BNG in 30 seconds
BNG uses habitats as a stand-in for wildlife. Before development, the site's habitats are scored in biodiversity units. After development, the site must be worth at least 10% more, and the gain must be secured for at least 30 years.
Measure what's there
A habitat survey records each habitat parcel, its size and its condition. The metric turns these into baseline units.
Plan what will be there
Retain, enhance and create habitat. New and improved habitat is discounted for how hard it is to create and how long it takes to mature.
Gain 10%, keep it 30 years
Post-development units must be at least baseline × 1.10, for each unit type separately: on-site, off-site, or as a last resort, through statutory credits.
What changed recently
- BNG becomes mandatory for major development.
- BNG becomes mandatory for small ("minor") development.
- Routine fixes to the metric documents and condition sheets. Defra says unit outputs did not change.
- Government response to the consultation on minor, medium and brownfield development.
- New exemptions for sites of 0.2 ha or less and for temporary development. The self-build exemption is removed. Minor development can go straight to off-site gains.
- BNG becomes mandatory for Nationally Significant Infrastructure Projects (NSIPs).
- Metric tool v1.1.0, then v1.1.1. The user guide adds guidance on spatial risk for projects that span several water catchments.
Does BNG apply to my site?
Answer one question at a time. The checker follows GOV.UK's exemptions guidance and the Small Sites Metric eligibility rules, and shows its reasoning at the end.
This is a guide, not a legal decision. Variations of existing permissions and phased developments have transitional rules, so check the planning practice guidance and talk to your local planning authority.
The basics
The rule fits on one line. It is applied separately to each of the three kinds of unit: area habitat units, hedgerow units and watercourse units. A surplus in one kind cannot make up a shortfall in another.
Who it applies to
| Development type | BNG mandatory from |
|---|---|
| Major development (Town and Country Planning Act 1990) | 12 February 2024 |
| Small ("minor") development | 2 April 2024 |
| Nationally Significant Infrastructure Projects (NSIPs) | Applications submitted on or after 2 November 2026 |
Exemptions (as at November 2026)
| Exemption | Detail | Lost if priority habitat is harmed? |
|---|---|---|
| Sites of 0.2 ha or less new | Red line area of 0.2 ha or below, for applications made from 6 Aug 2026 | Yes: does not apply if on-site priority habitat is harmed |
| De minimis | Affects less than 25 m² (5 m × 5 m) of habitat, and less than 5 m of hedgerow or other linear habitat | Yes |
| Temporary development new | The whole development is temporary, with permission for 5 years or less (applications from 6 Aug 2026) | Yes |
| Householder applications | Extensions, conservatories, loft conversions and similar | – |
| Self-build / custom build removed | Only applies to applications made before 6 Aug 2026 (or variations of a permission that was exempt) | – |
| Biodiversity gain sites | Development mainly to create habitat for another development's BNG | – |
| Others | High-speed rail, urgent Crown development, and development granted by a development order, including permitted development | – |
| Before mandatory BNG | Applications made before 12 Feb 2024, or before 2 Apr 2024 for small developments | – |
Three ways to deliver the gain: the biodiversity gain hierarchy
- 1On-site
Avoid harm first, then enhance and create habitat inside the red line boundary. - 2Off-site
Use your own land elsewhere, or buy units from a registered biodiversity gain site. From 6 Aug 2026, minor development can consider steps 1 and 2 together and go straight to off-site. - 3Statutory biodiversity credits
A last resort, bought from government. Two credits are needed for every unit.
Statutory credit prices (per credit, GOV.UK)
| Tier | Price per credit | Example habitats |
|---|---|---|
| A1 | £42,000 | All low distinctiveness; medium grassland, heathland, urban, cropland |
| A2 | £48,000 | Medium woodland; some high grassland and heathland |
| A3 | £66,000 | Wet woodland, upland birchwoods, littoral mud |
| A4 | £125,000 | Ponds, reservoirs, saltmarsh, various woodlands |
| A5 | £650,000 | High-distinctiveness lakes |
| H | £44,000 | Hedgerows |
| W | £230,000 | Watercourses |
2 credits are needed per unit. Prices are reviewed every 6 months, with at least 10 weeks' notice of changes. No VAT is charged.
The paperwork, in order
The legal agreement and monitoring plan are needed for significant on-site gains and for all off-site gains.
How the statutory metric works
Each habitat parcel becomes a number of biodiversity units: its size multiplied by its quality. New or improved habitat is then reduced for risk, that is, how hard it is to create and how long it takes to mature.
Yellow multipliers apply only to created and enhanced habitat. Off-site habitat adds a spatial risk multiplier. Watercourses add encroachment multipliers.
The four calculations
1 · Baseline and retained habitat
Hedgerows and watercourses use length in km instead of area. Watercourses add two more multipliers: watercourse encroachment and riparian zone encroachment.
2 · Created habitat
3 · Enhanced habitat
Only the improvement is discounted for risk. The starting value is kept in full.
where value = area × distinctiveness × condition
4 · The result
The tool reports this for on-site units, off-site units and the total. It also reports the units still needed: baseline × 1.10 − (on-site post-development units + off-site gain).
The multipliers
| Distinctiveness (set by habitat type) | |
|---|---|
| Very high | 8 |
| High | 6 |
| Medium | 4 |
| Low | 2 |
| Very low (hedgerow) | 1 |
| Very low (area) | 0 |
| Condition (from survey) | |
|---|---|
| Good | 3 |
| Fairly good | 2.5 |
| Moderate | 2 |
| Fairly poor | 1.5 |
| Poor | 1 |
| Condition assessment N/A | 1 |
| N/A – other | 0 |
| Strategic significance (location) | |
|---|---|
| High: in a local strategy, e.g. LNRS mapped measures | 1.15 |
| Medium: ecologically desirable but not in a strategy | 1.10 |
| Low | 1.00 |
Once a Local Nature Recovery Strategy (LNRS) is published, baseline habitats are scored Low, and the Medium category can no longer be used.
| Difficulty (by habitat; creation and enhancement differ) | |
|---|---|
| Low | 1 |
| Medium | 0.67 |
| High | 0.33 |
| Very high | 0.1 |
| Spatial risk (off-site only) | |
|---|---|
| Inside the same LPA or National Character Area | 1 |
| Neighbouring LPA or NCA | 0.75 |
| Further away | 0.5 |
Watercourses use waterbody and operational catchments instead, and intertidal habitats use Marine Plan Areas.
| Watercourse encroachment | |
|---|---|
| None | 1.0 |
| Minor | 0.8 |
| Major | 0.5 |
| Culvert | 0.68 |
Riparian zone width: 10 m for priority rivers, other rivers and streams, and canals; 5 m for ditches.
The time discount: "time to target condition"
Every habitat and condition pair has a standard number of years to reach the target condition. Value is discounted by 3.5% for each year, so the multiplier is 0.965years. Habitats marked "30+" are treated as 32 years, which gives 0.320.
| Years | 0 | 1 | 5 | 10 | 15 | 20 | 30+ |
|---|---|---|---|---|---|---|---|
| Multiplier | 1.000 | 0.965 | 0.837 | 0.700 | 0.586 | 0.490 | 0.320 |
For example, creating other neutral grassland in Moderate condition takes 5 years. Creating lowland meadows in Good condition takes 15 years. Creating habitat in advance of the losses reduces the time, and a delay increases it, but you must give evidence for either. Temporary losses can be left out of the calculation if the habitat type and condition are restored within 2 years. For NSIPs, this is 5 years for low or very low distinctiveness habitats.
Habitat lookup
Pick any of the 133 area habitats in the statutory metric to see its scores, as listed in tool v1.1.1.
The four metric rules
| Rule | In plain English |
|---|---|
| 1 | Follow the trading rules: lost habitat must be replaced with the same or better kind (see Trading rules). |
| 2 | Never add, swap or convert between area, hedgerow and watercourse units. Each needs its own 10%. |
| 3 | Use the statutory tool, or the Small Sites Metric where eligible. |
| 4 | Deviations are only allowed in exceptional ecological circumstances, and only if the decision maker agrees. |
Condition assessment
Condition is the survey judgement that most changes the result. Moving one parcel from Poor (1) to Moderate (2) doubles its units. The official workbook turns condition into checklists, and you can try three of them below.
The Statutory Biodiversity Metric Condition Assessments workbook (July 2025) has a selector sheet that matches each habitat to its criteria sheet. There is one sheet per habitat group (25 groups), each with a single-parcel and a 10-parcel version. Some habitats, such as cereal crops, bramble scrub and vegetated gardens, need no assessment ("Condition Assessment N/A"). The sheets score in three different ways:
| Style | Example | How it scores |
|---|---|---|
| Pass/fail count with an essential criterion | Low-distinctiveness grassland (modified grassland): 7 criteria. Criterion A, 6–8 species per m² including 2 or more forbs, is essential. | 6–7 passes including A = Good · 4–5 including A = Moderate · otherwise Poor |
| Points per indicator | Woodland: 13 indicators scored 3, 2 or 1 points each (age classes, browsing, invasives, native species, open space, regeneration, tree health, ground flora, structure, veteran trees, deadwood, disturbance and more) | Out of 39: 33+ = Good · 26–32 = Moderate · 25 or less = Poor |
| Failures by functional group | Hedgerows: 8 attributes (height, width, gaps, margin, enrichment, invasives, damage) in groups A–D, plus group E (tree class, tree health) for hedgerows with trees | Good: ≤2 failures and ≤1 per group · Moderate: ≤4 failures (≤5 with trees) and not both attributes failed in more than one group · otherwise Poor |
Try a condition sheet
Tick each criterion the parcel passes. The criteria are shortened from the official July 2025 sheets, so use the full official wording and footnotes in a real survey.
Trading rules
The trading rules stop someone destroying a rare habitat and replacing it with a large area of a common one. They only apply until the losses are covered (no net loss). The extra 10% can be any habitat.
| If the lost habitat is… | Area habitats: replace with | Hedgerows | Watercourses |
|---|---|---|---|
| Very high | The same habitat type (bespoke compensation possible) | Same habitat type | Same habitat type |
| High | The same habitat type | Same type or a higher band | Same habitat type |
| Medium | A medium habitat in the same broad type (e.g. grassland for grassland), or any higher-band habitat | Same band or higher | Same habitat type |
| Low | Same band or higher | Same band or higher | A higher band |
| Very low | No compensation needed | Same band or higher | – |
The Small Sites Metric (SSM)
A simpler version of the metric for small development. No ecologist is needed: a developer, project manager, gardener or landscape architect can complete it. Condition is fixed rather than surveyed, lengths are in metres, and there are fewer habitat types.
✓ Who can use it
Development that is not major and is one of:
- 1–9 dwellings on a site of 1 ha or less (under 0.5 ha if the number of dwellings is unknown)
- Commercial floorspace under 1,000 m², or a site under 1 ha
✗ Who cannot
- Sites with priority habitat (except some hedgerows and arable field margins)
- Sites with protected sites or European protected species present
- Minerals or waste development
Small developments can always use the full statutory metric instead.
In April 2026 the government decided not to extend the SSM to medium development, and not to make most of the proposed SSM changes for now. Because sites of 0.2 ha or less are now usually exempt, the SSM's main users will be minor sites between 0.2 ha and 1 ha. If off-site units are used with the SSM, the biodiversity gain plan must summarise how the trading rules were followed.
Will my site reach 10%?
A quick pre-check for on-site area habitats, using the habitat, condition, difficulty and time tables from statutory metric tool v1.1.1. Every number shows how it was worked out.
What's on the site now?
Add each habitat parcel, then say how much of it you'll keep unchanged and how much you'll improve. Whatever is left will be lost.
Improve existing habitat
One row appears for each parcel you said you'll improve. Choose the target condition, or a higher-distinctiveness habitat.
New habitat on the lost area
Include buildings and hard surfaces (Urban → Developed land; sealed surface) so the areas add up.
4 · Result
How accurate is the calculator?
The statutory calculation follows fixed rules, and every lookup it needs is in the tool's reference tabs: G-1 habitats, G-3 multipliers, G-4/G-5 time tables, G-6 hedgerows, G-7 watercourses and G-8 condition. This calculator uses those tables directly. To test it, we entered the same site into the official v1.1.1 workbook and into this calculator. The results matched exactly.
| Test site (2 ha, on-site only) | Official tool | This calculator |
|---|---|---|
| Baseline: 1.5 ha modified grassland (Poor) + 0.5 ha other neutral grassland (Moderate) | 7.00 | 7.00 |
| Retain 0.3 ha neutral grassland; enhance 0.2 ha from Moderate to Good; create 1.0 ha sealed surface, 0.3 ha vegetated garden and 0.2 ha neutral grassland (Moderate, medium strategic significance) | 6.61 | 6.61 |
| Net change | −5.54% | −5.54% |
| Units needed for 10% | 7.70 | 7.70 |
Load the worked example in the calculator to reproduce this.
What the official tool also checks
- Validation: created area must equal lost area (within 0.01 ha); enhancement cannot reduce condition or distinctiveness; some habitat and condition pairs are "Not Possible"; irreplaceable habitat produces no units. This calculator checks the first three.
- Trading rules by band and broad habitat, applied only up to no net loss. This is the most complex logic in the tool.
- Off-site units: the spatial risk multiplier applies only to the uplift, and only when the uplift is positive.
- Hedgerow and watercourse modules (same pattern, plus encroachment for rivers), and the individual tree helper, which works out area from trunk diameter (DBH).
Keeping up to date
Tool v1.1.0 and v1.1.1 both came out in November 2026. Defra says updates do not change unit outputs, but this calculator's tables are rebuilt and re-tested from each new workbook. Defra offers advance notice of changes to companies that use the tool's data in their own BNG software.
What's here, and what's coming
Planned in this order: explainers and the eligibility checker first, then the on-site calculator, the condition wizard, the other modules and finally import and export.
"Does BNG apply to me?"
A short question-by-question checker covering householder applications, 0.2 ha, de minimis, temporary development, priority habitat, NSIPs and Small Sites Metric eligibility.
"Will my site get 10%?"
Enter baseline habitats and what you'll keep, create or improve. A live bar shows progress to the 10% line and the units still needed.
Explain every number
Every unit total shows its working, e.g. "0.2 ha × 4 (medium) × 2 (moderate) × 1.10 (strategic) × 0.837 (5 years) × 1 (low difficulty) = 1.47".
Statutory credits estimate
If the gap were filled with credits: units × 2 credits × tier price. Shown as a last resort, as in the hierarchy.
Condition wizard
Three sheets work now. Next: every official sheet as yes/no questions with help text, the scoring rule, and "what would move this up a band".
Trading-rule checker
The calculator gives the rule for each lost parcel in plain English. Next: the full check, e.g. "You need 2.0 more units of medium or better grassland, or any high-distinctiveness habitat."
"What gets me to 10%?"
Suggest ways to close the gap (better condition, a different habitat, off-site units), with units per hectare, so a quick, easy habitat can be compared with a slow, difficult one.
Hedgerows, watercourses, off-site
The other two modules, then off-site units with the spatial risk multiplier.
GIS in, metric out
Import UKHab polygons (GeoPackage or CSV, matching the BNG Metric Mapper QGIS plugin) and export rows ready for the official tool; save projects.
Have a request? Suggest an idea.
Glossary
- Biodiversity unit
- The metric's measure of a habitat's value: size × distinctiveness × condition × strategic significance, with risk multipliers for new or improved habitat.
- Baseline
- The habitats on site before development, measured from a professional survey.
- Red line boundary
- The site boundary in the planning application. Everything inside it counts as on-site.
- Habitat parcel
- An area within the site holding one habitat type in one condition.
- Distinctiveness
- How rare and valuable a habitat type is, from very low (0) to very high (8). It is set automatically by habitat type.
- Condition
- How good an example of its type a parcel is, scored from the official condition sheets (Poor 1 to Good 3).
- Strategic significance
- Extra value for habitat in the right place, e.g. in a Local Nature Recovery Strategy's mapped measures (×1.15).
- LNRS
- Local Nature Recovery Strategy: a map and plan of where nature recovery matters most in an area.
- Difficulty multiplier
- A discount for how risky a habitat is to create or enhance (×1 to ×0.1).
- Temporal multiplier
- A discount for how long new habitat takes to reach its target condition (3.5% per year).
- Spatial risk multiplier
- A discount for off-site gains far from the development (×1, ×0.75 or ×0.5).
- Trading rules
- The rules on which habitats can replace which. The higher the distinctiveness, the stricter the rule.
- Irreplaceable habitat
- Habitat such as ancient woodland that cannot be replaced. It is outside the metric and needs bespoke compensation.
- Priority habitat
- Habitats of principal importance (Section 41, NERC Act 2006). Harming them removes the 0.2 ha, de minimis and temporary exemptions.
- Biodiversity gain plan
- The document showing how the 10% will be met. It is approved after permission and before work starts.
- HMMP
- Habitat Management and Monitoring Plan: how habitat will be managed and monitored for 30 years.
- Statutory credits
- Units bought from the government as a last resort. Two credits are needed per unit.
- NSIP
- Nationally Significant Infrastructure Project, consented by a Development Consent Order. BNG applies from 2 Nov 2026.
- SSM
- Small Sites Metric: the simplified metric for eligible small development.
- Competent person
- Someone with the skills to complete the full metric, usually an ecologist. The planning authority can reject calculations it doubts.
Sources (official only)
- Statutory biodiversity metric tools and guides (GOV.UK, Defra): calculation tool v1.1.1, user guide (Nov 2026), Small Sites Metric v1.2.3 and guide (July 2025), condition assessments (July 2025)
- Calculate biodiversity value with the statutory biodiversity metric (GOV.UK)
- Biodiversity net gain: exempt developments (GOV.UK)
- Understanding biodiversity net gain (GOV.UK)
- Biodiversity net gain collection (GOV.UK)
- Make on-site biodiversity gains as a developer (GOV.UK)
- Statutory biodiversity credit prices (GOV.UK)
- Government response: improving BNG for minor, medium and brownfield development (Defra, April 2026)
- BNG: what's changing and what it means for you (Defra blog, April 2026)
- The Statutory Biodiversity Metric: what it means (Defra blog)
- Biodiversity net gain: where to start (Natural England blog)
Contains public sector information licensed under the Open Government Licence v3.0. Metric values, tables and formulas were checked directly against the downloaded v1.1.1 workbook.