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How to boost your site’s biodiversity net gain

Words:
Stephen Cousins

With new developments now required to deliver a minimum 10% uplift in biodiversity, ecologists and landscape architects offer some tips on the best design strategies and methods for getting more nature on site

Mayfield Park in Manchester improved biodiversity by 90% and features 6.5-acres of parkland, with 142 trees and 120,000 shrubs and plants.
Mayfield Park in Manchester improved biodiversity by 90% and features 6.5-acres of parkland, with 142 trees and 120,000 shrubs and plants. Credit: Buro Happold

We live at a time when the world’s species are disappearing at an alarming rate and rapid biodiversity loss, caused by human-induced climate change, pollution and habitat destruction, is damaging pollination and food production. Abundant plants and animals in the UK have declined by an average 19% since the 1970s, according to the latest State of Nature report by conservation groups, and almost 1,500 native species are now threatened with extinction.

New Biodiversity Net Gain (BNG) planning legislation, which comes into force on 12 February, responds to this crisis by requiring new developments to demonstrate a minimum 10% uplift to biodiversity, compared to what existed before, through the creation and enhancement of diverse habitats.

The law only applies to England – Wales and Scotland are adopting slightly different approaches – and so far covers only larger building sites; those smaller than 0.5ha have a little longer to comply – until April 2024.

View of the biodiverse Redcliffe Way Courtyard looking towards St Mary Redcliffe church,
View of the biodiverse Redcliffe Way Courtyard looking towards St Mary Redcliffe church,

To calculate BNG, the government has produced the ‘statutory biodiversity metric’ formula, which measures any rise in biodiversity ‘units’ and the extent to which plans will improve a site’s ecological value.

If developers cannot create sufficient net gain on a site, they may either apply it to other land they own, or buy biodiversity units from a land manager. Statutory biodiversity credits can be bought, as a ‘last resort’, from the government, who will use it to invest in habitat creation elsewhere in England.

The 10% minimum is just the start, and an increasing number of councils and planning authorities are looking to go further. Research by property consultancy Carter Jonas found that three councils – Guildford, Brighton & Hove and Worthing – have adopted a policy of over 10%, and 17 have similar plans emerging in local plan reviews.

Architects and landscape architects will often take the lead in ensuring that biodiversity requirements are delivered and the following advice should help give nature its rightful place in design.

View set within the biodiverse Redcliffe Way Courtyard with play and places to rest set within biodiverse planting. Redcliffe Way housing scheme in Bristol was designed by McGregor Coxall, in collaboration with Groupwork architects, to achieve a biodiversity net gain of 58% on the site and to retain all of the trees. Designed by Groupwork Architects with McGregor Coxall (landscape architects).
View set within the biodiverse Redcliffe Way Courtyard with play and places to rest set within biodiverse planting. Redcliffe Way housing scheme in Bristol was designed by McGregor Coxall, in collaboration with Groupwork architects, to achieve a biodiversity net gain of 58% on the site and to retain all of the trees. Designed by Groupwork Architects with McGregor Coxall (landscape architects).

Scope out the site It is critical to understand the biodiversity baseline of the site environment and the adjacent context. An ecologist or biodiversity specialist should carry out surveys and measure the biodiversity value of the existing habitat to identify the constraints and opportunities.

Engage with consultants early ‘A key issue we come across is being brought into teams too late, when it’s difficult to affect change,’ explains Úna Breathnach-Hifearnáin, Aassociate at landscape architecture and environment design practice McGregor Coxall. ‘If buildings have already been positioned, and spaces are already overshadowed so not suitable for planting or growing, many opportunities are lost.’

Leave things as they are The main purpose of the net gain metric is to incentivise the retention of as much biodiversity value on site as possible. Habitats are graded based on criteria including distinctiveness and quantity, and retaining more of the most distinctive  habitats will make it easier to achieve a net gain. For example, a woodland scores more highly than a playing field because it is more diverse and supports more species. ‘If you remove woodland, you have to replace it with woodland, or something of higher value, which is really difficult,’ says Pernille Olsen, associate ecologist at Buro Happold. ‘The best thing is to leave the woodland alone; if you have a large master plan, integrate it into the design.’

Brownfield is more straightforward Urban sites typically incorporate extensive paved or hard-surfaced areas, reducing the likelihood of valuable habitats being lost and making it more straightforward to achieve a 10% uplift on what is already there. ‘If you start with a site that’s already a car park, and there’s very little greenery or maybe a couple of trees a bit of scrub, it’s going to be much easier to achieve net gain than on a site with woodland and water courses,’ points out Olsen. However, there are exceptions: open mosaic habitats characterised by areas of scrub and rocky ground might appear derelict, but are in fact very biodiverse. The Thames gateway area of London is one such example, thriving with different species of insects and invertebrates.

On greenfield sites where nature can’t be retained entirely, architects should prioritise building on habitats with a lower biodiversity value, such as arable land and agricultural pasture, not agricultural areas with high food production value, or on areas of scrub and woodland.

  • Fabrik is helping deliver the Whitehill & Bordon regeneration project in Hampshire with a focus on green spaces that weave the new town together and ensuring new development is sensitive to extensive woodland and trees.
    Fabrik is helping deliver the Whitehill & Bordon regeneration project in Hampshire with a focus on green spaces that weave the new town together and ensuring new development is sensitive to extensive woodland and trees. Credit: Frabrik Ltd
  • Fabrik is helping deliver the Whitehill & Bordon regeneration project in Hampshire with a focus on green spaces that weave the new town together and ensuring new development is sensitive to extensive woodland and trees.
    Fabrik is helping deliver the Whitehill & Bordon regeneration project in Hampshire with a focus on green spaces that weave the new town together and ensuring new development is sensitive to extensive woodland and trees. Credit: Fabrick Ltd
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Don’t just focus on vegetation Biodiversity often is equated with adding greenery, and while it is important to get the vegetation right, so is getting habitats for wildlife right. ‘Early engagement and assessment by an ecologist will determine which species need to be accommodated and the strategies required to support their habitats,’ says Michelle Sánchez Brajkovic, sustainability lead at architect RSHP.

Consider nature’s multi-functional benefits It’s important to understand the parallel economic and social benefits that can help justify the investment in biodiversity on a site. Nature can attenuate flooding, green walls and roofs can improve insulation to cut a building’s energy use, gardens and planting can increase footfall and business patronage, and street trees can improve air quality and cool urban spaces. ‘Consider features that can empower the community, an orchard, or community growing areas can bring really positive benefits, creating sensory spaces to relax in and an opportunity to get closer to nature,’ advises Breathnach-Hifearnáin.

Think long term Habitats introduced under BNG regulations must be maintained for a minimum of 30 years, which puts an onus on designing robust landscaping, plant species or green systems integrated into buildings. ‘Climbing green walls, with soil at the bottom and plants that climb up either cables or mesh, are lower maintenance than some green walls with individual plants in little modules and an irrigation network at the back,’ says Olsen. ‘With green roofs it’s about the species you select, how you prepare the substrate and the roof and the slope, and considering how they will get water. They are low maintenance, but not no maintenance.’

Picking species to suit site conditions is also key. Phase one of the Mayfield city centre regeneration project in Manchester achieved an impressive 90% uplift to biodiversity and includes tree species selected to be tolerant of drought and extreme weather brought on by climate change.

Avoid offsetting ‘The on site approach is preferred because it is more controlled and you can see the direct impacts of mitigation,’ says Tara Garraty, biodiversity specialist at Tunley Environmental. She adds that offsetting rules need more clarity, with Defra and Natural England still to launch a platform to help developers or landowners find viable, certified biodiversity credit sellers.

Look beyond BNG requirements BNG metrics only consider a specific set of enhancements for habitats and does not consider species, the interconnectivity of habitats or ‘Beta’ diversity related to the number of habitats in the same area.

Habitats don’t have red line boundaries, so projects should aim to tie into broader strategies to improve biodiversity across neighbourhoods, towns and cities, creating interconnected ecological networks that allow flora and fauna to travel and spread. BNG doesn’t take into account the need for bird or bat boxes, or the use of uninterrupted raised curbs and fences that prevent the movement of hedgehogs and other small creatures causing them to get run over on the roads. ‘This is an imperfect metric but progression to and through 10% net gain needs to be a key facet of progressive design and the creation of high-quality places,’ says Simon Greig, operating board director at landscape architect Fabrik.

 

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