The 18 m second staircase requirement, explained

The rule

New blocks of flats with a top storey 18 m or more in height are required to be served by more than one common stair. That is paragraph 3.30b of Approved Document B Volume 1, added by the 2026 amendment. The threshold was confirmed at 18 m after a consultation that had originally proposed 30 m, and it aligns the staircase trigger with the height threshold already used for the higher-risk building definition and the combustible materials ban of regulation 7(2). It does not align with the sprinkler requirement, which paragraph 7.4 sets at a top storey more than 11 m above ground level, so a block between 11 m and 18 m needs sprinklers but not a second stair.

How the height is measured

The relevant height is measured from ground level on the lowest side of the building to the floor surface of the top storey, ignoring any top storey that consists exclusively of plant rooms or machinery. This is the same measurement used for the higher-risk building test, which is deliberate: it means one number governs several thresholds rather than several near-miss numbers.

Two practical consequences follow. First, a roof terrace or plant deck above the top habitable storey does not push you over. Second, a sloping site measured from the wrong side can move a building across the threshold, so state which datum you used and show it on the section.

Transitional arrangements

The transitional provisions are the part that actually decides most live projects. The 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 building work either:

1. had started and was sufficiently progressed before that day; or
2. is started and sufficiently progressed within the period of 18 months beginning on that day.

"Sufficiently progressed" is not a formality. For a new building it means the pouring of concrete for the permanent placement of the trench, pad or raft foundations has started, or for piled foundations that the piling has started. For work to an existing building it is when the work itself has started; for a material change of use, when the work to effect the change has started. Not mobilisation, not enabling works, not a signed contract.

Why a second stair, and why 18 m

The single-stair block has been accepted in England for decades on the basis of a stay-put strategy: robust compartmentation means a fire is contained in the flat of origin, only that flat evacuates, and the single stair is therefore never carrying a full building evacuation. That logic depends completely on the compartmentation performing as assumed.

Where it does not (because of a defective external wall, a construction defect, or a fire that spreads outside the compartment), the strategy has to change to evacuation, and the single stair has to serve both escaping residents and attacking firefighters at the same time. That is the failure mode the requirement addresses, and 18 m is where the consequences of it become severe enough, and the firefighting operation slow enough, that redundancy is judged necessary.

What a second stair actually means for the design

Adding a stair is not just adding a stair. Expect the following to move:

• Plan efficiency. Two cores in a tower of modest floorplate can cost several percent of net saleable area, which is why the threshold was contested.
• Escape route geometry. Two stairs allow escape in two directions from the common corridor, which changes travel distance limits and often removes the need for a long single-direction travel justification.
• Smoke control. The corridor smoke control design changes when there are two protected routes to protect rather than one. It is rarely a simple duplication.
• Firefighting provisions. Which stair is the firefighting shaft, whether both need to be, and how the firefighting lift and riser are arranged.
• Evacuation strategy. A second stair does not automatically mean simultaneous evacuation, but it makes a phased or full evacuation option viable where it previously was not.

Existing buildings

The requirement applies to new buildings. It does not require a second stair to be added to an existing single-stair block. Existing tall single-stair buildings remain lawful, and the response to them runs through fire risk assessment, the safety case regime for higher-risk buildings, and remediation of any defects that undermine the stay-put strategy, not through retrofitting a core.

That said, where an existing single-stair building is being materially altered or extended, the position needs thinking about rather than assuming, and where a stay-put strategy has been suspended because of a defect, the single stair is doing a job it was never designed for. Interim measures follow from that, not from the staircase count.

Scotland, Wales and Northern Ireland

Building standards are devolved, and the 18 m staircase requirement described here is an England provision made through Approved Document B and the supporting regulations. Scotland works to the Technical Handbooks and has its own long-standing provisions on escape from tall residential buildings; Wales works to its own Approved Documents; Northern Ireland to its Technical Booklets. Check the relevant national document rather than assuming transfer.

Last reviewed 25 July 2026 against the editions named above. Standards are revised; check the current published edition before relying on anything here in a design.