Tom Kimber

Treading carefully – is the mandating of second staircases in new tall buildings on the horizon?

As part of an ongoing update to elements of Approved Document B, the Department for Levelling Up, Housing and Communities has completed a period of consultation, with the expected conclusions to recommend mandating second staircases in new residential buildings over 30 metres tall.

Such a ‘step change’ in the approach to designing tall buildings will have a significant impact on the plan and layouts, with the stair footprint and interactions requiring accommodating and variation from the established approach; with a single, central stair.

It will affect the internal Net to Gross ratio’s that determine a projects’ viability, including accounting for the area at ground floor, along with the additional emergency egress points to engage with the site and its context. There will likely be implications for the façade treatment and loss of high value perimeter, with potential further effect on other functions within the building, such as services runs, ancillary areas, etc.

Depending on the design, there are times when a second stair can be introduced within the existing stair volume, as an interlocking, or scissor stair. Unfortunately, while this increases escape capacity when appropriate, it is ruled out as a residential solution due to the open void and the need to ensure that the building is easily navigable by both the fire service and residents i.e. at least one staircase must be kept clear of smoke, if the other is overwhelmed.

The implications of the new regulations will require design teams to seek guidance from fire consultants earlier in the project programme, during the initial project work stages, to ensure that layouts are compliant.

Whilst not yet confirmed, it is highly anticipated that any transition period within the building regulations will be very short. Therefore, factoring in a second staircase at the very earliest stages of a project is important to limit the potential impacts on viability, site constraints, specifications and aesthetics.

In fact, since February, the Greater London Authority (GLA) is no longer accepting planning applications for residential towers over 30m, which rely on a single staircase means of escape, irrespective of Building Regulations requirements. The inference is that all new residential towers over 30m should be designed to accommodate two independent stair cores. However, without clear regulatory guidance, compliance is left to interpretation.

Unsurprisingly, we are seeing this hold up developments in London where sites are tight and two stairs are not spatially possible (without compromising viability at least). In anticipation of these changes, some clients with recently approved schemes with a single stair core, are looking to redesign layouts to incorporate a second –concerned that it may not be able to deliver the current scheme.

This move by the GLA reflects the general heightened awareness and concern for safety, for obvious reasons and by acting now, lives may be saved in future, so who can blame them? However, the GLA in acting unilaterally and in advance of the expected revisions to Building Regulations, has also left designers and developers in a quandary. They have also not taken the London Fire Brigade’s advice to implement the two-stair strategy in buildings over 18m – instead making the distinction at 30m.

Space planning and rationalisation of layouts is a key element of design development and at BoonBrown we already adopt a policy of reviewing layouts for optimisation, from the concept design stage, as part of our internal technical audit procedures.

In addition, as required by the ‘Gateways’ for high rise residential developments, introduced in 2021, the inclusion and interaction with specialist fire consultants during the feasibility, concept and planning stages of tall building is ever more important.

As a practice, with studios in London & the South West, we are already taking action in line with the above requirements, regardless of the likely formal rule changes. We will be reviewing all of our tall projects to ensure a robust and viable future.

You will be aware there are significant updates to Parts L (Thermal) and F (Ventilation), and two new documents, Parts O (Overheating) & S (Electric vehicle charging) of the building regulations that are coming into force on the 15 June 2022.

These are the most significant changes to the regulations in recent years.

Without going into the technical details of each change, Parts L & F have been revised and Part O introduced as an interim step to assist the Governments desire to get to Net Carbon Zero. Part S sets out parameters for electric vehicle charging.

For past changes to the regulations, there was an obligation to secure the building regulations application through the interim period by making a start on site.  Now however this has been increased to a ‘significant start’ on site.  Where previously the start on site could have been a very light touch, now the significant start is requiring that all the footings for the development are to be poured, or the site drainage for the scheme is to be installed.

We have been seeking clarity from local authority building control, private assessors and the NHBC and the uniform response is that if you have a site of 1 or 100 units, the footings or drainage of all the site will need to be installed by 15 June 2023 if you have a building regulation application running before this year’s deadline.

This would include all phases of a large, phased site, where previously if regulations changed then the plots not yet started would still be built under the older regulation set.

As a practice we are experiencing a desire from clients to submit building regulation applications prior to 15 June 2022 to ensure that the current regulations can be built to, giving the year’s grace to get the footings for the site poured.

The new regulation changes will now introduce a higher level of thermal performance and airtightness to the building fabric, improving the ventilation strategy and assessing and limiting the overheating (solar gains) within a building.

For example, in Approved Document L domestic external wall U values have been tightened to 0.18 W/m²/K, ground floors to 1.3 W/m²/K and roofs to 1.1 W/m²/K.

Within Approved Document F the air permeability for new build residential has been reduced to 5 from 10m³/m²h. Whole building ventilation rates have also increased.

These changes will not be overly difficult to achieve in themselves. As a practice we have been working with very similar U values over recent years on a wide range of projects where planning permissions have already required a betterment of the existing regulations, and in our experience with air tightness, with good site control most of our projects have achieved below 5m³/m²h in their air tests.

Approved document ‘O’ is principally directed at new build residential units, although does include live/work premises and is the driver for looking at the solar gains and improving the removing the excess heat within the home.

The known impact of this is that the likely input of a thermal modelling specialist at a far earlier point of the project. Currently, it is likely that the earliest point that many consultants are employed is once planning has been approved and a project is moving into the working drawing package stage.

The implications of the new regulations will require design teams to seek guidance from thermal modelling at a far earlier project stage.

If this isn’t actioned, then there is a risk that once planning is permitted and the next stage of the design development progresses, there will undoubtedly be the requirement to go back to planning to adjust the designs, adding in time and cost to the client.

Approved document S seems to regularise the addition of electric vehicle charging points on sites. It is the first approved document to my knowledge to introduce a financial cap as a defined figure. This does not appear to be related to an index linked increase, so time will tell how this document will be affected by the cost-of-living increases that the country is currently experiencing.