Mandatory biodiversity net gain: the statutory framework

Mandatory biodiversity net gain in England works through a condition that is deemed into planning permissions rather than through a policy an officer applies. Schedule 14 to the Environment Act 2021 inserted Schedule 7A into the Town and Country Planning Act 1990, and the effect is that most planning permissions carry a condition preventing development from beginning until a biodiversity gain plan has been submitted to and approved by the planning authority.

That is a structural difference from every other environmental requirement in planning. There is no negotiation about whether it applies. Either the development is exempt, or the plan has to be approved before anyone puts a spade in the ground.

Where the duty sits in statute

The operative provisions are in Schedule 7A to the 1990 Act. They set the general biodiversity gain objective: the biodiversity value attributable to the development must exceed the pre-development biodiversity value of the onsite habitat by at least 10 per cent, measured using the statutory biodiversity metric.

Three features of the drafting matter in practice.

The condition is deemed, so it appears on permissions whether or not the decision notice mentions it. Authorities normally do state it, but its absence does not remove it.

The gain is measured in biodiversity units produced by the statutory metric, not in area, tree numbers or any local proxy. A planting scheme that looks generous can score badly if it replaces distinctive habitat with amenity grassland.

And the obligation runs for at least 30 years from completion of the development, which is far longer than any construction programme and longer than most ownership periods.

When it applied from

The requirement applied to major development from 12 February 2024 and to small sites from 2 April 2024. Applications validated before those dates are generally outside the regime, which is why schemes with older permissions are still being built out without a gain plan.

Nationally significant infrastructure projects were dealt with on a separate timetable, and the position there has moved. Check the current gov.uk guidance rather than relying on a date quoted in an older note.

Exemptions written into the regulations

The exemptions are set out in regulations rather than in the Act, and they are narrower than developers hope.

• Development below the de minimis threshold, which turns on impacts to a very small area of on-site habitat and a short length of linear habitat
• Householder development, meaning extensions and alterations to a single dwellinghouse
• Self-build and custom build development within limits on the number of dwellings and the site area
• Development on a biodiversity gain site itself
• Urgent Crown development

The de minimis exemption is the one most often misapplied. It is assessed against habitat affected, not against site area or built footprint, and a small building placed on species-rich grassland is not exempt because the building is small. Establish the habitat baseline before concluding that the exemption applies.

Take the full list from the current regulations. The exemptions have been amended since the regime began, and a summary in an old article is not a safe basis for a programme.

The metric, and who is expected to run it

The statutory biodiversity metric assigns units to each habitat parcel by reference to its distinctiveness, condition, strategic importance and area, with separate calculations for hedgerows and watercourses. A small sites metric exists for qualifying schemes.

The Environment Act 2021 regime expects the metric to be completed by a competent person, and this is one of the report types where a licensed or competent professional is legally required rather than merely advisable. A Biodiversity Net Gain Assessment drafted by PlanScope is a starting point for that professional to check, complete and take responsibility for. It is not a substitute for their work, and no AI draft removes the need for a site survey.

Habitat condition assessment is the part that cannot be desk-based. Condition is judged against defined criteria in the field, in the right season, and a metric run on aerial imagery will not survive scrutiny. The habitat baseline usually comes out of a Preliminary Ecological Appraisal carried out at the right time of year.

Irreplaceable habitats sit outside the metric altogether. Ancient woodland, ancient and veteran trees, blanket bog and certain other habitats cannot be traded, and bespoke compensation arrangements apply.

The gain hierarchy

Deliver on site first. Where on-site delivery cannot achieve the required uplift, use off-site units, either on land you control or bought from a habitat bank registered on the biodiversity gain sites register. Statutory biodiversity credits sold by government are the last resort and are deliberately priced to be unattractive.

The trading rules constrain the substitution. Losses of higher distinctiveness habitat must be compensated with habitat of the same type or of higher distinctiveness, so a scheme cannot simply buy a larger area of a lower-value habitat. The detail of how that plays out on a real site is covered in our article on trading down and off-site sections of a BNG plan.

Spatial risk multipliers reduce the value of units delivered further from the development, which is why local off-site delivery is worth more than distant delivery of the same habitat.

Approval before development begins

The biodiversity gain plan is the document that discharges the deemed condition. It sets out the pre-development biodiversity value, the post-development value, the on-site and off-site gains relied on, any credits, and the steps taken to follow the hierarchy.

Timing is the operational risk. The plan must be approved before development begins, and the definition of beginning development is the ordinary planning one, so site clearance and demolition can trigger it. Programmes that assume the plan can be settled alongside early works are the ones that go wrong. Our article on the biodiversity gain plan covers the document itself in detail.

One drafting point worth knowing. The pre-development value is normally assessed at the date of the planning application, but where habitat has been degraded after 30 January 2020 without permission, the authority can assess the value as it was before that degradation. Clearing a site to improve the baseline does not work and it makes the ecologist's job harder.

Securing 30 years

On-site habitat is typically secured by planning condition and by a habitat management and monitoring plan running for 30 years. Off-site gains are secured through a section 106 agreement or a conservation covenant with a responsible body, and registered on the biodiversity gain sites register maintained by Natural England.

Price the whole 30 years. Establishment, replacement planting, monitoring reports at defined intervals and remedial works all cost money after the last unit is sold, and the management body needs a funding mechanism that survives the developer's exit.

Illustrative example: a 40-unit scheme on an arable field with a species-poor hedgerow along one boundary. Arable land scores low, so the on-site uplift from structural planting, a wildflower meadow and hedgerow enhancement gets close to the target. The residual shortfall is met with off-site units in the same local authority area, secured by agreement and registered. Had the same scheme sat on unimproved grassland, the on-site loss would have been far harder to replace and the off-site requirement several times larger.

Follow reform news on the planning law timeline, and note that regulations in this area continue to change.

This is general guidance, not legal advice; check the current regulations and confirm the position with your local planning authority or a qualified professional before relying on it. The statutory metric and the biodiversity gain plan require a competent person. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.