Second staircases in blocks of flats: the 18m rule in Approved Document B (2026)

The new paragraph 3.30 says flats should be served by more than one common stair in two cases. First, where a storey doesn't meet the rules for a single escape route, or for a small single stair building. That was already the position.

Second, and this is the change, where the building has a top storey 18 metres or more in height. Below 18 metres, a single common stair is still acceptable, provided every flat is separated from it by a protected lobby or corridor, and the travel distance in one direction is no more than 7.5 metres. For small buildings with a top storey up to 11 metres, and no more than three storeys above the ground storey, Diagram 3.9 allows a simpler single stair arrangement.

So how is 18 metres measured? Diagram D6 in Appendix D measures the height of the top storey from the upper surface of its floor down to ground level on the lowest side of the building. It's the floor of the top storey that counts, not the roof.

And rooftop plant areas, and any top storey that's only plant rooms, are left out. On a sloping site, the lowest side of the building sets the ground level, so a building can cross the threshold on one side even if it doesn't on the other.

The amendment took effect on 30 September 2026. The previous guidance continues to apply where a building notice, an initial notice, or a full plans application was given to the relevant authority before that date, and the work was sufficiently progressed either before that date, or within 18 months beginning on it, which takes you to the end of March 2028. For a new building, sufficiently progressed means the concrete for the foundations has started to be poured, or permanent piling has started.

For work to an existing building, it means the work has started.

With two stairs, the common corridor is designed so that a fire in one flat can't cut people off from both of them. Diagram 3.8 shows the arrangements. The corridor between the two storey exits can be up to 30 metres long.

A flat in a dead end can be up to 7.5 metres from the point where escape is possible in two directions. And the corridor is divided by self-closing fire doorsets, positioned so smoke can't reach both stairs. If the maximum travel distance is no more than 15 metres, the central door can be left out.

Two stairs only count if they're genuinely alternatives. Paragraph 3.31 says interlocked stairs, two stairs wound around each other in a single shaft, count as a single escape route. And an escape route should never pass through one stair enclosure to reach the other.

It may pass through a stair's protected lobby, but a lobby should not give direct access to more than one stair. In a building with a storey 18 metres or more in height, each common stair should be at least 1100 millimetres wide between the walls or balustrades, and the smoke control that protects the stairs must be designed with both in mind.

The other half of the 2026 amendment supports evacuation lifts. Paragraph 3.29 says that where an evacuation lift is provided, it should sit inside an evacuation shaft, which contains three things: a protected stairway, the evacuation lift itself, and an evacuation lift lobby. The lobby doubles as a refuge, a protected place to wait for the lift.

It must open directly onto the protected stairway, and must not be entered directly from any flat, maisonette, storage room or electrical equipment room. The evacuation shaft gets the same fire protection as the stairway, and any smoke control for the stair must protect the lift and its lobby too.