The free BNG calculator for UK landowners
A back-of-envelope estimator to give you an indicative idea of whether biodiversity net gain applies to your land, roughly how many BNG units you might need to find, and a sensible cost range across on-site, off-site, and statutory credit routes. Play with house numbers, habitat type, and plot sizes — the numbers update live. Treat the output as a first-look rule of thumb, not a planning submission.
What is biodiversity net gain, in plain English?
BNG is a legal duty introduced under the Environment Act 2021. Most new development in England now has to leave the natural world at least 10% better off than it was before, and prove it on a spreadsheet, for 30 years. Below is what that actually means for a small site.
Every habitat on the site is scored. Arable and improved grassland score low; ancient woodland, species-rich grassland and hedgerows score high. Multiply by condition (poor / moderate / good) and area to get the baseline number of "biodiversity units".
After the development is built, the new on-site habitats are scored again. The post-development score has to be at least 110% of the baseline. The gap between what you've got and the 10% uplift target is what you must mitigate.
Deliver gain on the development site first (cheapest). If you can't, buy off-site units from a habitat bank (mid-priced). Statutory credits sold by the government are a deliberately expensive last resort.
For the long-form definition, see our full BNG explainer.
Do I need BNG for my site?
If you're still figuring out whether biodiversity net gain even applies to you, here's a sixty-second yes/no check before you spend any money on consultants.
- You're applying for planning permission for one or more homes in England after April 2024
- You're doing a non-residential development larger than 25 m² of impacted habitat
- You're a minor commercial scheme under 1,000 m² — the small sites metric applies, but the 10% target still bites
- Your site contains hedgerows, mature trees, scrub or grassland of any quality — these count even when small
- You're selling a self-build plot to a future occupier who will build their own home (narrow exemption)
- You're extending an existing house
- The total impacted habitat is under 25 m² (a de minimis exemption applies)
- Your application is permitted development, not full planning permission
- The site is exclusively hard-standing with no vegetation — but check, even car parks usually have a verge that counts
Note: exemptions are narrow and the rules are evolving. The calculator above is the fastest way to find out the actual scale of liability if BNG does apply to you — drop your numbers in and you'll know in about ten seconds.
What does BNG typically cost?
Indicative 2025 unit costs for a small UK site. The calculator above will give you a tailored range based on your habitat, condition and built footprint — the table here is just for context.
On-site BNG via thoughtful planting is dramatically cheaper than buying off-site units. The calculator's "Land kept as habitat" slider lets you play with how much of your site you'd keep wild — see what difference it makes to the shortfall and budget.
A free BNG feasibility tool, not a paid consultation
Most landowners only need a rough indication before they decide whether to commission a proper ecological survey. This calculator gives you that indication in about ten seconds, for free, without an email address or a sales call.
Every slider re-runs the metric and re-prices the shortfall. Watch how a different house footprint, a smaller garden, or more retained habitat changes the bill in real time.
Try semis, detacheds, and bungalows. Swap improved grass for woodland. See what an extra 20% retained land does. Nothing is saved, nothing is sent — explore until the numbers click.
We tell you up front what this misses: hedgerows, watercourses, spatial significance, time-to-target factors. For an actual application you'll need an ecologist. For a first look, this gets you there.
Biodiversity net gain, answered
The questions landowners actually ask before booking an ecologist.
Do I need biodiversity net gain (BNG) for my site?
Most new planning permissions in England since February 2024 require a 10% biodiversity net gain, secured for 30 years. Small sites (1–9 homes, or non-residential under 1,000 m²) were brought in from April 2024. Self-build plots sold to a future occupier who builds their own home are exempt, as are house extensions and very small de minimis sites (under 25 m² of habitat). The BNG calculator above gives you an instant feasibility check for your land.
How much does BNG cost a typical small site?
For a four-home rural plot, BNG usually adds £5,000–£15,000 in survey, metric, and planting costs. On top of that you may need to buy off-site biodiversity units at £20,000–£42,000 per unit if you can't deliver enough net gain on your own land. Statutory credits sold by the government start at £42,000 per unit and are deliberately priced to be a last resort. The calculator gives you a likely cost range tailored to your habitat, condition, and built footprint.
Is BNG required for self-build plots?
Self-build and custom-build plots are exempt from BNG when the plot is sold to the future occupier who will build their own home. If you build the house and then sell it on the open market, the exemption does not apply. The exemption is narrow — speak to your planning authority before relying on it.
How is BNG measured?
The site is surveyed against the government's Statutory Biodiversity Metric (Metric 4.0). Habitats are scored on type, condition, area, strategic significance, and how hard they are to recreate. That gives you a baseline number of biodiversity units. After the development is built, the new on-site habitats are scored again. The post-development score has to be at least 110% of the baseline.
What's the difference between on-site, off-site, and statutory credit BNG?
On-site BNG means creating or enhancing habitat inside your red line — wildflower meadow, native hedges, ponds. It's the cheapest and policy-preferred route. Off-site BNG means delivering the units on other land — your own land elsewhere, or units bought from a habitat bank. Statutory credits are a government-sold backstop, priced high (£42,000+ per unit) to discourage their use. Almost every landowner should try on-site first.
Can BNG be a source of income for my land?
Yes. If you own land that is hard to develop but could be enhanced ecologically — low-grade pasture, scrubby corners, field margins — you can sell biodiversity units into the national BNG register. Pricing today sits around £20,000–£35,000 per unit, and a few acres of habitat creation can generate 20+ units. The catch is a 30-year management commitment locked in by Section 106 or Conservation Covenant.
Is this BNG calculator official?
No. This is a free rule-of-thumb tool built to give landowners and small developers a quick, honest first look at the numbers. For an actual planning submission you'll need a licensed ecologist to do a UKHab survey and run the proper Defra Metric 4.0 spreadsheet. Expect £1,500–£4,000 for a small-site survey and metric report.
How accurate is the cost estimate?
The calculator uses 2025 unit costs and Metric 4.0 logic, so the cost range it shows is realistic for most small sites. It does not model hedgerows, watercourses, spatial significance multipliers, time-to-target factors, or post-2030 management costs. Treat the output as a sensible budget range to start with — not a quote.
When the numbers feel real, here's what to do
Send us your site boundary and we'll come back with a planning read — including a rough BNG strategy — as part of a free landowner appraisal.
Start free site assessment →