Approved Document B is written to be safe, simple and applicable to buildings that behave the way most buildings behave. That makes it an excellent default and a poor fit for anything ambitious.
If your design needs longer travel distances, a large open-plan volume, an atrium, fewer stairs or a layout Approved Document B does not contemplate, there are two routes forward. This explains both.
Three routes to compliance
The legal duty is to satisfy the functional requirements in Part B of Schedule 1 to the Building Regulations 2010. There is more than one way to demonstrate that you have.
Route 1: Approved Document B
Statutory guidance. Prescriptive, tabulated and straightforward to apply. Follow the recommendations and you have a strong presumption of compliance.
Its limits are stated in the document itself. Approved Document B notes that following the guidance does not guarantee compliance and that it may not provide appropriate guidance where a case is unusual in terms of design, setting, use, scale or technology. It lists non-standard conditions including buildings with unusual occupancies or high levels of complexity, very large or very tall buildings, large timber buildings and some buildings incorporating modern construction methods.
Use it when: the building is conventional, the occupancy is conventional and the design does not need anything Approved Document B does not offer.
Route 2: BS 9999:2017 or BS 9991:2024
British Standards giving a risk-based, code-based approach. More sophisticated than Approved Document B, considerably less onerous than full fire engineering.
- BS 9999:2017, fire safety in the design, management and use of buildings. For buildings other than dwellings
- BS 9991:2024, fire safety in the design, management and use of residential buildings
These are the workhorses of design flexibility. Most projects that need more than Approved Document B allows need one of these, not full fire engineering.
Use them when: you need flexibility that can be justified through a structured risk assessment and compensatory features, rather than through bespoke modelling.
Route 3: Fire engineering under BS 7974
BS 7974, application of fire safety engineering principles to the design of buildings. Performance-based analysis: quantified assessment of fire development, smoke movement, tenability, structural response and occupant evacuation, typically involving computational modelling.
Use it when: the design cannot be justified within any code or where the building is genuinely unusual in scale, geometry or use.
How BS 9999 creates flexibility
This is worth understanding in detail because it is where most real-world design freedom comes from.
BS 9999 requires a risk profile to be established for the building, combining two things.
Occupancy characteristic
| Characteristic | Description | Examples |
|---|---|---|
| A | Occupants awake and familiar with the building | Office and industrial premises |
| B | Occupants awake and unfamiliar with the building | Shops, exhibitions, museums, leisure centres, other assembly buildings |
| C | Occupants likely to be asleep, subdivided into Ci long-term individual occupancy, Cii long-term managed occupancy, Ciii short-term occupancy | Ci individual flats without 24 hour management on site, Cii serviced flats, halls of residence, boarding school sleeping areas, Ciii hotels |
| D | Occupants receiving medical care | Hospitals, residential care facilities. Outside the scope of BS 9999 |
Note that occupancy characteristics Ci and Cii are covered in more depth in BS 9991.
Fire growth rate
| Category | Rate | Description | Typical examples |
|---|---|---|---|
| 1 | Slow | Evenly distributed low level fire load, small discrete packets of fuel or material of limited combustibility | Reception areas, concourses without concession outlets, halls with limited fire load such as sports stadia and foyers |
| 2 | Medium | Evenly distributed low to mid-level fire load, mixed combustible materials | Offices, lounges, classrooms, auditoria, seating areas, galleries, car parks |
| 3 | Fast | Stacked combustibles excluding high rack storage, process, manufacturing or storage of combustible materials | Shop sales areas, workshops, factories, small storage buildings |
| 4 | Ultra-fast | Medium to large quantities of combustible materials, high racked storage, flammable liquids and gases or where rapid uncontrolled fire growth could occur | Warehousing, processing plants, car stackers without fire separation between stacked cars |
The risk profile
Combining the two produces a profile: A1 to A4, B1 to B4, C1 to C4.
That profile then drives travel distances, exit widths, door widths, compartment sizes and fire resistance periods throughout the standard.
Two constraints matter:
- A4, B4 and C4 are unacceptable within the scope of BS 9999
- C3 is unacceptable under many circumstances unless special precautions are taken
The sprinkler mechanism
This is the key that unlocks most designs.
BS 9999 provides that where sprinkler systems are installed, the fire growth rate can be reduced by one level. An unsprinklered shop at B3 becomes B2. An unsprinklered office at A2 becomes A1. An unacceptable B4 becomes an acceptable B3.
The standard's own example is direct: the provision of an automatic sprinkler installation permits a reduction in fire growth rate, allowing larger travel distances, smaller doors, larger compartments, reduced fire resistance periods and other provisions.
Two important conditions. Where only part of a building is sprinklered, the reduction applies only to the sprinklered rooms and associated corridors and linking spaces must either be sprinklered too or be separated by fire-resisting construction. And where sprinklers are used to change the risk profile, only systems installed to BS EN 12845 for new systems or BS 5306-2 for existing systems may be used to adjust the fire resistance periods in the standard's tables. Watermist may be used as an alternative where relevant fire test protocols exist, designed to BS 8489-1, subject to that standard's limitations.
Fire risk management levels
BS 9999 also ties design to management. It establishes management system levels associated with each risk profile and devotes a full section to designing so that a building can be managed.
This is the part clients most often overlook. Flexibility obtained under BS 9999 is frequently conditional on a defined standard of fire safety management being sustained for the life of the building. If it is not, the design basis quietly evaporates. The fire strategy must state the assumption and the regulation 38 handover must communicate it.
What flexibility actually looks like
Common design objectives and how they are typically achieved:
| Design objective | Typical route |
|---|---|
| Extended travel distances | BS 9999 risk profile with sprinklers reducing fire growth rate or fire engineered analysis |
| Larger compartments | BS 9999 with sprinklers |
| Reduced fire resistance periods | BS 9999 risk profile, with sprinklers installed to BS EN 12845 or BS 5306-2 |
| Open-plan flats and apartments | BS 9991 open-plan flat design provisions, typically with sprinklers and enhanced detection |
| Atria | BS 9999 or fire engineering, usually with smoke control modelling |
| Single-stair buildings | BS 9991 for residential, subject to its provisions and current height limits, carefully scrutinised |
| Reduced stair widths | BS 9999 exit capacity based on risk profile and evacuation strategy |
| Phased evacuation | BS 9999, with associated detection, voice alarm and management requirements |
| Unusual geometry or occupancy | BS 7974 fire engineering |
Note that the second staircase requirement for new residential buildings in England over 18 metres takes effect on 30 September 2026, which materially affects single-stair residential design. Confirm the current position before designing to it.
Fire engineering under BS 7974
Where no code accommodates the design, performance-based analysis is the route. Typical components:
- Fire dynamics: design fire scenarios, heat release rates, growth curves
- Smoke movement: computational fluid dynamics modelling of smoke filling, layer height and visibility
- Tenability analysis: whether conditions on escape routes remain survivable for the required period
- Evacuation modelling: pre-movement time, travel time, queuing and total evacuation time
- Structural fire response: performance of the structure under a realistic rather than standard fire
- Comparative analysis: showing the engineered solution performs at least as well as the code-compliant alternative
Two things clients consistently underestimate.
Cost and programme. Modelling takes time and specialist software and the analysis must be defensible to a reviewer who may commission peer review.
The management burden. Engineered solutions typically depend on systems performing as modelled for the building's life. Smoke control that is never tested or sprinklers that are isolated for works and not reinstated, invalidate the basis of approval.
Practical guidance for design teams
Decide the route early. Retrofitting BS 9999 onto a design developed against Approved Document B rarely works cleanly because the two use different parameters.
Do not mix codes. Cherry-picking the most favourable parameters from Approved Document B, BS 9999 and BS 9991 is one of the fastest routes to rejection. The standards are internally coherent packages. Pick one and apply it.
Get the risk profile right. It drives everything downstream. Where a building has multiple uses, BS 9999 requires assessment of the reliance each occupancy type places on the prescribed measures and where two profiles rely on a common measure such as fire resistance, the more conservative profile governs that measure.
Be honest about the fire growth rate. BS 9999 asks for justification where a specific rate is selected and notes that warehouses in particular vary widely. Selecting an optimistic rate to make the numbers work is a false economy.
State what the flexibility depends on. Sprinklers, smoke control, management level, staff numbers, detection category. If it is load-bearing for the design, it must be in the strategy and in the regulation 38 handover.
Remember the building has to be operated. A design that requires a management level the eventual operator cannot sustain is not a good design however elegant the modelling.
Frequently asked questions
Is BS 9999 a legal alternative to Approved Document B? Both are means of demonstrating compliance with the functional requirements of Part B. Neither is itself the law. Building control bodies routinely accept BS 9999 and BS 9991 designs, provided they are applied properly and consistently.
Which applies to residential buildings, BS 9999 or BS 9991? BS 9991:2024 is the residential standard. BS 9999:2017 covers buildings other than dwellings and expressly notes that occupancy characteristics Ci and Cii are covered in more depth in BS 9991. Mixed-use buildings need both, applied to the appropriate parts.
Do sprinklers automatically allow longer travel distances? Under BS 9999, sprinklers reduce the fire growth rate by one category, which changes the risk profile, which changes the permitted travel distance. The mechanism is indirect and the sprinkler system must be installed to the standard the analysis assumed.
When do I need full fire engineering rather than BS 9999? When the design cannot be justified within a code. Large atria, unusual geometries, very large undivided volumes, non-standard occupancies and designs relying on quantified smoke or evacuation performance.
Does a fire-engineered design cost more to run? Often, yes. Engineered solutions typically depend on active systems and defined management standards, both of which carry ongoing cost. That should be part of the decision, not a surprise after handover.
Can an existing building be reassessed under BS 9999? Yes and it is a common and useful exercise. Reassessing an existing building against BS 9999 rather than Approved Document B sometimes demonstrates that an apparent deficiency is acceptable given the actual risk profile.
