Is Biodiversity Net Gain failing our trees?
A deep dive into Defra's proposals
The UK government is tweaking its Biodiversity Net Gain (BNG) policy – the new regime that aims to ensure that developments leave the natural environment in a better state than they found it. But is it all going to plan? Canopy, a coalition of grassroots tree campaigners, isn't so sure. They've been digging into the details of Defra's consultation, "Improving the implementation of Biodiversity Net Gain for minor, medium and brownfield development", May 2025, and they've got some serious concerns, especially about the impact on individual trees.
What's the big picture?
Canopy fears that the proposed changes could lead to a decline in Individual trees habitat, particularly in our urban areas, and questions whether developments on brownfield land are being properly assessed.
The value of Individual trees habitat
Here's the crux of the issue: the BNG metric doesn't always accurately reflect the value of individual trees. Think about it: a tree's ‘habitat area’ – the biomass it supports through its roots, branches and canopy, and the life it shelters – is almost always larger than the ground it stands on.
When Natural England devised the area calculation methodology for Individual trees habitat, its approach was to convert the Root Protection Area (RPA) of this habitat into its equivalent habitat area. The thinking was that creating a proxy for the average 1:5 root-to-shoot ratio would roughly capture the tree’s biomass.
Canopy's analysis of over 15,000 trees shows that a tree's habitat area will exceed its RPA by an average of 34%. This difference is even more pronounced (nearly 50%) when considering small and medium-sized trees, which make up the majority (78%) of all trees. This means Individual trees habitat has an outsized impact, especially on smaller development sites under 1 hectare, which make up 84% of all developments subject to BNG assessment.
Inconsistent application: Trees falling through the cracks
Canopy's research shows BNG isn't always being applied to Individual trees habitat consistently. They've found examples of:
- Failing to include on-site Urban or Rural tree habitat in the BNG calculation
- Subsuming Urban or Rural tree habitat into other habitats
- Overvaluing replacement trees
- Claiming a de minimis exemption despite the clear presence of on-site trees
- Inconsistencies between the arboricultural evidence and the BNG evidence.
They also found that small sites have a 17% higher biodiversity value than large sites. and so should not be exempt from BNG.
Exemptions: A licence to chop?
The consultation DEFRA is proposing worrying new exemptions for minor developments (mostly those on sites of under 1 hectare), which would raise the de minimis threshold – the level below which BNG requirements are applicable. Canopy believe this is a major problem. Data shows that development sites of under 0.1 hectares account for 25% of all developments that require assessment under the current BNG regime. Removing a quarter of sites from the BNG regime undermines the entire system.
This could mean more trees, especially urban trees, are lost without proper mitigation.
Other red flags
Canopy also raises concerns about:
- Relaxing the Biodiversity Gain Hierarchy: This would remove the incentive to improve on-site habitats and encourage local offsetting schemes.
- The Small Sites Metric: The SSM tends to overvalue baseline habitats by some 10% when compared to the SBMCT. This is because the baseline habitat condition is fixed at moderate.
- Disapplying the Spatial Risk Multiplier (SRM): This will remove the incentive to provide ‘local’ off-site habitat for these small sites.
- Assessing spatial risk using Local Nature Recovery Strategy (LNRS) areas: LPAs are the most appropriate bodies upon which to base the selection of the SRM, not LNRS sites.
What about brownfield sites?
The report also highlights the importance of properly identifying Open Mosaic Habitat (OMH) on brownfield land. OMH is a priority habitat often found on brownfield sites targeted for development, but its high value and limited offsetting opportunities can create challenges for developers. The report insists that sites are accurately assessed before designation as brownfield land. Sites that meet the National Planning Policy Framework (NPPF) definition of previously developed land need to be surveyed to identify baseline habitats so that high-value habitats like OMH are excluded from the brownfield designation.
Recommendations: A way forward
Canopy urges Defra to:
- properly value Individual trees habitat: include them consistently in BNG calculations, using accurate data.
- reject broad exemptions: don't let smaller developments off the hook.
- maintain the biodiversity gain hierarchy: keep the incentive for on-site improvements.
- provide clearer guidance: make the BNG process more accessible for everyone, not just experts.
Is BNG delivering on its promises?
This question hangs over Defra's proposals. Unless the Government addresses these concerns, Biodiversity Net Gain risks becoming a net loss for our trees and therefore our wildlife, especially in the urban areas where they're needed most.