already in development, the 2015 edition (with its 2021 amendment) may remain the applicable reference, depending on when the building regulation application was or will be submitted.

The letter was signed by Philip White, Chief Inspector of Buildings and Director of Building Safety, and is addressed to the Construction Leadership Council.

Relationship to BS EN 12101

BS 9991 references the BS EN 12101 series throughout - specifically for the specification, testing, and performance of smoke ventilation components. Key relationships:

Component Certification Requirements

BS 9991:2024 requires that smoke ventilation components meet specific certification standards. Products tested only to BS 476 (the older British Standard fire test) are no longer sufficient, components must carry UKCA or CE marking under the relevant harmonised standard.

Reaction to fire, A2-s1, d0. All smoke ventilation components installed in common escape routes must achieve at least A2-s1, d0 classification under EN 13501-1. This means limited combustibility, minimal smoke production, and no burning droplets.

Damper durability, C10,000. Smoke control dampers must achieve a minimum operations classification of C10,000 under BS EN 12101-8. This reflects the cumulative cycle count expected over a building’s life, including weekly testing.

Damper leakage, reduced leakage S. Dampers must achieve reduced leakage class S at the 1000 Pa reference test pressure.

AOV certification, BS EN 12101-2. Natural smoke vents must be tested and declared to BS EN 12101-2. The declared aerodynamic free area (Aa) is the value to use in vent area calculations, not the geometric opening dimension.

Fan certification, BS EN 12101-3. Extract fans in mechanical smoke ventilation systems must be tested and declared to BS EN 12101-3, including performance at elevated temperature.

Key Changes in BS 9991:2024

Smoke control annex became normative. In BS 9991:2015 the smoke control annex was informative, guidance rather than a requirement. In the 2024 edition it is normative, meaning compliance is mandatory rather than best practice. This is arguably the most structurally significant change in the 2024 revision.

AOV height threshold reduced from ~30m to 18m. Natural AOVs opening directly from corridors or lobbies to the external environment are now only permitted in buildings with a top storey below 18 metres. Above 18m, natural smoke shafts remain an option up to 30m; above 30m, mechanical ventilation is required.

1000 Pa test reference pressure for dampers. The reference test pressure for smoke control damper leakage classification was raised from 500 Pa to 1000 Pa. This makes leakage testing considerably more demanding:

• Dampers certified at 500 Pa may not achieve the same leakage class when re-tested at 1000 Pa
• When replacing or re-specifying dampers in buildings designed to earlier editions, confirm the test pressure used in the product’s certification
• New-build specifications should reference 1000 Pa certification from the outset

AOVs into shafts must be smoke control dampers. The 2024 edition clarifies that AOVs opening into smoke shafts or ducts must be specified as smoke control dampers, not fire doorsets or other components not designed for this purpose.

Site-fitted actuators prohibited. Fitting actuators to standard components to create a smoke vent or damper is no longer acceptable. Products must be tested and certified as complete assemblies.

See BS EN 12101-8 for the damper leakage classification system and BS EN 12101-2 for natural smoke vent certification.

June 2025 Corrigendum A1

In June 2025 BSI issued Corrigendum A1 to BS 9991:2024, making a number of technical corrections and clarifications. The smoke control changes are:

• Dead-end corridors in multi-stair buildings are limited to a maximum of 7.5 metres travel distance to the nearest staircase. Where corridors exceed this, a two-shaft mechanical ventilation system is required (Annex F.3.2.2).
• Buildings above 60 metres must consider seasonal climatic effects on smoke ventilation system performance, with reference to BS EN 12101-13.

Other corrections in A1 cover escape staircase design (winder steps now permitted in non-firefighting stairs), final exit sizing, fire-resisting construction around utility cupboards, and evacuation lift controls. Designers working to BS 9991:2024 should confirm they are referencing the version incorporating Corrigendum A1.

Height-Based System Selection

Building height is the primary driver of system type selection under BS 9991:2024. The thresholds are more stringent than the 2015 edition.

Top storey below 18 metres, natural AOVs opening directly from corridors or lobbies to the outside are permitted. Natural smoke ventilation is acceptable for common corridors and lobbies provided vent areas, positioning, and inlet provisions meet the standard’s requirements.

18 metres to 30 metres, natural AOVs are not permitted. Natural smoke shafts remain an option in this height band, but the design must demonstrate adequate shaft performance. The 2024 edition provides more detailed requirements for this band, reflecting the increased evacuation challenge.

30 metres to 60 metres, natural smoke shafts are no longer acceptable. Mechanical smoke ventilation (BS EN 12101-3 fans) or a pressure differential system (BS EN 12101-6) is required.

Above 60 metres, mechanical systems are required and must additionally consider seasonal climatic effects on performance, with reference to BS EN 12101-13 (added by Corrigendum A1, June 2025).

Buildings requiring stair pressurisation, where the number of stairs, occupancy type, or evacuation strategy demands it, stairwell pressurisation to BS EN 12101-6 is required. The 2024 revision strengthens the criteria for when pressurisation applies and what performance it must achieve.

Single-stair buildings, increasingly common in urban infill and Build-to-Rent schemes, require particularly careful attention. The single-stair configuration means the escape route is also the firefighter access route, and the smoke ventilation design must reflect this dual function.

Corridor Ventilation

Common corridors, the escape routes connecting flat front doors to the protected stair, are one of the areas where BS 9991:2024 provides the most specific guidance.

For natural corridor ventilation using AOVs, the 2024 revision sets out updated requirements on:

• Vent positioning, high-level vents should be positioned to provide effective cross-ventilation. Dead ends where smoke could accumulate require particular attention.
• Vent area, vents must meet the standard’s requirements expressed in terms of aerodynamic free area (not geometric free area). Products must carry BS EN 12101-2 certification; the aerodynamic free area is stated on the product’s declaration of performance. Use the Free Area Calculator to check the declared Aa against the 1.5 m² corridor and 1.0 m² stair targets.
• Corridor geometry, where corridors are ventilated by a single vent at one end, the maximum corridor length affects whether a natural solution is viable or whether a mechanical approach is required.
• Inlet air provision, a high-level outlet vent cannot function without an adequate low-level inlet air path. The inlet must be explicitly designed, not assumed. Inlet air typically enters via a lobby or stair, and the flow path must be unobstructed.

Mechanical corridor extract, typically via a smoke shaft drawing from each floor, requires fans tested and certified to BS EN 12101-3, with ductwork meeting BS EN 12101-7 and dampers meeting BS EN 12101-8. Smoke shafts must have a minimum dimension of 850mm in any direction and a cross-sectional area between 0.6m² and 1.0m².

In multi-stair buildings, dead-end corridors are limited to a maximum of 7.5 metres travel distance from the furthest point to the nearest staircase. Corridors exceeding this require a two-shaft mechanical ventilation system as set out in Annex F.3.2.2 of BS 9991:2024 (confirmed in Corrigendum A1).

Lobby and Stairwell Ventilation

Lobbies, where provided between the corridor and the protected stair, require their own ventilation provision. A lobby vent (typically an AOV window or duct outlet) must be sized to handle the smoke it may receive, and inlet provision must be considered separately from the corridor inlet.

Stairwells, natural stairwell ventilation typically uses a single AOV at the top of the stair, sized to meet BS 9991’s requirements for the stair geometry. In taller buildings, stair pressurisation replaces natural ventilation. Key design considerations include:

• The vent area at the head of the stair must be adequate to prevent smoke logging during a fire event
• Control of the stair head AOV must be integrated with the building’s smoke control panel
• In buildings where the stair serves as a firefighter access route, the system must be capable of manual override by the fire service

Cause and Effect, Control, and Commissioning

BS 9991 requires that smoke ventilation systems operate automatically in response to defined trigger conditions and that the cause-and-effect logic is documented, reviewed, and tested. Key requirements:

• Cause-and-effect matrix, the system’s response to each possible trigger condition (smoke detector activation, call point activation, fire alarm signal) must be documented and agreed at design stage
• Smoke control panel. The system must be controlled via a dedicated smoke control panel meeting the relevant standard, with manual override capability for the fire service
• Commissioning. The system must be formally commissioned by a competent person, with the commissioning record forming part of the building’s fire safety documentation. BS 9991:2024 reinforces the importance of complete commissioning records; missing records represent a significant compliance gap

For new buildings, the cause-and-effect matrix and commissioning requirements should be specified at design stage. For existing buildings, confirming that commissioning records exist and reflect the system as installed is an important early step in any compliance review.

See the Vent Trade commissioning guide for further detail on the commissioning process.

Management and Maintenance

Unlike BS EN 12101, which focuses on product performance, BS 9991 also governs ongoing management. The 2024 edition aligns with the SCA’s maintenance guidance structure, which defines three distinct inspection roles:

Nominated Person, daily checks. A member of the building management team must check the smoke control panel every day to confirm normal operational status, no faults, no alarm conditions, normal power. The check must be recorded in the building’s smoke control logbook. This is a formal requirement in BS 9991:2024, not optional best practice.

Competent Person, weekly and monthly testing. A competent person (with specific smoke control knowledge) carries out weekly and monthly functional tests, including activating the system through the control panel and confirming correct response. Test results are logged.

Specialist Contractor, quarterly and annual inspections. A specialist smoke control contractor carries out quarterly visual inspections and an annual full-system inspection, including physical operation of AOVs and dampers, fan performance checks, and verification of the cause-and-effect sequence.

All maintenance records must be kept available for building control and fire service inspection. Under the Building Safety Act 2022, higher-risk residential buildings (those over 18 m or 7 storeys) have additional legal obligations for safety case documentation, maintenance logs form part of the golden thread of building information. See Approved Document B for the wider regulatory context.

See also: Free Area Calculator · Smoke Ventilation Glossary

Related Standards

• BS 9999 - The equivalent standard for non-residential buildings
• BS 7346 - Components for smoke and heat control systems
• BS EN 12101 - Overview - The product standard family referenced throughout BS 9991
• Approved Document B & Building Safety Act - UK regulatory framework

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