Since 2024, most new developments in England must deliver a measurable gain for nature — not just avoid harm. Here's what Biodiversity Net Gain actually requires, how the units are calculated, and how to evidence it without derailing your programme.
What is Biodiversity Net Gain?
Biodiversity Net Gain (BNG) is an approach to development that leaves the natural environment in a measurably better state than before. It's no longer a voluntary best practice under the Environment Act 2021; BNG became a mandatory condition of planning permission in England, per gov.uk's biodiversity net gain guidance.
The headline requirement is simple to state: every applicable development must deliver a minimum 10% net gain in biodiversity, measured and maintained for at least 30 years.
Like the increasing planning-authority requirement for a Whole-Life Carbon Assessment, BNG has moved from a nice-to-have to a hard condition of permission on larger schemes.
When did it become mandatory?
BNG became a statutory requirement for most developments in England on a rolling basis:
- 12 February 2024 — major developments under the Town and Country Planning Act 1990.
- 2 April 2024 — small sites (minor developments).
- 6 August 2026 — scope narrowed. Development on a site no larger than 0.2 hectares became exempt, provided it does not impact onsite priority habitat, and the self-build and custom build exemption was withdrawn.
- Later phases — Nationally Significant Infrastructure Projects (NSIPs) follow on their own timeline.
Check scope before anything else. The 6 August 2026 exemption took a substantial share of development out of the regime, and it applies only to applications made on or after that date. Our guide to the 0.2 hectare exemption sets out the threshold, the priority habitat carve-out and the transitional rules; if the site is still in scope, which metric you may use is a separate question with its own conditions.
If your planning application falls in scope, you cannot discharge planning without demonstrating the 10% gain.
How biodiversity is measured — the statutory metric
You can't manage what you can't measure, so BNG is quantified using Defra's Statutory Biodiversity Metric, which replaced Biodiversity Metric 4.0 — calculations from 4.0 or earlier are not accepted in planning applications. It converts habitats into biodiversity units based on:
- Size — the area of the habitat (or length, for linear features).
- Quality (condition) — how good the habitat is, from poor to good.
- Distinctiveness — how ecologically valuable the habitat type is.
- Strategic significance — whether it's in a location identified as a priority for nature.
Crucially, three things are measured separately, and each must not fall below baseline:
| Measured separately | Examples |
|---|---|
| Area habitats | Grassland, woodland, scrub, and similar |
| Hedgerows | Field boundaries, tree lines |
| Watercourses | Rivers and streams |
Each category needs its own 10% gain — a surplus of area habitat units can't cover a hedgerow shortfall.
What is a biodiversity unit?
A biodiversity unit is the metric's single currency. For habitat that is already there, the calculation is a straight multiplication:
Baseline units = area (ha) × distinctiveness × condition × strategic significance
Each term is a number read off Defra's own tables, not a judgement call:
| Term | What it is | Range |
|---|---|---|
| Distinctiveness | Fixed per habitat type | 0, 2, 4, 6 or 8 |
| Condition | Scored against the metric's criteria for that habitat | 1 (poor) to 3 (good) |
| Strategic significance | Whether the parcel is a local priority for nature | 1.0, 1.1 or 1.15 |
The consequence is the thing most people miss: area barely matters on its own. One hectare of modified grassland in poor condition with no strategic significance is 1 × 2 × 1 × 1 = 2 units. One hectare of lowland meadow in good condition is 1 × 8 × 3 × 1 = 24 units. The same hectare on a drawing, twelve times the obligation.
Habitat you create is worth less than habitat you keep
For new or improved habitat, two further multipliers apply, and both of them cut:
Post-development units = area × distinctiveness × condition × strategic significance × temporal × technical difficulty
- Temporal risk discounts the gain by 3.5% a year for every year until the habitat reaches its target condition. Fifteen years out, the multiplier is 0.586. Thirty or more years out it bottoms out at 0.32.
- Technical difficulty reflects how reliably that habitat can be created or improved at all: 1.0 for low, 0.67 medium, 0.33 high, 0.1 very high.
Put those together and the arithmetic gets brutal. Destroying that hectare of good-condition lowland meadow costs you 24 units. Recreating a hectare of good-condition lowland meadow earns you 24 × 0.586 × 0.33 = 4.64 units — because it takes 15 years to get there and meadow creation is rated high difficulty.
You would need more than five hectares of new meadow just to stand still, before adding the 10%.
Work out your own units. Our free BNG calculator runs Defra's Statutory Biodiversity Metric on your habitat parcels and returns baseline, post-development and net-gain units for area habitats, hedgerows and watercourses separately. It is a planning aid for understanding your position early — off-site gains, trading rules and statutory credits are not modelled, and your formal submission must be made with Defra's own tool.
The maths that catches people out. The 10% gain is calculated against your pre-development baseline biodiversity units not against zero. Clear a good-quality habitat and you create a large deficit you then have to make up, plus 10%. Surveying and protecting what's already there is almost always cheaper than recreating it.
What data you need before you can calculate
The metric is a spreadsheet; the work is getting defensible inputs into it. For the baseline you need, for every parcel inside the red line:
- A habitat classification. Each parcel identified as a habitat type the metric recognises — in practice a UK Habitat Classification survey by a competent ecologist. The metric has no "unknown" row.
- A condition assessment. Scored against the published criteria for that specific habitat type, not a general impression. Most habitats have their own criteria sheet and their own set of permitted condition bands.
- Areas and lengths. Hectares for area habitats, kilometres for hedgerows and watercourses, taken off a GIS layer rather than estimated. Units scale linearly with this number, so a sloppy boundary is a sloppy result.
- A strategic significance category. Whether the parcel sits in an area identified as a priority for nature, normally from the Local Nature Recovery Strategy or the local plan.
For the post-development side you additionally need, per proposed parcel: the target habitat type, the target condition, and the number of years to reach it — which, as above, is what the temporal multiplier eats.
Two practical consequences. Surveys are seasonal, so a parcel you cannot survey in the right window cannot be baselined this year. And you cannot defer this to the end of design: the baseline shapes the layout, because every unit you avoid losing is a unit you do not have to recreate at a 0.33 multiplier.
Watercourses — the module that catches people out
Rivers and streams are not area habitats measured in a different unit. They are a separate module with a separate 10% test, and this is where otherwise-complete applications fail.
Three rules make watercourses unforgiving:
- No cross-subsidy. A surplus of area habitat units cannot make up a watercourse shortfall. Each module stands alone.
- Like-for-like trading only. Every watercourse type in the metric requires the same habitat type in replacement. There is no trading down to something cheaper, and a culverted reach requires better distinctiveness than it had.
- Creation is rated high difficulty. Creating a river or stream carries the 0.33 multiplier, on top of the temporal discount.
Work a short reach through. Two hundred metres of "other rivers and streams" in moderate condition is 0.2 × 6 × 2 × 1 = 2.4 units.
| What you do with that 200 m | Units |
|---|---|
| Leave it as it is (baseline) | 2.4 |
| Create 200 m of the same elsewhere | 0.66 |
| Enhance it in place from moderate to good condition | 2.09 |
Recreating it returns about a quarter of what you lost. And note the third row: improving the condition of the same reach from moderate to good — a real, funded intervention — still lands below the 2.4 you started with, because the temporal and difficulty multipliers apply to enhancement too. On watercourses, condition uplift alone will usually not even hold baseline. You need length as well.
The practical rule: route the development away from the watercourse. It is almost always cheaper than any of the alternatives.
The biodiversity gain hierarchy
Like the carbon reduction hierarchy, BNG has an order of preference to deliver as high up the list as possible:
- On-site — achieve the gain within the red-line boundary of your development (retained and enhanced habitats, new planting, green roofs, SuDS).
- Off-site — if you can't hit 10% on-site, deliver or buy units on other land (your own or through the habitat market).
- Statutory biodiversity credits are a last resort: credits bought from the government. These are deliberately priced to be the least attractive option.
Going off-site also carries a cost the brochures tend to skip. A spatial risk multiplier reduces the value of units delivered away from the impact site: full value inside the same local planning authority or National Character Area, 0.75 in a neighbouring one, 0.5 beyond that. Buy units two counties away and half of what you paid for evaporates.
Statutory biodiversity credits, and what they cost
Statutory biodiversity credits are units sold by the government when you genuinely cannot deliver the gain on-site or off-site. They are priced to be the option nobody chooses.
| Tier | Price per credit | Covers |
|---|---|---|
| A1 | £42,000 | All low distinctiveness; medium-distinctiveness grassland, heathland, urban, cropland, individual trees |
| A2 | £48,000 | Medium woodland and intertidal sediment; high-distinctiveness heathland and calcareous grassland |
| A3 | £66,000 | Wet woodland, felled woodland, upland birchwoods, several intertidal sediment types |
| A4 | £125,000 | Priority ponds, saltmarsh, coastal lagoons, maritime cliffs, floodplain wetland mosaic |
| A5 | £650,000 | High-distinctiveness lakes |
| H | £44,000 | All hedgerows |
| W | £230,000 | All watercourses |
Those are the headline prices, and they are not what you pay. A spatial risk multiplier applies to credits as well, and it doubles the quantity: you must buy two credits for every one biodiversity unit of shortfall. So the real cost of a unit is twice the table figure — £84,000 at the cheapest tier.
Return to the 200 m of river above. Lose those 2.4 units entirely and cover them with credits: 2.4 × 2 = 4.8 credits at £230,000 = £1,104,000. For two hundred metres of ordinary stream.
Two further things worth knowing before anyone budgets off this table. Prices are reviewed every six months, with at least ten weeks' notice of a change — so a figure quoted at RIBA Stage 2 may not be the figure at submission. And credits are only available once you have demonstrated that on-site and off-site options were properly exhausted; they are not a line item you can simply elect.
What developers must do — the practical steps
- Survey the baseline. Commission a suitably qualified ecologist to assess existing habitats and calculate baseline units using the statutory metric. Do this early; it shapes your whole layout.
- Design to retain and avoid. The cheapest units are the ones you never lose. Keep valuable habitats, hedgerows, watercourses and trees where you can.
- Calculate the gain. Model your post-development habitats and confirm you reach at least 110% of baseline units for each of the areas, hedgerows and watercourses.
- Prepare a Biodiversity Gain Plan. Set out the baseline, the proposed habitats, how you meet the 10%, and how gains are secured and maintained for 30 years.
- Secure it legally. On-site and off-site gains are secured through planning conditions, planning obligations (Section 106) or conservation covenants, with long-term management and monitoring.
Common mistakes that cost time and money
- Surveying too late. Ecological surveys are seasonal. Missing a survey window can delay a scheme by months.
- Clearing habitat before baselining. Removing habitat before the baseline is agreed upon can mean it's still counted as if present (the "degradation" rule) or simply lands you with a bigger deficit.
- Treating the three modules as one pot. Areas, hedgerows and watercourses each need their own 10%. A scheme can be comfortably in surplus overall and still fail.
- Pricing creation as though it delivers face value. Temporal and technical-difficulty multipliers routinely cut new habitat to a third or a quarter of its paper units. Budgets built on paper units come up short.
- Ignoring the 30-year commitment. BNG isn't a one-off at planning; it's a three-decade management and monitoring obligation that must be resourced and funded — the same discipline a live Environmental Management Plan is built for, rather than a document filed once and forgotten.
- Treating it as separate from carbon. Low-carbon, nature-positive design decisions often overlap; the teams and data should talk to each other.
How BNG and carbon fit together
BNG and carbon reporting are separate regimes, but they pull in the same direction and increasingly land on the same desk. A contractor or developer who can evidence both a credible carbon record (PAS 2080, PPN 006) and a compliant Biodiversity Gain Plan is telling clients and planners a coherent sustainability story and doing it from one set of project data rather than two disconnected spreadsheets — which is exactly what VerdCore's BNG reporting software is built for.