Can England’s new ‘biodiversity net gain’ law reverse declines in nature?

"We had a vision to create the most sustainable, carbon-efficient building that we could develop, and also to deliver as much net gain in biodiversity as well," says Emma Payne, project manager at Eden, an office block in Salford which is set to complete next year. If the architect's renderings are anything to go by, it will make a distinct departure from more traditional glass and steel office blocks. Instead, Eden will be draped head to toe in what will be Europe's largest living wall. Payne says some 350,000 plants will attach to the building on grids (the plants are currently sprouting in a nursery in Chichester). There will also be numerous bird boxes and insect hotels.

Thanks to this profusion of plant life, Muse Developments, the firm behind the design, claims it will increase biodiversity levels on the site by (a very specific) 174 per cent. Previously a car park, the new building will bring an abundance of vertical vegetation to an area that was concreted over. A management team will be employed year-​round to plant new bulbs and keep things nicely pruned. Payne adds that irrigation will be provided by rainwater collected on the roof, which should be ample thanks to "our Manchester climate", she quips.

Eden might be a somewhat unusual building, but the developer's goal of increasing biodiversity on the site will soon become the norm in England (other UK nations are mulling similar ideas). As part of the 2021 Environment Act, the concept of biodiversity net gain – or BNG – will come into force in November 2023 and will affect most kinds of construction.

The new legislation will require developers to calculate how much biodiversity (essentially, the variety of living things) is on a given site as part of their planning application. They then need to show how they're going to mitigate any damage caused by construction and increase total biodiversity by at least 10 per cent. Furthermore, they must put together a plan to manage the new habitat for at least 30 years. (Some firms, like Muse Developments who are behind the Eden building, are choosing to comply before the requirement becomes mandatory).

In a world where rates of biodiversity are declining rapidly, this seems like a welcome new law. Crucially, it goes further than current planning legislation, which is based on a 'do no harm' approach. By making developers go further, and increase the number of plants and animals in a place, this could perhaps contribute to the restoration of nature. However, as with so many policies, turning a good idea into practice is easier said than done.

Defra, the UK's Department for Environment, Food and Rural Affairs, has played a key role in drafting the legislation and defining how it will work. A spokesperson explains the essential idea as: "An approach to development that leaves biodiversity in a measurably better state than before. This means protecting existing habitats and ensuring that lost or degraded habitats are compensated for by enhancing or creating habitats that are of greater value to wildlife and people."

It is understood that pretty much all residential and commercial developments will be affected, as will most infrastructure projects too (though very small sites might not need to comply).

Once the law is in force, developers will need to include their plans to generate BNGs as part of planning permission applications and show how they'll preserve the habitats for 30 years. It will then, in most cases, be down to local planning authorities to verify whether the developer has proposed a reasonable plan for increasing biodiversity. Defra says it has provided £4.18m to local authorities to help them prepare.

When it comes to generating BNGs, a developer has two options. The first is to implement them on site. For example, if you were building a new housing estate on what was previously arable land, you would have to plant a variety of trees, dig in ponds, add wildflower meadows and so on, to increase the area's biodiversity compared to what was there before.

Alternatively, developers can buy offsetting credits. In this model, a landholder will offer up an equivalent patch of land to be returned to nature. Offsetting will also need to be done nearby, preferably within the same local planning authority area.

In practice, this new legislation will require developers to conduct an ecological survey of any site they plan to build on. Ecological consultants will carry out an analysis of the land to figure out what state it's currently in, and whether it is poor, average or good in terms of biodiversity. They will need to find out what species are living there, then plug their data into the Biodiversity Metric, a calculator which can be found on the Natural England website. After adding in all their data, the metric tells the developer the current state of biodiversity, so they can then figure out exactly how to increase that total by 10 per cent. Ecologists and landscapers will be able to advise on what species and habitats need to be created.

Evidently, sites like Eden in Salford, which can claim a 174 per cent BNG, are unusual. In most cases, it will involve calculating metrics for farmers' fields, brownfield sites or country parks that landholders are looking to sell off. It is worth noting that some kinds of biodiversity are deemed irreplaceable – things like ancient woodlands or hedgerows. These cannot be torn down so developers will need to plan around them.

When policymakers and industry experts were first fleshing out ideas for BNG, "there was an initial, big, big push to doing onsite restoration and putting that biodiversity back on the site", recalls Jason Reeves of the Chartered Institute of Ecology and Environmental Management. The idea was to improve local wildlife habitats and restore them to something resembling their 'natural state' (as far as anything can be natural in a country where the landscape has been altered by humans for millennia).

The trouble with doing BNG onsite, however, is that it requires a 30-year management plan. And this raises all sorts of issues. Will people remember to maintain a wildflower meadow, or will it all get mown down after a few years? Can a small woodland by a housing estate ever become a haven for wildlife if people are having barbecues and parties in there, fly-tipping, walking their dogs and letting cats roam free?

These issues mean there are a lot of doubts about how effective onsite biodiversity can be. Reeves wonders if will be "one of those things which will stretch local authorities; will they have the capacity to keep going back and checking? Or even to provide the management for 30 years?"

As a result, there is a big debate within the ecology profession about whether "you bundle up all of those little onsite ones and you do one really great off-site restoration [and] actually do something better for nature", Reeves explains. The idea is that biodiversity thrives when it has more space to spread and do its thing. While it is nice for residents of a new housing estate to have a small nature reserve nearby, BNG will probably have more of an impact when it's done at the landscape scale. If all the small nature reserves on housing estates could instead be bunched together, far more plants and animals would have the space to thrive.

To do this, developers would buy biodiversity credits – paying for someone else to provide and manage the BNG on a different site.

One company which is trying to meet this demand for offsetting is the Environment Bank. James Cross, the organisation's CEO, explains. "We are building what we call habitat banks," he says, with the hope to create at least one in every local planning authority in England. These habitat banks, which must be at least 10 hectares in size (but are often a lot larger) are leased or even bought off farmers and landowners. The Environment Bank will then take responsibility for turning this land into woodland, wild meadows, wetlands or whatever is most appropriate. "It's really good for nature, because it's bigger, it's better and it's more joined up," he reckons.

This does seem like a win-win. Developers can hand over the creation and management of the land to experts and can use more of the land they've paid for to build on. Meanwhile, landowners can generate an income from land which might not otherwise be very productive. This should all be a boon for nature, providing larger reserves for more plants and animals to thrive. Cross says his organisation has already signed deals on thousands of hectares across the country and is planning to increase this in future.

Yet not everyone is convinced BNG is going to be quite as effective as hoped. Ian Rotherham is emeritus professor of ecology at Sheffield Hallam University. "I think [BNG] is very attractive to a lot of stakeholders. But I think the danger is that it promises everything, and it gets everybody off the hook," he argues.

Part of the problem, he believes, is that we can never claim there's been no 'net loss' in biodiversity when we build on a site. A lot of the natural landscape in countries like England looks the way it does because of millennia of human interventions. The plants and animals living on any single site are "there because of very specific conditions. And you can't replicate that." This means that trying to replace lost hedgerows, trees or ponds within the site or externally, inevitably causes a loss which can't really be replaced.

Another problem is that the law requires developers to pay for ecological surveys and submit their findings to planning authorities. Rotherham characterises this as letting developers be "the poacher and the gamekeeper". Since it is developers who commission and pay for ecological reports, we need to trust that this process has been done honestly. "Unfortunately, some of my colleagues [in the ecology profession] are prepared to accept that money, not to give an unbiased point of view, but to give a story that is plausible and sufficient to get the planning consent." Essentially, some ecologists might downplay how biodiverse a site is, so the developer can get the go-ahead to build there and make fewer BNG contributions.

He emphasises that most ecological consultancies take a balanced and ethical stance. But he says he has seen plenty of cases where the results of these surveys missed "obvious" markers of biodiversity.

Another weakness for this kind of legislation is that it requires local planning authorities to be able to understand ecologists' reports and assess whether they are accurate. However, LPAs have faced years of underfunding, and many do not have in-house ecologists to verify whether BNG assessments are accurate.

Rotherham is conscious of the charge of 'nimbyism' ('not in my back yard'), where people living in attractive rural areas fight off any new developments. People do of course need to live somewhere, and infrastructure needs to be built. Nonetheless, he is convinced we should be taking more creative approaches to building, especially increasing the use of 'brownfield' sites in city centres where there's little biodiversity to disrupt anyway. He also thinks building is possible in green belts (protected areas of nature encircling cities), but at much lower density and with more eco-friendly designs.

"I suppose it's the million-dollar question," says Jason Reeves of the CIEEM, when asked if he thinks this new law really will increase biodiversity in England. "I have to be an optimist; I have to hope that it will."

He points to reasons to be hopeful. First, in his experience, most developers "want to do the right thing, but don't want to disadvantage themselves either". So long as there's a level playing field which all developers and construction firms must follow, he believes they are generally happy to make the idea work. He also notes that "when you add more green space, you can sell houses for more money" – another motivation for the construction industry. There also does seem to be commitment from central government, which will hopefully mean the policy is enacted for the long term.

However, Reeves notes that, at least in terms of footprint, property development has far smaller impact on England's biodiversity than agriculture, which takes up far more land, and turns it into monocultures. Without wider efforts to reintroduce biodiversity across the landscape, adding 10 per cent to compensate on newly built sites can only ever be a small part of the solution.

Back in Salford, the Eden building continues to rise from the ground, and the first green wall sections were to be tested over the course of this summer.

Emma Payne, the project manager, explains building and attaching the green wall is actually quite straightforward, not unlike installing a regular façade. On completion, the vertical wall will be continually pruned, so it will remain green throughout the year, but with different seasonal colour showing through.

"There's going to be a constant wash of different colours," Payne predicts, with "pollinating receptors for the bees and nectar for the birds. This is a living garden, essentially."

It is going to take many years for the eventual impact of the biodiversity net gains legislation to be seen on a grand scale – and for us to judge if it works. Nevertheless, it is surely a step in the right direction, and must be better than 'business as usual'. Whether it is enough to tackle the more widespread decline in biodiversity is another question entirely.