01 What BNG is, and the 10% rule
Biodiversity net gain (BNG) is a requirement, introduced by the Environment Act 2021, that most new development leaves biodiversity measurably better off: at least a 10% increase in biodiversity value over the pre-development baseline, secured and maintained for at least 30 years. It is a planning condition on the scheme, not a voluntary commitment.
It became mandatory for major development under the Town and Country Planning Act on 12 February 2024, for small sites on 2 April 2024, and is being extended to Nationally Significant Infrastructure Projects (NSIPs). A handful of exemptions exist, for example genuinely de minimis impacts, but for most schemes BNG is now a line that has to be answered in the application, so on any developable site you should expect to see it addressed.
02 How the statutory metric works
Biodiversity value is measured in "units" using the government's statutory biodiversity metric. A habitat's units are a function of its area, its distinctiveness (habitat type), its condition, and its strategic significance (whether it is in a locally important location), with connectivity also considered. Hedgerows and watercourses are measured as separate unit types, so a single site can carry three distinct unit accounts.
Created or enhanced habitats are scored at a target condition, then discounted by risk multipliers for how difficult that habitat is to create and how long it takes to mature - a time-to-target penalty - because a newly created habitat is not worth the same as an established one on day one. That discount is why replacing a high-distinctiveness habitat with new planting rarely balances the metric on a like-for-like area basis.
03 On-site, off-site, or statutory credits
BNG follows a mitigation hierarchy. First avoid and reduce habitat loss; then deliver the required units on-site; then, where on-site delivery falls short, secure off-site units (for example from a habitat bank); and only as a last resort buy statutory biodiversity credits from the government. The order matters, because each step down the hierarchy tends to cost more and signal more constraint.
Statutory credits are deliberately priced high to push delivery towards real habitat on-site or off-site, so heavy reliance on them is a sign the scheme is struggling to meet its obligation through habitat. When you read a metric calculation, the split across these three routes tells you as much about deliverability as the headline unit figure does.
04 What BNG costs a scheme
On-site BNG consumes developable area - landscape buffers, ponds, species-rich grassland - and brings a 30-year management commitment. On a tightly packed logistics platform that land take competes directly with lettable area, so the metric is not just an ecology question, it is a yield question.
Off-site units bought from habitat banks vary widely by habitat type and region, but are commonly in the order of tens of thousands of pounds per unit, with high-distinctiveness habitats costing materially more; treat any figure as approximate and market-dependent. Statutory credits are tiered and set above market on purpose. The honest driver of cost is how many units the scheme is short and which habitats it loses - so the cost question is really a metric question.
05 The 30-year tail, and irreplaceable habitats
BNG is not a one-off cost. Units must be secured for at least 30 years, through a planning condition or obligation (such as a section 106 agreement) or a conservation covenant, and delivered against a Habitat Management and Monitoring Plan (HMMP). That is an ongoing management and monitoring liability that belongs in the appraisal, not just a day-one capital cost.
Irreplaceable habitats such as ancient woodland sit outside the metric: their loss generally cannot be offset and is treated as a planning red line, so their presence is a constraint, not a unit calculation. If a site carries irreplaceable habitat on the ground a building wants to occupy, that is a developability question to resolve before price, not a number to trade in the metric.
06 Red flags to catch before you bid
Some findings should change your bid, not just your notes. Watch for: no BNG assessment or metric calculation in the data room at all; a large off-site unit requirement the scheme cannot meet on-site; reliance on statutory credits to balance the metric; loss of high-distinctiveness or irreplaceable habitat; and a 30-year management and monitoring cost that has not been carried into the appraisal.
Each is a cost or a deliverability question. The aim before a bid is to know the unit shortfall, the likely route to meet it, and the long-term liability, rather than discovering them after exchange - and to make sure none of them are buried in an appendix.
07 Frequently asked questions
What is biodiversity net gain?
Biodiversity net gain, or BNG, is a planning requirement under the Environment Act 2021 for most development to leave biodiversity measurably better off than before. In practice that means delivering at least a 10% increase in biodiversity value over the site's pre-development baseline, measured with the statutory biodiversity metric and secured for at least 30 years.
Is biodiversity net gain mandatory?
Yes. A minimum 10% BNG became mandatory for major development under the Town and Country Planning Act on 12 February 2024, and for small sites on 2 April 2024, with extension to Nationally Significant Infrastructure Projects following. Only limited exemptions apply, so most schemes must demonstrate how they meet the 10% requirement as part of the planning application.
How much does biodiversity net gain cost?
It depends on how many biodiversity units the scheme is short after on-site measures and which habitats it loses. On-site delivery costs developable land plus 30-year management. Off-site units from habitat banks commonly run to tens of thousands of pounds each, more for high-distinctiveness habitats, and statutory credits are priced above the market on purpose as a last resort.
How long does biodiversity net gain have to be maintained?
The habitats delivered for BNG must be secured and managed for at least 30 years. That is enforced through a planning condition or obligation, such as a section 106 agreement, or a conservation covenant, with delivery tracked against a habitat management and monitoring plan, so BNG is a long-term liability rather than a one-off cost.
Turn the BNG position into a number, in hours
Plumb reads the biodiversity metric and ecology reports in a deal's data room alongside the planning and site material, pulls out the baseline and post-development units, the on-site versus off-site split and the 30-year management commitment, and flags where the scheme relies on off-site units or statutory credits - every point cited back to the page it came from, so your read stands up in committee.