The second staircase rule: what changes on 30 September 2026

What changes on 30 September 2026?

From 30 September 2026, new residential buildings in England above 18m must be designed with a second staircase. The date has been fixed on the industry's calendar since the government confirmed the mandate and the transition window, and it lands three months after the guidance in Approved Document B and the design codes that support it, including BS 9991:2024, had already made twin-stair design the default expectation for tall residential schemes.

Transitional provisions protect schemes that are sufficiently progressed under the earlier rules. For everything else over the threshold, the second stair is now a fact of the massing, the core, the net-to-gross and the viability appraisal, from the first sketch.

Who is caught, and who is transitioned?

The mandate applies to new residential buildings above 18m in England. The threshold deliberately rhymes with the higher-risk building definition, so a scheme crossing it usually collects both consequences at once: two staircases in the design, and the full Gateway 2 regime in the delivery.

Transition matters enormously and turns on how far a scheme had progressed before the deadline. Projects that were sufficiently advanced can complete under the single-stair basis they were designed to; the exact cut-off conditions are set out in the transitional provisions and deserve careful legal reading on any marginal scheme, because getting it wrong means a core redesign at the worst possible moment.

What does a second staircase do to a tall residential scheme?

Three things, and every developer pro forma in the country has now priced them. It consumes floor area on every level, which stresses net-to-gross efficiency precisely where tall residential is most sensitive. It adds cost, both directly and through the structural and services coordination that follows a second core. And it removes a design freedom that single-stair buildings relied on, which is why the transition produced two visible market behaviours.

The first is threshold avoidance: schemes redesigned to sit just below 18m, avoiding the stair and the gateway regime together. It is legal, it is rational, and it deserves the scrutiny it gets, because a building optimised against a threshold is not necessarily a building optimised for the people in it. The second is mid-process adaptation: schemes already in the system adding the stair during design development or even during Gateway 2 determination. Vivere Residences in Manchester, tracked on our Gateway Projects Tracker, added a second staircase to its hotel element during the approval process and still went from June 2026 approval to a July start on site.

How does this interact with Gateway 2?

Directly. A scheme above 18m submitting for building control approval after the mandate must demonstrate the two-stair design, and the fire strategy, evacuation approach and smoke control all flow from it. The BSR expects a fully resolved, coordinated design at submission, and the staircase strategy sits at the centre of that resolution: it is exactly the kind of safety-critical, structure-shaping decision that cannot be left to "develop during construction".

Expect the autumn 2026 submissions spike to test this. The Building Safety Levy goes live on 1 October 2026, one day after the staircase mandate, and the industry expects a rush of Gateway 2 applications lodged ahead of both. Applications assembled in a rush, on designs finalised against a moving threshold, are precisely the applications that fail validation or determination. The schemes that clear cleanly will be the ones that treated 30 September as a design deadline, not a submission deadline.