Small sites and the April 2024 biodiversity commencement
From 2 April 2024 a small site permission carries a biodiversity gain condition, so development cannot begin until the council approves a gain plan. Survey timing is the constraint most self-builders underestimate.
Mandatory biodiversity net gain reached small sites in England on 2 April 2024, seven weeks after it applied to major development on 12 February 2024. From that date a planning permission for a small site carries a biodiversity gain condition, which means development cannot lawfully begin until the local planning authority has approved a biodiversity gain plan showing at least a 10 per cent gain measured with the statutory metric. For a self-builder putting up one or two houses, that is a survey, a calculation and an approval step that did not exist before.
What counts as a small site
The definition is set out in the regulations and it is not the same as the definition of minor development that planners use in other contexts.
For residential development, a small site is one where the number of dwellings is between one and nine on a site of less than one hectare, or, where the number of dwellings is not known, a site of less than half a hectare.
For non-residential development, it is a site where the floorspace to be created is less than 1,000 m², or where the site area is less than one hectare.
Read those against your scheme carefully, because a site of 0.9 hectares delivering nine houses is a small site while the same site delivering ten is not, and the difference changes both the metric available and the level of evidence expected.
What the April 2024 date actually did
The commencement did not change the substance of the requirement. It extended the same regime to smaller schemes: a minimum 10 per cent gain over the pre-development biodiversity value, secured for at least 30 years where the plan relies on created or enhanced habitat, with a biodiversity gain plan approved before development begins.
Two dates matter for any given application.
Whether the application was made before or after the relevant commencement date, since transitional provisions turned on that.
And the pre-development biodiversity value date, which is fixed by the regulations and is not simply the date somebody walked the site. Clearing vegetation in advance to lower the baseline does not work, because the regime provides for the value to be assessed as at an earlier date where degradation has taken place.
As at the date of writing, the small sites regime has been in force since 2 April 2024. The extension of the regime to nationally significant infrastructure was legislated for separately, and its commencement should be checked on gov.uk rather than assumed.
The small sites metric and when it can be used
Defra publishes a simplified small sites metric alongside the full statutory metric, intended to reduce the burden on schemes where the ecological picture is straightforward.
Eligibility is conditional. The simplified version is available only where the site meets stated criteria, which relate to the absence of certain priority habitats, the absence of protected species or designated sites, and limits on the extent of habitat involved. Where any of those are present, the full statutory metric applies.
Check the current eligibility conditions on gov.uk before assuming the simplified route is open, because the criteria have been refined since launch. And note the practical point that establishing eligibility itself requires an ecological assessment, so the simplified metric reduces the calculation work rather than removing the survey.
Whichever version applies, the Environment Act 2021 regime expects the metric to be completed by a competent person. A Biodiversity Net Gain Assessment drafted with AI assistance is a starting point for that person and never a substitute for them.
Exemptions worth checking before you spend anything
Several exemptions exist and two of them matter to small developers.
The de minimis exemption applies where the development affects habitat below stated thresholds, expressed as a small area of on-site habitat and a short length of linear habitat. It is narrower than it sounds, and a scheme that removes a hedgerow will rarely qualify.
The self-build and custom housebuilding exemption applies to development consisting of no more than nine dwellings on a site of no more than half a hectare, where the dwellings are exclusively self-build or custom build. Every limb has to be satisfied, including the exclusivity requirement, so a scheme with one open market plot alongside three self-build plots falls outside it.
Householder applications are also outside the regime, which is why an extension does not trigger a gain plan while a new dwelling in the same garden does.
Confirm any exemption in writing with the council before relying on it, and keep the correspondence. The condition is deemed to be attached to the permission, so the evidence that it does not bite is worth having on file when a purchaser's solicitor asks.
The evidence a gain plan needs
A gain plan is a short document standing on a longer piece of survey work.
- A habitat baseline survey identifying habitat types and condition, carried out to the relevant methodology and at a time of year when the habitats can be identified.
- The metric calculation showing pre-development and post-development biodiversity value.
- Plans showing retained, enhanced and created habitats.
- A habitat management and monitoring plan covering the 30 year period where created or enhanced habitat is relied on.
- The mechanism securing the gain, whether a condition, a planning obligation or a conservation covenant.
Timing is the constraint most often underestimated. Botanical surveys need the growing season, and a baseline walked in January will either be caveated heavily or will have to be repeated. Where protected species may be present, a Preliminary Ecological Appraisal should come first, since its findings determine whether further survey work is needed and when it can be done.
Securing gain on a constrained plot
Small urban plots are often mostly hard surface, which produces a low baseline and makes the 10 per cent easy to achieve in absolute terms. Rural plots with diverse grassland or a mature hedgerow are the difficult ones, because the baseline is high and the trading rules require replacement habitat of the same broad type at the same or better distinctiveness.
Practical measures that work on a small site include retaining and extending hedgerows rather than replacing them, converting amenity grassland to a sward of native grasses and wildflowers with a proper management regime, adding native shrub and tree planting with room to mature, and creating a small pond where hydrology allows.
Measures that disappoint on the metric include ornamental planting, individual specimen trees in hard landscape, and green roofs on a small footprint, all of which are worth doing and none of which moves the number much.
Where on-site gain cannot be achieved, the hierarchy runs to off-site units registered on the biodiversity gain sites register, and then to statutory biodiversity credits as a last resort. Credits are priced to be the least attractive option, which is deliberate.
Where small site schemes come unstuck
Four failures account for most of the trouble.
A baseline survey carried out at the wrong time of year, producing a plan the council will not approve without a repeat visit in the growing season.
An optimistic condition assessment that an ecologist reviewing for the council disagrees with, which changes the required gain.
A 30 year management commitment that nobody has worked out how to fund or who will carry out after the plot is sold.
And a gain plan left until after the permission is granted, when the applicant discovers that development cannot begin until it is approved.
The fix for all four is the same: treat the ecology work as part of the pre-application programme rather than as a condition to be discharged later.
Follow changes to the regime on the planning law timeline, and enter your postcode on PlanScope to see which reports your plot is likely to need. 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. This is general guidance rather than legal advice, so verify the current requirements on gov.uk and confirm the position with your local planning authority before relying on it.
Frequently asked questions
What counts as a small site for biodiversity net gain?
For residential development, between one and nine dwellings on a site of less than one hectare, or a site of less than half a hectare where the number of dwellings is not known. For non-residential development, less than 1,000 square metres of floorspace or a site area of less than one hectare. The definition differs from minor development elsewhere in planning.
Can I use the small sites metric?
Only where the site meets the stated eligibility conditions, which relate to the absence of certain priority habitats, protected species and designated sites, and to limits on the extent of habitat involved. Establishing eligibility still needs an ecological assessment, so the simplified metric reduces the calculation work rather than removing the survey.
Does the self-build exemption apply to my scheme?
Only if every limb is satisfied: no more than nine dwellings, a site of no more than half a hectare, and dwellings that are exclusively self-build or custom build. One open market plot alongside self-build plots takes the scheme outside it. Confirm any exemption in writing with the council and keep the correspondence on file.
Can I clear the site before the survey to lower the baseline?
No. The regulations provide for the pre-development biodiversity value to be assessed as at an earlier date where habitat has been degraded, so clearance in advance does not reduce the requirement. It does make the assessment harder and more expensive, and it creates a problem when the council asks what was there before.
When does the gain plan have to be approved?
Before development begins. The biodiversity gain condition is deemed to be attached to the permission, so starting work without an approved plan is a breach. That is why the survey and metric work belongs in the pre-application programme rather than being left as a condition to discharge afterwards.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.