BS 9999 vs Approved Document B: What's the Difference?

Approved Document B and BS 9999 are both routes to demonstrating compliance with Part B of the Building Regulations, and specifiers frequently need to understand how they relate to each other, because a single project can end up referencing both — Approved Document B for most of the building, and BS 9999 for a specific area where its more flexible, risk-based approach offers a better fit.

A Simpler Document vs a Risk-Based Code

Approved Document B is essentially a simplified, prescriptive guidance document: it provides tabulated answers — this fire resistance period, this maximum travel distance, this minimum stair width — based on broad categories of building height and purpose group. BS 9999, Code of Practice for Fire Safety in the Design, Management and Use of Buildings, takes a more sophisticated, risk-based approach. It introduces the concept of a building's management level — essentially, how well-managed the fire safety systems and procedures in the building are expected to be over its life — and allows some parameters, including escape distances and compartment sizes, to vary based on that management level and on more detailed engineering judgement than Approved Document B's simpler tables permit.

When Specifiers Choose BS 9999

BS 9999 is commonly used on larger, more complex non-domestic buildings — deep-plan offices, large mixed-use developments, buildings with atria or unusual geometries — where Approved Document B's simpler tables produce an overly conservative result, or don't cleanly address the building's configuration at all. It is also frequently the reference point for smoke control strategy in atria and large-volume spaces, an area where Approved Document B is comparatively thin and BS 9999 provides more developed guidance on smoke reservoirs, curtain boundaries and extraction design.

Fire Doors and Curtains Under Each

For fire doors, the practical difference specifiers encounter is usually in the detail of when a particular fire resistance period is triggered, rather than in the door product itself — a door tested to EN 1634-1 and classified to EN 13501-2 satisfies either route, because both ultimately reference the same European test and classification framework for the product evidence. For fire curtains, BS 9999 is typically the more useful reference, since it deals more explicitly with large-volume compartmentation and smoke control strategies where curtains are most often specified, whereas Approved Document B tends to push these scenarios towards a fire engineered assessment in any case.

Can They Be Mixed on One Project?

Yes, and it is common practice — a building might follow Approved Document B for its standard office floors and stair cores, while using BS 9999's more developed atrium and smoke control guidance for a central atrium space, or BS 9999's management-level approach to justify a longer travel distance in a specific zone. What matters is that the chosen approach for each part of the building is applied consistently and agreed with building control and the project's fire engineer, rather than cherry-picking the most favourable figure from each document without a coherent overall strategy.

Specification Takeaways

• —Confirm early in design which guidance route — Approved Document B, BS 9999, or a bespoke fire engineered strategy — applies to which part of the building.
• —For atria, large-volume spaces and complex geometries, expect BS 9999 to be the more directly useful reference for fire and smoke curtain strategy.
• —Specify fire doors and curtains tested to EN 1634-1 / EN 12101-1 and classified to EN 13501-2 regardless of which guidance route is followed — the product evidence base is the same either way.
• —Document which route was used for which zone in the fire strategy report, so future alterations or extensions are assessed against a clear baseline.

Whichever route your project follows, BÖLDT's EN 1634-1 tested, EN 13501-2 classified fire doors and fire curtains provide the same underlying evidence base referenced by both Approved Document B and BS 9999.