Sprinklers, stairs and the residential height thresholds

The sprinkler requirement residential height threshold in England sits at 11 m to the floor of the top storey for blocks of flats, and the second staircase threshold sits at 18 m. Both are set in Approved Document B rather than in planning policy, and both have moved in the last few years. They matter to planning because crossing either one changes the core, the plan depth, the number of units and often the elevations, and a design that sits 300 mm above a threshold is a design that gets redrawn.

Measure early. The cost of finding out at technical design stage is a planning amendment on top of everything else.

Height thresholds in the fire guidance

Approved Document B is guidance supporting the Building Regulations rather than a regulation in its own right, so compliance can be demonstrated another way. In practice, for residential buildings, the thresholds in it are treated as requirements by building control bodies and by funders.

Verify every figure against the current published edition of Approved Document B before you rely on it. These thresholds have changed more than once since 2018 and at least one has been the subject of consultation on a further change.

Where suppression becomes a requirement

The sprinkler threshold for blocks of flats was reduced to 11 m in 2020, from a much higher figure. For most schemes that means any block of four storeys or more will need suppression, because 11 m to the top floor surface is reached quickly once floor-to-floor heights and a commercial or undercroft ground floor are taken into account.

The design consequences run further than the pipework. A sprinkler installation needs a water supply with adequate flow and pressure, which often means a tank and pump set, and that plant needs a room with access, drainage and an acoustic separation from habitable rooms. On a tight urban site that room competes with the cycle store, the substation and the bin store for the same ground floor frontage.

Two knock-on effects worth pricing at design stage. A tank and pump room at ground level consumes lettable or saleable area, which affects the unit count and the viability position. And the water undertaker's confirmation of available flow can take weeks, so an incoming main upgrade discovered late is both a cost and a programme item.

For houses, the threshold is lower in a different sense: a dwellinghouse with a floor more than 7.5 m above ground level is expected to have a sprinkler system, which catches four storey townhouses and some three storey houses on sloping sites.

The second staircase position and transitional dates

A requirement for a second staircase in new residential buildings with a top storey above 18 m was introduced through an amendment to Approved Document B following consultation, after an initial proposal at a higher threshold was revised downwards. Transitional arrangements applied to schemes already in progress, defined by reference to a date and to the progress made by that date.

Those transitional arrangements were time-limited, and the window for relying on them has been closing. Check the current edition of Approved Document B and its transitional provisions on gov.uk before you assume a scheme qualifies, because the position depends on dates and on what counts as sufficient progress, and that is not something to take from a secondary source.

What the requirement does to a design is substantial. A second core on a residential building of this height typically consumes a meaningful percentage of each floor plate, affects the efficiency ratio, and may change the number of units per core and therefore the corridor arrangement. On a single-stair tower designed before the change, retrofitting a second stair is often not possible within the existing envelope, and the scheme is redesigned rather than amended.

The consequences for an existing permission: a changed core is unlikely to be a non-material amendment, so expect a section 73 application or a fresh application, with the fire statement updated and the Building Safety Regulator reconsulted.

Measuring height for these purposes

Height is measured to the upper surface of the floor of the topmost storey, not to the ridge, the parapet or the top of the lift overrun. Storeys consisting exclusively of plant are generally excluded, as are rooftop plant areas.

The measurement is taken from ground level, and on a sloping site Approved Document B takes ground level on the lowest side of the building. That is the detail that catches schemes on falling ground, where the same building measures under the threshold from the high side and over it from the low.

Three practical consequences:

1. A level change during design development, or a revised site strategy that drops the ground around one elevation, can push a compliant building over a threshold without anyone changing a floor level.
2. Mezzanines and double-height spaces need care in deciding what counts as a storey.
3. Basements are treated differently from storeys above ground, so a scheme with a basement car park is not automatically closer to a threshold.

Record the measurement on the drawings, with the datum, the ground level used and the top floor surface level dimensioned. Building control will check it and so will the fire engineer, and an undocumented assumption is the thing that unravels at Gateway Two.

Planning consequences of a core change

This is where a Building Regulations threshold becomes a planning problem.

Adding a core changes the plan form, usually the elevations, and often the roof, because a second stair needs its own smoke ventilation and its own head. It changes the unit mix, because the lost area comes out of the smallest units first, and that can change the affordable housing quantum and the policy compliance position on mix. It changes the daylight and sunlight results for the scheme and for neighbours, because the massing has moved. And it changes the Fire Statement, which has to be reissued, with the author's competence declaration current for the revised scheme. That statement requires an author with the necessary fire engineering competence under the Building Safety Act framework and Article 9A of the procedure order, and a drafted document is a starting point for that professional rather than a substitute.

The full engineered strategy follows at the next stage. A Fire Safety Strategy to the relevant British Standard is a fire engineering document and needs the same competence applied in more depth.

Where the scheme is phased, moving a threshold in one block can have knock-on effects on the delivery programme and on obligation triggers, and how authorities treat phase boundaries is covered in the note on phasing a large scheme without gaming thresholds. The design rationale for the core arrangement belongs in the Design and Access Statement, because a reviewer who cannot see why the plan has two stairs will ask about efficiency instead.

Checking the current Approved Document

A short discipline that avoids most of the trouble in this area.

Download the current Approved Document B Volume 1 from gov.uk at the start of each project rather than reusing a saved copy. Note the publication date and any amendment date on the cover, and record both in the project file. Read the transitional provisions, which sit at the front and are frequently skipped. Then write the applicable thresholds into the design brief as hard constraints with the source edition cited.

Where a scheme straddles a transitional date, take advice rather than forming a view from a trade article. The conditions attached to transitional relief usually turn on both a date and a state of progress, and both have to be satisfied.

One honest uncertainty: the direction of travel on the suppression threshold and on external wall materials has been towards lower heights, and further change has been consulted on. Designing to the current minimum on a scheme that will not reach site for two years carries risk.

Designing with headroom above the threshold

The practical options, in order of how often they work.

Stay clearly below the threshold. Not 150 mm below, because a level change will eat that. Half a metre of headroom on the top floor surface level is a reasonable margin on a flat site and more on a slope.

Accept the threshold and design for it from the start. A scheme planned around suppression and two stairs from day one is more efficient than one that has them inserted, because the plan form, the riser positions and the core arrangement are designed together.

Or redistribute height across the site. On a scheme with several blocks, keeping one block below the threshold and taking the height elsewhere can be better than pushing every block just over it. That is a masterplanning decision, not a Building Regulations one, which is why it has to happen before the planning application rather than after.

What rarely works is value engineering a scheme downwards late in the day to drop under a threshold. By the time the floor-to-floor heights are reduced to achieve it, the ceiling heights, the services zones and the quality of the units have all suffered, and the saving is smaller than it looked.

This is general guidance, not legal advice; check the position with your building control body, your fire engineer or another qualified professional before relying on it. To see which documents a residential scheme of this height is likely to need, enter the postcode on PlanScope.

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