Is there a trade-off between density and safety?

The list of challenges around developing high-rise and high-density homes in urban areas is long and full of complexities, from first concept to planning to construction and on to handover. The bad news is that this list is getting longer as two entirely unrelated events continue to loom over the residential property sector.

The Grenfell Tower fire and the pandemic both prompted rapid responses, the former bringing new safety priorities for high-rise residential buildings and the latter raising questions about urban lifestyles and the value of space, inside and outside the home. Grenfell has already resulted in changes to the design and delivery of residential towers, and both that and the pandemic may have continuing impacts.

The post-Grenfell regulatory landscape for tall residential buildings has seen architects move to non-combustible facade materials, with rainscreen cladding still used, typically with mineral fibre and solid metal panels. One alternative coming under consideration is curtain walling. But Simon Lay, director of fire and risk advisor OFR Consultants, urges some caution. ‘It is an area that needs to be looked at quite carefully,’ he says, ‘because the guidance around curtain walling is not as well advanced as it is around other walling systems.’

New products, particularly for waterproofing and membranes, have helped resolve key challenges around non-combustibility but, Lay believes, bigger advances in walling systems are some way off. ‘There won’t be massive leaps in technology because the testing and assurance for the non-fire aspects of some materials is a major challenge,’ he says. The return of timber – for structural systems or facade detailing – looks similarly remote. ‘We can crack the safety of timber with more research and I don’t think it’s a product we should give up on,’ he says. ‘But in the short to medium term, a combination of legislation and a nervous insurance industry are near insurmountable obstacles. Tall residential buildings are complex and if you use innovative materials or methods you exponentially increase the complexity, and that means you really can’t be designing with a great reliance on existing statutory guidance.’

That complexity extends to the single staircase. The option for a single staircase remains in recent revisions to statutory guidance and the draft revision of the British Standard, although ongoing government-backed research could change that. But at planning, while some proposals are being approved, others are encountering disapproval from planners, the London Fire Brigade and local campaign groups. 

Whether single-stair schemes progress may depend on the level of detail and analyses committed to each individual application, says Lay. ‘Anyone trying a light-touch approach to fire safety of a single-stair, tall residential building at planning stage will rightly risk rejection,’ he adds.

Adding a second staircase to a building, however, brings its own challenges, as Lay explains. ‘Once you start going to smaller sites – and these are the sort of sites that typify our congested cities – you end up with smaller footprints and staircases very close together,’ he says. ‘That creates challenges when it comes to things like smoke control. For example, there is the potential to protect one staircase while flooding the other with smoke. Unintended consequences can come from adding a second staircase that have to be really thought through. It is not a case of safety being definitively delivered.’

Already, some developers are indicating that sites could become unviable if two staircases becomes the norm. ‘It tends to be developers that have pocket city centre sites that require a more elegant form,’ says Lay. ‘You can’t do that in a commercial way and have more than one staircase, so you would see a significant dip in some areas.’ Instead, he argues, attention should focus on appropriate, protected egress capacity, taking into account the needs of those with mobility impairments, using robust and considered analyses of the escape, and firefighting challenges.

Doubling up
As debate continues, some are already changing course. Rory O’Hagan of Assael Architecture says: ‘As a practice, we are increasingly being asked to incorporate enhanced safety features into our designs, such as a second staircase on tall towers.’ He points out that many global locations, such as New York, already set more stringent requirements. ‘With over 200 consented tall buildings in the London pipeline alone, architects and developers will need to pivot to work within this new framework,’ he says.

Jo Cowen, of Jo Cowen Architects adds: ‘As a practice, we believe you should be designing every building with a dual-core strategy and shouldn’t only be relying on counter measures, such as sprinklers and pressurised systems.’ Dual cores feature in schemes like the practice’s upcoming Kingston Gate, in Kingston, south-west London, an 18-storey mixed-use development with 386 build-to-rent homes.

Inevitably, the addition of a second staircase means some loss of units. Cowen estimates that the conventional net-to-gross ratio of around 80 per cent drops closer to 70 per cent. ‘It has cost-and-return implications for developers but we feel we’re better to be ahead of the curve,’ she explains, pointing out that this has been especially relevant for long-term institutional products like build-to-rent. Another factor affecting cost and return is provision of amenity space, a nice-to-have that became essential during the pandemic. ‘Build-to-rent assets have been phenomenally successful through the pandemic,’ Cowen says, ‘because they are purpose-designed communities, generally with quite a lot of amenity space beyond the apartment and professional management.’ That includes private external amenity space, communal social and workspace and such features as apps to connect neighbours. 

This model has attractions for residents of all tenures, but the question of viability remains. The answer, argues Cowen, is to build even taller. ‘We need to allow a bit more height on some of these schemes, providing we’re creating dual cores, cladding buildings properly and ensuring durability and safety,’ she says. ‘That has got to be the give; viability is still one of the greatest challenges.’

Going lower
High density doesn’t have to mean high rise though, and Cowen also points to locations where her practice is delivering density with two-storey housing, including Eddington, in Cambridge. 

Here a 25 dwellings per hectare (DPH) outline consent was revised by the practice to create a 42 DPH development, by swapping conventional roads and pavements for intimate, green and largely car-free liveable streets, with family homes in staggered terraces. 

Its density enables the scheme to deliver economic, social and environmental benefits, including efficient service runs, ready interaction with neighbours and amenities. ‘Density doesn’t result in a loss of quality,’ says Cowen. ‘The opposite applies, as it gives us more money to spend on the quality of the home and the spaces in between, promoting living landscape-led streets and sustainability.’

Sustainability policies are having their own impact on urban residential development, as Simon Henley of Henley Halebrown points out. ‘A string of requirements in London, including sustainable urban drainage, the Urban Greening Factor tool, and provision for play and amenity space, are making it increasingly difficult to achieve high density with high rise,’ he says. ‘Greater London Authority policies like the Urban Greening Factor may prove to be a crucial tool in managing density.’

Henley sees advantages to medium rise in this context. ‘A good dimension seems to be six or seven storeys,’ he says. ‘It feels a comfortable scale to live and allows you to have a relationship with the public realm. You can hear your children playing in a courtyard. There’s a strong sense of community and strong relationship with the natural world.’

The practice’s medium-rise additions to the Frampton Park Estate in Hackney are an example, providing 45 mixed-tenure homes for Hackney Council in Taylor Court, Chatto Court and nearby Wilmott Court. 

The buildings connect residents to nature and neighbourhood via loggias, comprising precast‑concrete columns and balconies, while Wilmott Court has a secluded, high-level courtyard, marking the entrances to eight houses occupying its two top storeys. 

The overall scheme achieves a density of 269 DPH and, importantly, is a sensitive response to redevelopment of two small sites – formerly occupied by a pub and a small housing block – on the fringe of an existing estate. 

Government policy has put the focus firmly on delivering quantity of homes to satisfy demand, rather than qualities like these. ‘One of the challenges is to find the appropriate level of development because there’s a risk – politics aside – that in the process of delivering “units”, you don’t actually deliver good homes,’ Henley says. That is a lesson that has been given added emphasis by events.