Biodiversity net gain: where the small sites line falls

The biodiversity net gain small sites threshold is set by a statutory definition rather than by judgement. For residential development, a small site means between one and nine dwellings on a site of less than one hectare, or where the number of dwellings is not known, a site smaller than half a hectare. For non-residential development, it means floorspace under 1,000 square metres or a site area under one hectare. Fall inside that, and a simplified version of the statutory metric is available and the mandatory gain regime applies in its small sites form. Fall outside, and you are on the full metric with the requirements that go with it.

The regime itself came into force for major development on 12 February 2024 and for small sites on 2 April 2024, under the biodiversity gain condition inserted into planning legislation by the Environment Act 2021.

The statutory definition of a small site

Read the definition against your own scheme carefully, because the residential test has two limbs and they behave differently.

Where the number of dwellings is known, the test is one to nine dwellings and a site area under one hectare. Both have to be satisfied. Nine houses spread across 1.2 hectares is not a small site.

Where the number of dwellings is not known, as in an outline application with the quantum reserved, the test is site area under half a hectare. That is a materially tighter figure, and it is a reason to fix the unit numbers in the application description where you can.

For non-residential development the test is floorspace under 1,000 square metres or site area under one hectare. Note that this limb is expressed differently from the residential one, so check the current wording rather than assuming symmetry.

Mixed schemes need care. A scheme with eight dwellings and 400 square metres of commercial floorspace has to be tested against both limbs, and the outcome is not always intuitive.

Exemptions and how narrowly they are read

Several exemptions sit alongside the small sites definition, and they are drawn tightly.

The de minimis exemption applies where the development impacts less than 25 square metres of on-site habitat and less than five metres of on-site linear habitat such as hedgerow. That is a small footprint, roughly a modest garden building. Both limbs must be satisfied.

Householder applications are exempt, which covers extensions and works within the curtilage of a dwellinghouse.

Self-build and custom build development is exempt where the scheme consists exclusively of self-build or custom build dwellings, has no more than nine dwellings, and sits on a site no larger than half a hectare. All three conditions apply, and the exclusivity requirement means a scheme mixing a self-build plot with a market plot loses it.

Development on a biodiversity gain site is exempt, as is urgent Crown development.

Two practical points. Exemptions are claimed on the application form and the council checks them, so a wrongly claimed exemption produces a validation problem rather than a quiet pass. And a section 73 application can bring a permission into the regime where the original was outside it, depending on the dates involved, which is a trap on older consents being varied now.

The metric version to use on a small site

Qualifying small sites can use the simplified small sites version of the statutory metric, which asks for less habitat detail and is designed to be usable without a full ecological survey in straightforward cases.

Its availability is conditional. It cannot be used where the site contains priority habitat other than specified types such as hedgerows and arable field margins, where the site falls within or adjacent to certain designated areas, or where the development affects European protected species requiring a licence. Where any of those apply, the full metric is needed even though the scheme is small.

Use the version of the metric current at the date of the biodiversity gain plan, and say which version you used. Metric versions are published and superseded, and a plan prepared on a superseded version is a common reason for refusal to approve.

A Biodiversity Net Gain Assessment under the Environment Act 2021 regime expects the statutory metric to be completed by a competent person, and the biodiversity gain plan has to be approved by the local planning authority. An AI-assisted draft is a starting point for that person rather than a substitute for them.

Pre-development baseline and its timing

The baseline is the habitat on site immediately before development, valued in biodiversity units using the metric.

Two timing rules matter more than anything else in the regime.

The baseline is normally taken at the date of the application, or an earlier date agreed with the authority. So a site cleared the week before you apply has a lower baseline, which looks advantageous.

And that is exactly what the degradation rule prevents. Where habitat has been degraded after 30 January 2020 without planning permission, the baseline is taken as at the date immediately before the degradation, on the basis of the best available evidence. Authorities use historic aerial imagery to check, and the imagery is easy to obtain.

The practical instruction is simple. Do not clear the site. Not the scrub, not the hedge, not the rough grassland. A morning with a flail can add months and real cost to the scheme, and it is visible from the air.

Survey timing is the other programme risk. Habitat condition assessment needs to be done when vegetation is identifiable, broadly April to September for grassland, so an autumn instruction can mean waiting. A Preliminary Ecological Appraisal commissioned early picks up both the habitat baseline and any protected species issue in the same visit.

The biodiversity gain plan and approval

Permission granted subject to the biodiversity gain condition cannot be implemented until the authority has approved a biodiversity gain plan. This catches developers who expect to deal with it later.

The plan sets out the pre-development baseline, the post-development on-site value, any off-site units, any statutory credits, and how the 10 per cent gain is achieved. On-site gains have to be secured and maintained for at least 30 years, through a condition, a planning obligation or a conservation covenant, and significant on-site habitat has to be covered by a habitat management and monitoring plan.

Allow time. The plan is a separate approval after permission, and councils have limited ecological capacity. Where the gain is delivered off site, the units have to be registered on the statutory register and allocated to your development before the plan can be approved, which is a further step with its own lead time.

Off-site units and credits at small scale

Three routes deliver the gain, in a fixed order of preference: on site, off site through registered units, and statutory credits as a last resort.

On a nine unit scheme, on-site delivery is often achievable because the baseline is low. A site of improved grassland or hardstanding starts from a small number of units, and a well designed scheme with native hedgerow, a modest area of species-rich grassland and some tree planting can exceed the requirement without losing developable area.

Where it cannot, off-site units are bought from a registered gain site, priced per unit and varying substantially by habitat type and region. Statutory credits from government are deliberately priced above market rates to encourage the other routes, and using them requires evidence that the other options were pursued.

Two small-scale cautions. Fractions of units are traded, but some providers set minimum transaction sizes that make a small purchase disproportionately expensive. And the requirement is measured by habitat type, so losing a hedgerow means replacing hedgerow units specifically, not simply buying the cheapest available.

Programme consequences for a nine unit scheme

Set against a scheme that would once have gone from instruction to submission in eight weeks, the regime adds:

1. An ecology instruction at the start rather than after design freeze.
2. A habitat survey timed to the season, which can be the binding constraint.
3. A metric calculation and a design iteration to accommodate on-site habitat.
4. Registration and allocation of off-site units where needed, before the gain plan can be approved.
5. A gain plan approval after permission and before commencement.
6. A 30 year management and monitoring obligation, with the cost provided for.

The last one is the one small developers underestimate. Thirty years of management for a hedgerow and a wildflower strip has to be funded and secured, usually through the obligation, and the commuted sum is calculated on a realistic cost. Other requirements that appear as schemes cross thresholds follow a similar pattern, as set out in statutory consultees that appear as a scheme grows and in housing mix and space standards as schemes grow.

This is general guidance, not legal advice. Thresholds, exemptions and metric versions are set in legislation and revised, so check the current position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.