The move towards mandatory second staircases in residential buildings over 18 metres is already reshaping how schemes are drawn, costed and procured in the UK. While the intent is to harden life-safety and evacuation resilience, design teams say the additional vertical circulation is forcing rethinks of cores, risers and facades. Developers and housebuilders are testing whether to absorb the extra space in larger footprints, reduce unit counts, or adjust massing to hold net-to-gross. Contractors are examining programme effects from structural changes and extra fire-protection systems, alongside retender risk where designs are revisited mid-pipeline. Planning consultants are flagging that some live applications may need amendments if the height line is crossed. And across the supply chain, everything from fire doors and pressurisation fans to stair balustrades and smoke shafts features in revised take-offs.
TL;DR
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– Additional stairs above 18m are driving bigger cores, tighter floorplates and pressure on net-to-gross, with knock‑on effects for layouts, lift strategies and facades.
– Costs are moving in both design time and construction scope, with core structure, fire protection and services rerouting among the main drivers.
– Live schemes near the threshold face planning, building control and programme risk, making early engagement and clear height verification essential.
– Teams are exploring scissor or dual-stair solutions, rebalancing unit mixes and allowing for contingencies where viability is sensitive.
Design shifts: cores, lifts and net-to-gross
/> A second stair changes the geometry of typical floors. Many residential blocks have relied on a single protected stair with a compact core; adding another stair either introduces a second protected enclosure or a dual-stair configuration within a larger core. That, in turn, reorders lifts, risers and refuse/plant rooms, and can compress apartment frontages. Architects are reporting that some schemes can hold overall massing by widening cores and subtly moving structure, while others trade a modest increase in building length or depth for less loss of saleable area. Where facades were tightly optimised, introducing another stair can shift window positions and service penetrations, prompting recladding and detailing revisions.
Lift strategy is in the frame as well. Although the staircase change does not itself mandate more lifts, proximity rules, evacuation planning and accessibility ambitions often lead to a re-check of numbers, car sizes and positions. Fire engineering approaches, including smoke ventilation, pressurisation and refuge planning, will need to be retested to maintain tenable conditions with the new arrangement. Mechanical and electrical teams also point to riser competition: a second stair can displace or split vertical services, changing fan sizes, duct routes and plant rooms. Structurally, fresh openings, additional walls and different load paths may ripple into reinforcement, core wall thicknesses and temporary works.
For housebuilders working with repetitive blocks, the second stair requirement encourages a family of floorplates rather than a single optimised module. Build-to-rent operators are weighing the operational benefits of enhanced egress and perceived resident confidence against reduced net internal area. Student and later living typologies, which often use compact cores, are re-examining corridor widths, cluster layouts and the feasibility of scissor stair options. Across the board, the earlier the second stair is baked into a concept, the cleaner the downstream coordination.
Cost, programme and procurement: where the money moves
/> Direct costs are relatively visible: extra stair flights, landings, door sets, wall linings, smoke control or pressurisation equipment, and additional fire-stopping. Indirect costs can be larger: recoordination time, planning and building control submissions, revised structural frames, altered facade packages and the effect on preliminaries from longer programmes. Where floorplates tighten, small changes in unit count or mix can move revenue assumptions, stressing viability for schemes with thin margins. Lenders and funders are asking for sensitivity runs that show the impact of core growth on net internal area and yield, which in turn influences contingency levels carried into tender.
A plausible on-the-ground scenario is a regional city residential tower at concept stage, initially optimised around a single, compact core. Once the second stair is introduced, the design team reworks the typical floor to keep daylighting and stack efficiencies, which shortens some apartment frontages and increases the core width. The facade supplier is asked to reprice adjusted module spans and movement joints, while the MEP consultant re-routes risers and re-sizes smoke control fans. The contractor flags programme risk tied to design freeze, advising that early packages for structure, stairs and facade will need later release dates. The client, seeking to preserve viability, rebalances the mix and explores modest massing adjustments within planning parameters.
Procurement responses vary. Some contractors are inserting provisional sums for staircase and smoke control scope where designs are not fixed. Others are pricing two options to give clients a costed choice between separate stairs or a dual-stair core. Supply chain capacity for compliant door sets, pressurisation systems and enclosure linings is also in view, with lead-times becoming a planning consideration on tighter programmes. Market chatter suggests tenders that had been paused for redesign are re-emerging with clearer scopes and firmer allowances.
Planning, compliance and the road ahead
/> For live projects close to the 18‑metre threshold, the detail of how height is measured and interpreted is now business-critical. Design teams are double-checking levels, datum points and top-storey definitions early, and engaging building control to avoid late surprises. Planning-wise, adding a second stair or widening a core can be a material change, even if the building envelope does not grow, so applicants are preparing to justify layout moves and any subtle massing shifts. Fire strategies are being refreshed to align evacuation planning, refuge points and firefighting access with the new arrangement, with documentation tightened for approval routes.
Devolved administrations may take different paths and timescales, so developers with cross-border portfolios are tracking regional guidance closely. For refurbishments and change-of-use schemes, applicability and proportionality tests will be decisive, and most teams are seeking early, written advice. In the meantime, professional indemnity considerations are leading many consultants to adopt the two-stair approach as a baseline for new tall residential concepts, even ahead of full formalisation, to reduce redesign risk and align with client expectations.
# What to watch next
/> The near-term focus is on clarity, capacity and how schemes adapt, with several signals worth tracking.
– How planning authorities handle applications where a second stair changes internal layouts but not overall massing.
– Whether supply chains for compliant stair enclosures, door sets and smoke control kit can scale without pushing lead‑times materially.
– How lenders and valuers reflect net-to-gross movement in funding decisions on marginal schemes.
– The degree to which regional policies align on the 18m line and any accompanying technical interpretations.
# Caveats
/> There is still uncertainty around precise scope, timing and transitional arrangements, which can differ by scheme status and jurisdiction. The headline height threshold is clear, but how height is determined on particular sites, including sloping ground or mixed-use podiums, can change outcomes and should be confirmed early. Cost and viability impacts are highly scheme-specific, depending on floorplate geometry, facade strategy and planning constraints. Nothing here should be read as compliance advice; dutyholders should seek project-specific guidance from their fire engineer, building control body and legal advisers.
The direction of travel is towards earlier integration of dual-stair strategies in tall residential design and more conservative allowances in budgets and programmes. The key question is how quickly the market can normalise layouts and procurement around the second stair while keeping schemes fundable and deliverable.
FAQ
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What does “over 18 metres” mean in practice?
Height is determined by building regulations definitions rather than rough storey counts, and the measurement method can affect whether a scheme is caught. The datum and which floor is considered the top storey are important details. Teams should confirm the approach with their fire engineer and building control body at concept stage to avoid late redesigns.
# Which building types are most likely to be affected?
/> New multi-residential buildings that exceed the height line are the primary focus. Mixed-use schemes with residential elements above that level may also be in scope depending on configuration. Non-residential tall buildings are subject to different rules, but some clients may adopt similar design principles as good practice.
# Do projects already in planning have to change?
/> Many clients and authorities are encouraging alignment with the two-stair approach on schemes near or above the threshold, but treatment can vary. Transitional arrangements and case-by-case judgments are common, so early dialogue with planning and building control is advisable. Where internal layouts change materially, an amendment or resubmission may be needed.
# How much extra cost should be allowed?
/> There is no single figure that fits all projects. Cost drivers include the added stair structure and enclosures, smoke control measures, revised facades, reworked services and extended design time. Sensitivity testing and contingencies are being used to manage uncertainty while designs are refined.
# What design choices are teams considering to accommodate a second stair?
/> Options include two separate protected stairs or a dual-stair configuration within a larger core, each with different space and coordination implications. Designers are revisiting lift positions, riser locations and apartment frontages to preserve daylighting and efficiency. The right solution depends on the floorplate, structural grid, facade system and planning constraints.
