Second staircases become mandatory on 30 September. The cost is not the staircase

It is the net to gross. A second core takes saleable area out of every floor of the building, and on a scheme already consented that loss has nowhere to go.

From 30 September 2026, new blocks of flats in England over 18 metres must have a second staircase. The requirement has been coming for some time, and most developers with tall buildings in the pipeline know about it. What is less well understood is the transitional test, and what the change actually does to a viability appraisal.

The transitional test has two limbs

A scheme escapes the requirement if a building notice, an initial notice, or a building control approval application with full plans was given to the relevant authority before 30 September, and the work is sufficiently progressed either before that date or within 18 months after it.

Sufficiently progressed has a specific meaning. It is the start of foundation work, which means pouring concrete for permanent placement or piling, or for an existing building the start of the works or the material change of use. It is not site set-up, and it is not a ceremonial dig.

So a scheme with full plans in and no realistic prospect of foundations inside 18 months is not protected, whatever the application date says.

Why it costs more than the staircase

Pricing the second stair core as a construction item is straightforward and it is not the expensive part. The cost is the area it takes.

• Net to gross. A second core, its lobby and its protected route come off the saleable area on every residential floor. On a tall, sender building that ratio is already tight, and this is the item that pushes it.

• Unit count. Where the loss is enough to remove a unit per floor, the effect on revenue runs through the whole appraisal, not just the build cost.

• Redesign. Substantial enough on most schemes to need the design team back, and often enough to require a new planning application, or at least a section 73 amendment.

• Programme. All of the above takes months, and the transitional clock is running.

The delay effect is not theoretical. The requirement has been associated with holding up delivery of at least 38,000 homes in London alone.

Every scheme we have priced over 18 metres has had two staircases since 2019. The regulation is catching up with what competent design was already doing. So if your scheme has a single core and is over 18 metres, the question is not what the second staircase costs to build. It is what the area it takes does to the unit count, and what that does to the revenue line.

What we do with it

On any tall residential scheme we are asked to appraise now, the first question is which side of 30 September it sits, and the second is whether the foundation test can honestly be met inside 18 months. Only then is there a build cost to talk about.

Where a scheme is protected but marginal, we price it both ways. An appraisal that assumes protection on a programme nobody believes is not prudent, it is optimistic with a cost plan attached.

What this means for your scheme

• Over 18 metres and residential: check the foundation date, not just the application date.

• Sufficiently progressed means foundations started, either before 30 September or within 18 months of it.

• Price the area loss, not the staircase. The net to gross is where the money is.

• If the scheme is caught, assume the design team is going back to the drawing board and programme for it.

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Sources. The Building Regulations amendment introducing the second staircase requirement, in force 30 September 2026, and accompanying guidance. Legal commentary summarised and linked, not reproduced.