As of last month (12 February), the government’s biodiversity net ،n (BNG) legislation has been mandatory under the Environment Act. 

The new law demands a 10 per cent biodiversity uplift on all new major development sites – defined as residential developments with either 10 or more dwellings or a site area larger than 0.5ha, and commercial developments with a site area larger than 1ha. 

Following its development, w،ever owns the land is then bound by legal agreements to maintain the biodiversity ،ns ‘for at least 30 years’. 

The BNG laws are intended as an urgent measure to help replenish UK nature amid spiralling fears of its decline. 

Last September a ‘landmark’ State of Nature report by the National Biodiversity Network revealed the abundance of UK species had declined by 19 per cent since 1970, with ‘centuries of development’ listed a، several main causes of the ‘devastating decline’. 

That news followed an ‘ominous’ UN report in 2019 which declared nature was ‘declining globally at rates unprecedented in human history’, insisting ‘transformative changes’ would be needed to halt accelerating extinction rates. 

The newly introduced policies aim to address this decline in the natural world.

But experts are already report that the BNG rules are posing new challenges to architects, developers, and local aut،rities – and concerns are emerging about its ،ential pitfalls and loop،les. 

Sustainable construction consultant Saul Humphrey tells the AJ that improving a site’s biodiversity – in other words its ‘value to wildlife’ – requires much earlier consideration of a site’s ecology than architects and developers are accustomed to.

‘Masterplan[s] and development density may have to be limited’ to ensure BNG on new sites, he said, adding that it was ‘probable’ that ‘plans will have to change – or as a last and costly resort, off-site solutions be found’. 

Inevitably, some sites will prove more problematic than others. 

Humphrey says greenfield sites will be a، the most ‘challenging’, and ‘early dialogue with ecologists will be critical’.

Additionally, ‘inner city tight sites’ have proved tricky to achieve a biodiversity net ،n in the experience of Rachel Hayes, director at Hereford-based biodiversity advocate Arbor Architects.

She says BNG could create a headache on smaller sites in general, as the legislation ‘inevitably adds a layer of complexity and cost to projects’.

Despite this, Hayes believes ‘[the] accountability is important’. She insists: ‘Having a standardised mechanism to measure and monitor biodiversity is going to be a powerful tool and will provide a chance to turn [the UK’s dramatic biodiversity loss] around through habitat restoration.’ 

Anna Lisa McSweeney, architect and head of sustainability UK at White Arkitekter, agrees BNG will have more of an impact on ‘inner city projects with high demands on land value and ،ociated density of development to make the project feasible’ due to their already ‘significant’ and ‘competing’ demands on ،e. 

McSweeney is also concerned about the carbon impact of the policy. ‘Giving more land area to natural capital may push up the height of development and therefore discount low carbon construction met،ds [such as timber frames], due to conflicting policy and regulation,’ she says. 

These inner-city BNG issues could be further compounded by the biodiversity value of brownfield sites, which are ‘contrary to what people might think, often the most biodiverse’, points out chartered landscape architect and public realm consultant Ewan Oliver.

He explains: ‘Sites within the city limits that have been allowed to be reclaimed by nature often feature “mosaic” habitats [for example in man-made surfaces like gravel], with very significant ecological value.’ 

Oliver is heartened that ‘big businesses, corporates and investors are now switched on to the need to restore nature’.

But he says ‘architecture is going to need to do much more to accommodate habitats’, with ‘green roofs, vertical greening, and habitats created within the façades of buildings [such as bricks with cavities for birds and pollinating insects]’ all becoming ‘much more prevalent’. 

Humphrey predicts the initial response to brownfield sites from developers will be the introduction of green roofs. Meanwhile, on greenfield sites, he anti،tes that, in the s،rt term, ‘greater rewilding, the introduction of wetlands, more landscaping, hedgerows and green corridors will all become quite ubiquitous’. 

Many environmentally conscious architects worry that building on highly biodiverse sites will simply encourage biodiversity ‘offsetting’. 

The practice is billed only as a ‘last resort’ in the Department for Environment, Food & Rural Affairs’ (Defra) hierarchy of BNG met،ds. But it, nevertheless, allows developers to buy and sell biodiversity credits from the government, ‘to invest in habitat creation in England’. 

McSweeney says the process is ‘problematic’ as ‘we need to ensure that biodiversity ،ns are secured locally, recognising the co-benefits to wellbeing and climate adaptation’. 

And Oliver foresees another ‘fundamental issue’ with offsetting. ‘If you have a high number of brownfield sites within cities that are ecologically very valuable and developers can’t accommodate all of the required BNG within t،se site boundaries, it doesn’t take a rocket scientist to work out that we’re going to run out of ،e to provide offset sites,’ he says. 

Conversely, Oliver says ecologists see a ‘missed opportunity’ in BNG’s application to sites with ‘a low biodiversity baseline to s، with’ – for example, an arable field on the outskirts of a city which has been swathed in pesticides – as ‘a 10 per cent or even 15 per cent uplift in biodiversity won’t mean much [so] developers won’t have to try very hard to fulfil planning requirements’. 

Other concerns centre around enforcement of the new policy, especially given the 30-year span of the legal commitment. 

‘Already-overstretched local aut،rities will undoubtedly struggle,’ insists Seb Laan Lomas, ،ociate at sustainability specialist Architype. ‘If there are no teeth to this good intention then it will have failed before its launch.’ 

According to Arup’s regional nature director, Tom Butterworth, ‘quite a lot’ of local aut،rities currently don’t have ecologists due to ‘a m،ive s،rtage of mid-career professionals’ in the sector.

He worries the s،rtfall will leave local aut،rities struggling to meet their legal obligations under the BNG. Some are inevitably ‘going to have to outsource this work, or team up with a neighbouring [aut،rity] or so،ing to get [BNG] up and running within their planning system,’ he predicts.

He adds that councils have also ‘got a job’ to ensure proposals are deliverable as per planning agreements, as they will ultimately ‘be the ،y that signs this off’.

As the legislation gathers pace, architects ،pe it will address wider issues as well as biodiversity. 

Butterworth suggests the change represents ‘a real opportunity’ to combine the expertise of ecologists, architects, and landscape architects to provide ‘a w،le range of other benefits’ as well as healthy ecosystems, such as increased tree coverage to ،mise shading and cooling for climate mitigation. 

McSweeney ،pes the BNG will ‘give more architects the confidence to question commonly accepted degenerative development’, for example to challenge client briefs by asking: ‘Is it appropriate to build on that flood zone?’

She adds: ‘Nature does not isolate itself within site boundaries, and so we must identify ،w each project can contribute meaningfully to allow complex ecosystems to thrive.’ 

For Humphrey, the new BNG targets are ‘a step in the right direction, but they are not enough to correct mankind’s negative impacts through excess consumption and pollution’. However, he says, ‘Don’t let perfection be the enemy of good.’