TL;DR
- Biodiversity net gain (BNG) requires most new development in England to deliver at least a 10% increase in biodiversity value relative to the site's pre-development habitat. (GOV.UK biodiversity net gain)
- It became mandatory for major development for planning applications made on or after 12 February 2024, and was extended to small sites from 2 April 2024. The framework comes from the Environment Act 2021, via Schedule 7A of the Town and Country Planning Act 1990. (GOV.UK)
- Habitat enhancements secured for BNG must be maintained for at least 30 years after the development is completed. (GOV.UK)
- Some development is exempt, including householder development, self-build and custom-build of up to 9 dwellings on a site of up to 0.5 hectares, and de minimis impacts below the stated thresholds. Check the current exemption criteria on GOV.UK for your specific case. (GOV.UK)
- For an appraisal, BNG is a cost and a long-term commitment: on-site habitat, off-site units, or statutory credits, plus 30 years of monitoring and maintenance. The figures below are illustrative.
Biodiversity net gain is one of the biggest changes to the English development process in years, and it now applies to most schemes, not just large ones. If you are appraising a site, BNG is a line you have to understand, because it adds cost, can constrain your layout, and creates a commitment that lasts three decades. Here is what it is and how to treat it.
This is general information, not legal, ecological or investment advice. BNG is a technical statutory regime, the detail and thresholds can change, and the right figure for any site depends on an ecological assessment, so take specialist advice and confirm the current rules on GOV.UK before relying on anything here. Every figure below is illustrative.
What biodiversity net gain is
GOV.UK describes biodiversity net gain as "a way of creating and improving biodiversity by requiring development to have a positive impact ('net gain') on biodiversity." In practice the rule is that development must deliver at least a 10% increase in biodiversity value relative to the pre-development biodiversity value of the onsite habitat. (GOV.UK) Biodiversity value is measured using a statutory metric that scores habitats by type, condition and size, so the "10%" is a calculated figure, not a guess.
The law and the dates
BNG was established through the Environment Act 2021, specifically through Schedule 7A of the Town and Country Planning Act 1990, which the Environment Act inserted. (GOV.UK) The commencement was staged:
- 12 February 2024: mandatory BNG took effect for major development (for applications made on or after that date).
- 2 April 2024: the requirement extended to small sites (non-major development).
(GOV.UK) Because the trigger is the date the application is made, the position can differ between schemes that look similar, so check which regime applies to your application.
The 30-year commitment
BNG is not a one-off box-tick at consent. Significant habitat enhancements delivered for BNG must be maintained for at least 30 years after the development is completed. (GOV.UK) That long horizon matters for developers because it means BNG is secured through legal mechanisms (such as a planning condition, a planning obligation, or a conservation covenant) and carries ongoing monitoring and maintenance responsibilities or costs that outlast the build itself.
The delivery hierarchy: on-site, off-site, then credits
There are three broad ways to deliver the gain, generally considered in this order:
- On-site: create or enhance habitat within the development itself. This is usually preferred but uses land that could otherwise be developed, so it interacts directly with your density and layout.
- Off-site: deliver or buy biodiversity units elsewhere if you cannot achieve the full gain on-site.
- Statutory biodiversity credits: a last-resort option to buy credits from the government where on-site and off-site delivery is not possible.
The mix you choose changes both the cost and the developable area, which is exactly why BNG is a feasibility input, not a post-consent detail.
Exemptions
Not every project is caught. GOV.UK lists exemptions including:
- Householder development.
- Self-build and custom-build development of no more than 9 dwellings on a site no larger than 0.5 hectares.
- De minimis impacts below the stated thresholds (for example, where the development does not impact more than a small specified area of onsite habitat).
- Certain development under permitted development rights.
(GOV.UK) Exemption criteria are specific and can be updated, so confirm against the current GOV.UK guidance for your exact scheme rather than assuming an exemption applies.
How BNG hits your appraisal
In a residual land value appraisal, BNG shows up in more than one place:
- Direct cost: the ecological survey and metric assessment, the cost of creating on-site habitat, or the price of off-site units or statutory credits. This sits in your statutory and professional costs.
- Lost developable area: land given over to on-site habitat is land you are not building saleable units on, which can reduce GDV. That trade-off can be the more expensive one.
- Long-tail liability: 30 years of monitoring and maintenance, secured by legal mechanisms, which a buyer or funder will want to see properly provided for.
Illustrative example: a small scheme that has to set aside part of the site for on-site habitat and commission an ecological assessment carries both a direct cost and a reduction in the number of plots, and both come off the residual land value rather than off your fixed profit. Model BNG as early as you model build cost, not as an afterthought. Our feasibility appraisal guide shows where statutory costs like this sit in the full model.
FAQ
How much biodiversity net gain is required?
At least a 10% increase in biodiversity value relative to the pre-development value of the onsite habitat, measured using the statutory metric. (GOV.UK)
When did BNG become mandatory?
For major development, for planning applications made on or after 12 February 2024; for small sites (non-major development), from 2 April 2024. It is established through the Environment Act 2021. (GOV.UK)
How long must the biodiversity gain be maintained?
Significant habitat enhancements must be maintained for at least 30 years after the development is completed. (GOV.UK)
Is my development exempt from BNG?
Some development is exempt, including householder development, qualifying self-build and custom-build of up to 9 dwellings on up to 0.5 hectares, and de minimis impacts below the thresholds. The criteria are specific, so check the current GOV.UK guidance against your exact scheme. (GOV.UK)
Before you appraise a site
Biodiversity net gain is now a standard part of the development picture in England, with a direct cost, a potential hit to developable area, and a 30-year commitment behind it. Establish early whether your scheme is caught or exempt, get an ecological assessment to drive the metric, and put both the delivery cost and any lost area into your appraisal. Sanity-check the planning and site picture with the planning check and the site feasibility tool, and model the full scheme end to end with the feasibility wizard.