Chapter 10: Building Safety
11 is the new 18

Why the 18m threshold under the
Building Safety Act risks being reduced to 11m

While 18m has long been the critical threshold for building safety regulations, developers are increasingly treating 11m as the new red line. Lanpro’s Regional Director, Diane Ellis, examines how mounting regulatory pressures are squeezing mid-rise development out of the market – with profound implications for housing density, urban design and the UK's ability to meet its housing targets.

18m (or seven storeys) is the height at which a residential-led scheme can fall into the higher-risk building category, bringing a high level of scrutiny and legal responsibilities under the Building Safety Act (BSA). The 18m threshold is widely known as the tipping point for more stringent regulations across the development sector.

And yet it’s 11m, rather than 18, which is now becoming the relevant threshold in the minds of many developers. In law, a separate cluster of rules and operational expectations for these ‘relevant buildings’ that sit in the middle ground between mid-rise and high-rise, has made 11m the cut-off point for many schemes.

Whilst the regulations for these relevant buildings are not as stringent as those for higher-risk buildings, many developers are seeking to avoid the increased regulation, cost and risk brought by legislation of developing these mid-rise buildings altogether, by developing buildings below the 11m threshold.

Why 11m matters

For multi-occupied residential buildings over 11m, the Fire Safety (England) Regulations 2022 impose additional ongoing duties, including fire door checks.

Furthermore, in Approved Document B, the Government has moved key life safety provisions down to this height – most notably lowering the trigger for sprinklers in new blocks of flats from 30m to 11m, banning the use of combustible materials in the external walls, and adding wayfinding signage expectations for the fire service above 11m. And more changes are coming through the Fire Safety (Residential Evacuation Plans) (England) Regulations 2025, for example, which will impose ever tighter restrictions on buildings over 11m.

Conversations about the 11m limit often refer to fire engine’s ladder length. The reality is more nuanced than a single piece of equipment, but the underlying point is that operational assumptions do change at 11m, as well as at 18m.

Wary of falling foul of building safety regulations and concerned that the regulations that apply at 18m could, in future, be applied at 11m, many developers are now treating 11m as a risk line, even when not strictly necessary.

The viability issue

Below 18m, a scheme can usually retain some flexibility in design development and programme while working through conventional building control routes. Above 18m (and within the higher-risk definition), a three-stage ‘gateway’ process is required at planning stage, pre-construction and pre-occupation. Gateway 2 requires that the developer secures Building Safety Regulator building control approval before construction begins and then manages design change through a more formal process. Delays in the system and the need to front-load all design work introduce uncertainty, longer lead-in times, increased costs and changes in cash-flow. This is exacerbated by the need for buildings over 18m to provide a second staircase and lift. The result is that residential developers will chose either to build just below 18m (to avoid the impacts of the BSA) or considerably above, at a point at which the additional costs and delays can be justified through the financial benefits of building tall.

Over the long term, we may well see skylines dominated by buildings of over 30m or under 18m, but very little to scale between the two, as would be expected in a traditional skyline. The absence of medium-height buildings would be greater still if 11m became the threshold.

Additionally, there is a growing disconnect between the statutory 12-week determination period for Gateway 2 new-build applications and the reality on the ground. The commercial impact of delay has been

immediate. NHBC registrations in London fell sharply in Q2 2025, with developers citing the building safety regime, specifically Gateway 2, as the cause of the delays. Again, these figures would be higher still if 11m were to become the threshold.

Skylines and density

Here in the north-west, city skylines are changing rapidly, particularly in Manchester, but there is evidence of a growing gap between schemes that stay comfortably below key thresholds and schemes that go high enough to carry the additional regulatory overhead. I have heard about a scheme that had been consulted on at 19 storeys but is being revised to six storeys with Gateway 2 delays cited as the reason.

Masterplans and local plans, following good urban design principles, assume a certain distribution of heights, for example taller markers on corners, mid-rise blocks along key routes, density stepping down to existing neighbourhoods. If mid-rise is squeezed out, we lose height but also the geometry that makes many urban extensions, regeneration areas and transport-led schemes work.

There is a wider strategic problem too. The NPPF says plans and decisions should make efficient use of land by achieving appropriate densities, balancing this against local context, design quality, infrastructure capacity and viability. Where there is (or will be) a shortage of land to meet housing needs, it says it is especially important

to avoid low densities and ensure sites make optimal use of their potential, including through minimum density standards in town and city centres and other locations well served by public transport, seeking a significant uplift in average density unless there are strong reasons why that would be inappropriate.

Centre for Cities has recently made the same point in national economic terms: Flat Britain: The urban density gap and how to close it estimates that if British cities matched the density of comparable French or Japanese cities, their urban cores could accommodate at least 2.3 million additional homes. It also suggests that if London’s Zones 1 to 3 were as dense as Paris, the capital alone would have around 500,000 more homes. This extends beyond London: the report argues the largest shortfall sits in the UK’s major regional cities, with 12 big cities outside London accounting for over half of the total gap.

If the market responds to uncertainty by spreading buildings out and flattening them, pressure does not disappear. It relocates – onto land take, infrastructure and the political friction around the Green Belt.

Design quality is being squeezed

Another consequence of gradually lowering building heights is the impact on design, streetscape and amenity. For the scheme to remain viable, developers and designers look for efficiencies. The price of land generally remains the same whether a low-rise or high-rise building is developed on it. However, a low-rise scheme would mean a lower return, resulting in the need to make cost savings elsewhere.

What needs to change

I do not think anyone in the sector begrudges the principle of building safety, but there is clearly a need for delays and viability challenges to be addressed.

Uncertainty is one of the greatest threats to viability and results both from delays in decision-making and confusion from ever-changing regulations for residential buildings over 11m and 18m. At present, building below 11m, rather than the wider-known threshold of 18m, significantly simplifies the process and increases certainty, building above this adds uncertainty and considerable costs, both to the planning and development sector and the residents desperately in need of a home.

www.lanpro.co.uk