Second staircase requirement checker (18 m rule)
The 2026 amendment to Approved Document B Volume 1 adds a recommendation that blocks of flats with a top storey 18 m or more in height are served by more than one common stair. It is paragraph 3.30b, and it takes effect on 30 September 2026.
Note the wording: 18 m or more, not "above 18 m". A building whose top storey sits at exactly 18.0 m is in scope.
The 2019 edition continues to apply where a building notice, initial notice or building control approval application with full plans was given to the relevant authority before 30 September 2026, and the work either had started and was sufficiently progressed before that day, or is started and sufficiently progressed within the period of 18 months beginning on that day.
Second staircase required
At 21 m the top storey is 18 m or more and the building is in scope. You currently show 1 staircase. A second, independent common stair is required under paragraph 3.30b, with its own protected lobby arrangement and its own route to a final exit.
Working
| Step | Value |
|---|---|
| Is the building a block of flats?Blocks of flats (including build to rent)ADB Volume 1, paragraph 3.30b | Yes |
| Top storey above ground level21 m against a 18 m or more thresholdADB Volume 1, Diagram D6 | 18 m or more |
What this means
- At 18 m or more a block of flats is also a higher-risk building, so the Gateway 1 to 3 regime, the golden thread and the accountable person duties all apply. That is a separate test under a separate instrument: Building Act 1984 section 120D with SI 2023/275 regulation 2 for design and construction, and Building Safety Act 2022 section 65 with SI 2023/275 regulation 8 for occupation. Paragraph 3.30b is guidance on requirement B1 and derives nothing from either. The tests agree here only because this building is over 18 m.
- BS 9991:2024 is now the residential design standard and was published without a transition period. If you are still working to BS 9991:2015, expect that to be questioned.
Sources
- Approved Document B, Volume 1, paragraph 3.30b (2026 amendment)Flats should be served by more than one common stair where the building has a top storey of 18 m or more in height, measured as shown in Diagram D6 of Appendix D. In force 30 September 2026.
- Approved Document B, Volume 1: 2026 amendments, transitional provisionsThe 2019 edition continues to apply where a building notice or initial notice was given to, or a building control approval application with full plans made to, the relevant authority before 30 September 2026, and the work either had started and was sufficiently progressed before that day, or is started and sufficiently progressed within the period of 18 months beginning on that day. Approved Document B states the period, not an end date. 30 March 2028 is derived from it, not printed in the document.
- BS 9991:2024Fire safety in the design, management and use of residential buildings. Published November 2024, with corrigendum A1 in June 2025. No transition period was given.
- Building Act 1984, section 120D (inserted by the Building Safety Act 2022, section 31), with SI 2023/275 regulation 2Higher-risk building for the design and construction regime: at least 18 m in height or at least 7 storeys, and of a description specified in regulations. Cited here only because a block of flats caught by paragraph 3.30b is also caught by this, so Gateway 2 applies to the same scheme. It is not the authority for the staircase provision, and its storey limb has no counterpart in paragraph 3.30b.
This is an engineering aid, not engineering advice. The result above is produced from the inputs you gave and the guidance cited. It does not account for everything a competent fire engineer would consider on a real building, and it is not a substitute for reading the current published standard. Check every number before it goes into a design.
Questions people ask
When does the second staircase rule come into force?
30 September 2026. The 2019 edition continues to apply to schemes where a building notice, initial notice or full plans application reached the relevant authority before that date, provided the work is started and sufficiently progressed either before that day or within the period of 18 months beginning on it. Approved Document B expresses the transition as that period rather than as a calendar date, so treat 30 March 2028 as a date you have derived rather than one the document prints.
What is the second staircase height threshold?
18 m or more, measured to the floor surface of the top storey above ground level as shown in Diagram D6. The wording matters: paragraph 3.30b says "18m or more in height", not "more than 18m", so a building at exactly 18.0 m is caught. The figure is the same 18 m used for higher-risk buildings, but the two tests are not the same test and do not always agree. The higher-risk definition is "at least 18 m in height <em>or</em> at least 7 storeys"; paragraph 3.30b has no storey limb at all. A six or seven storey block that comes in under 18 m, which is common, is a higher-risk building and is not caught by 3.30b on height grounds. Paragraph 3.30a can still require a second stair on escape route grounds, and that is a separate assessment.
What counts as sufficiently progressed?
For a new building, work is treated as sufficiently progressed when the pouring of concrete for the permanent placement of the trench, pad or raft foundations has started, or, for piled foundations, when the piling has started. For work to an existing building it is when the work itself has started, and for a material change of use it is when the work to effect the change has started.
Does the second staircase rule apply to student accommodation?
Not directly. Paragraph 3.30b sits in Approved Document B Volume 1, which covers dwellings, and the 2026 amendment describes it as a recommendation for blocks of flats. Purpose-built student accommodation, HMOs, care homes and hotels are designed under Volume 2 or BS 9991:2024, which have their own provisions on single stairs. The design logic is similar and many building control bodies will expect equivalent treatment, but citing paragraph 3.30b for a hotel is citing the wrong document. Agree the basis of design in writing early.
Does an existing single-stair building need a second staircase?
No. The requirement applies to new building work. An existing building is assessed against the fire strategy it was built to, plus the Fire Safety Order duties on the responsible person. A material alteration or a change of use can bring the new provisions into play, so check before you assume.
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