What new biodiversity law means for housing projects in England

New homes for us often come at the expense of living space for wildlife. But, from the new year, a new law in England means developers will have to make sure their projects deliver 10% more nature. It's called biodiversity net gain.

Conservation groups are broadly supportive but worried about how natural gain will be measured and how it will be policed.

At a part-built housing estate near Aylesbury in Buckinghamshire, Helen Nyul, group biodiversity manager, from the big housebuilder Barratts showed me what it means in practice.

Some of it within the newly built environment: "We've got the house marten nesting cups which are on the side of the front end of our building here. We've got swift nesting bricks.

"We've also installed hedgehog highways, which are simple little holes underneath fences in people's back gardens, giving hedgehogs a much wider expanse of garden roaming."

And some of it beside the estate: "A nature pond that used to be an old fishing lake. But we've enhanced it and now it's got lots of birds coming to visit every day.

To make the calculation of 'gain' ecologists assess a site prior to development and give it a score depending on the variety and rarity of the species.

If the building site is on a standard arable field, the existing wildlife might be quite meagre so improving on it may be straightforward.

But if trees, hedgerows, wetlands or even scrubland are bulldozed then making up for the loss and adding 10% is much more challenging. So biodiversity offsetting is allowed.

A field of barley a few miles from Milton Keynes has been selected to enjoy natural regeneration as a payback for damage elsewhere.

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Fields will be 'bursting with life'

Emma Toovey is chief ecology officer at Environment Bank, a company which sells biodiversity units to developers who can't do enough for nature within their own development. Right now they've just begun planting hedges, but much more is planned across 38 hectares.

"These fields are going to be species-rich meadows filled bursting with life, beautiful wildflowers, buzzing with insects," she said. "We're going to have a big area of scrub planting, which sounds unusual, but it's also fantastic for wildlife."

But will it just be a question of plant, pocket the money and walk away? Ms Toovey insisted there is rigorous follow-up.

"We undertake surveys annually to make sure that what we've promised on the ground is actually happening," she said.

"And then we report back to the local planning authorities to tell them how it's going. And if things aren't quite going to plan, we can adjust what we do on the land to make sure that all of these commitments that we've made on behalf of the developer actually happen."

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Conservation groups would like to see more than 10% uplift

Conservation groups are pleased the bill recognises the need to compensate nature for the harms of development but, perhaps unsurprisingly, would like to see more than 10% uplift.

Also they worry local authorities just don't have enough trained ecologists on staff or the money to buy the expertise which will be demanded by the new law.

Sophus zu Ermgassen, an ecological economist from the University of Oxford, said: "Local authorities will be responsible for the overall outcomes of biodiversity net gain.

"So it's completely essential that they have the resources and credible monitoring and enforcement mechanisms to be able to do their job. And they don't have the resources to do those things at the moment."

Both the government and the Labour Party are promising hundreds of thousands of new homes.

Get biodiversity net gain right and it could prove we can have thriving wildlife and a booming building sector. Get it wrong and there will be even less space for nature.