Second staircases above 18 metres and what changed

New residential buildings in England above 18 metres are expected to have two staircases. The threshold came down from the 30 metres originally consulted on, and the change was made through statutory guidance in Approved Document B rather than through planning policy, which is why it surprised so many schemes that already had planning permission. Verify the current text before designing to any figure, because the guidance has been amended more than once and transitional arrangements have their own end dates.

For a developer the planning consequence is simple to state and hard to absorb. A consented single stair tower above the threshold cannot be built as consented, and the redesign changes the thing the permission approved.

The requirement and the buildings it affects

The requirement applies to new residential buildings, measured by the height used in the building safety regime, which is taken to the floor level of the top storey rather than to the roof or parapet. That measurement point catches buildings that look shorter than 18 metres on an elevation and misses some that look taller, and arguments about mezzanines, plant levels and the definition of a storey are common.

It sits alongside, but is separate from, the higher-risk building definition in the Building Safety Act 2022 regime, which uses 18 metres or seven storeys with at least two residential units. The two thresholds overlap in most cases without being identical, so check both.

Existing buildings are treated differently from new ones, and the guidance deals with material alterations separately. A scheme adding storeys to an existing block needs specific advice rather than an assumption either way.

Why the threshold moved

The consultation that followed the Grenfell Tower Inquiry proposed a second staircase above 30 metres. Fire and rescue services, the fire engineering profession and several London bodies argued for a lower figure, on the basis that evacuation and firefighting access become difficult well below 30 metres, and the government confirmed 18 metres in 2023. The amendment to Approved Document B followed, with a transitional window for schemes already in progress.

That sequence matters when advising a client. The requirement arrived through building regulations guidance, so it binds at building control rather than at planning, and a planning permission granted before the change is not protected from it.

Effect on core layout and unit count

A second stair costs floor area on every storey, and the loss compounds. The core grows, the lobby arrangement changes, and the ratio of sellable area to gross internal area falls. On a small footprint tower the effect can be severe enough to make the scheme unviable, which is why several consented point blocks were redesigned as wider, lower buildings.

The design responses used in practice are these:

• Increasing the footprint so the core is a smaller proportion of each floor plate
• Reducing height below the threshold and recovering units elsewhere on the site
• Reconfiguring from a point block to a linear block with two cores
• Accepting fewer, larger units on a floor plate that no longer supports the original mix
• Moving to a scissor stair arrangement where the geometry allows it

Each of those changes the elevations, the massing and often the affordable housing offer, so each has a planning consequence as well as a technical one.

Transitional arrangements and how they worked

The amendment carried transitional provisions allowing schemes that had reached a defined stage before the cut-off date to proceed under the previous guidance, provided work then started within a set period. The detail of what counted as reaching that stage, and how much progress was needed, is in the guidance itself and has been the subject of considerable debate.

Those windows were designed to close. By late 2026 the practical position for most schemes is that the requirement applies, and a developer relying on transitional protection needs documentary evidence of the qualifying step and of the work done since. Check the current text rather than a note written when the change was announced. Reform news of this kind is tracked on the planning law timeline, and the primary source is the published guidance itself.

Schemes consented before the change

A planning permission approves drawings. If the building those drawings show cannot lawfully be built, the permission has limited value, and the route back depends on how much the design has to move.

Minor changes may be capable of a non-material amendment under section 96A, which suits a small adjustment to a core that does not change the external appearance. Changes that alter the elevations, the footprint or the unit numbers usually need a section 73 application to vary the approved plans condition, and where the scheme changes in substance a fresh application is the honest route.

The test that matters for section 73 is whether the variation would amount to a fundamental change to the development permitted. Adding a second stair inside an unchanged envelope is usually fine. Widening the building by three metres and losing eight flats is usually not.

Re-planning an existing permission

Sequence the work so the fire engineering leads the planning submission rather than following it. The order that works is: confirm the height measurement and whether the requirement applies, develop the stair strategy with the fire engineer, test it against the planning parameters that are fixed, then choose the planning route.

Update the supporting documents together. The fire statement submitted at Planning Gateway One describes the escape strategy in planning terms, so a change of stair arrangement makes the original statement out of date. The developing fire safety strategy carries the detail through to the later gateways, and the two should tell the same story. Where affordable housing or viability was agreed on the original unit mix, expect the section 106 to need varying as well.

Consultation is worth doing properly. A revised scheme that is taller in the middle or wider at the edges will draw objections from people who accepted the first version, and an explanation that the change is driven by fire safety guidance is one officers and objectors both understand.

Checking the current guidance text

Three habits keep advice safe on this topic. Read the published Approved Document rather than a summary of it, because summaries written at announcement are now out of date. Record the version and date you relied on in the design record, since that record feeds the golden thread for a higher-risk building. And ask the fire engineer to confirm the height measurement in writing, because the difference between 17.8 and 18.2 metres is the whole question.

One honest uncertainty. Guidance in this area has changed several times since 2023 and further amendment is plausible, so a scheme with a long programme should be designed with some tolerance rather than to the current minimum.

PlanScope drafts the planning narrative documents and flags where facts are missing. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Fire statements and fire safety strategies require someone with the relevant fire engineering competence, and an AI draft is a starting point for that professional rather than a substitute.

This is general guidance, not legal advice. Check the current Approved Document B text and the position with the Building Safety Regulator and your local planning authority before relying on it. Enter your postcode on PlanScope to see which documents your scheme is likely to need.