Biodiversity net gain on small sites and the exemptions

Biodiversity net gain applies to almost every planning application for a new dwelling, and the small sites route is a lighter version of the same duty rather than a way out of it. The biodiversity gain condition was inserted into the Town and Country Planning Act 1990 by the Environment Act 2021, took effect for major development on 12 February 2024 and for small sites on 2 April 2024. If you are building one house on a plot, you are in scope unless a listed exemption applies. Most self-builders discover this at validation.

What the gain condition actually requires

Every permission granted subject to the condition carries the same shape of obligation. The development must deliver at least a 10 per cent increase in biodiversity value measured by the statutory metric, and that value has to be maintained for 30 years. Before you can start work, the local planning authority must approve a biodiversity gain plan.

The condition is deemed. It attaches automatically to the grant of permission whether or not the council writes it out in the decision notice. You do not get to argue about whether it applies once permission is issued. The only questions are whether your development is exempt and how you will meet the target.

The small sites definition and the dates

A small site for these purposes is residential development of between one and nine dwellings on a site smaller than one hectare, or, where the number of dwellings is not known, a site of less than 0.5 hectares. For non-residential development it is floorspace of less than 1,000 m2 on a site of less than one hectare.

The date matters for applications made before the regime started. Permissions granted on applications submitted before the relevant date are outside the regime, and that transitional position is still worth checking on older consents you are about to implement. If your scheme straddles the boundary between minor and major, two dwellings on one plot sets out what else changes with the unit count.

Exemptions and how narrow they are

The Biodiversity Gain Requirements (Exemptions) Regulations 2024 list the exempt cases. Four of them come up regularly on small schemes.

• De minimis development, where the works affect less than 25 m2 of onsite habitat that has a biodiversity value above zero, and less than five metres of linear habitat such as hedgerow.
• Householder applications, so an extension, a loft conversion or a garden building on your own house is out of scope.
• Self-build and custom housebuilding, for up to nine dwellings on a site no larger than 0.5 hectares, where the development meets the statutory definition of self-build or custom housebuilding.
• Development on a biodiversity gain site, which is land already registered to deliver gains for someone else.

The de minimis test is tighter than it sounds. A plot of mown amenity grassland has a biodiversity value above zero, so clearing 200 m2 of it takes you outside de minimis even though nothing of ecological interest is being lost. Hard standing, buildings and sealed surfaces score zero, which is why infill on a concrete yard sometimes does qualify.

The self-build exemption is the one most often claimed and most often lost, because it turns on the statutory definition rather than on the fact that you are building a house for yourself. The interaction with other reliefs is set out in self-build exemptions across CIL and biodiversity net gain, and the two regimes do not use the same test.

Permitted development is not subject to the gain condition, which is a real distinction for barn conversions and office conversions taken through prior approval. Permission in principle behaves differently again, because the condition attaches at technical details consent rather than at the first stage, and permission in principle and the small site limit explains where that route sits for a plot of this size.

The small sites metric and when you can use it

Defra publishes a simplified small sites metric alongside the full statutory metric. It asks for less habitat detail and produces a unit figure in the same currency.

You can use it only where the development qualifies as a small site, where priority habitat is not present other than limited hedgerow and watercourse cases, and where no protected species or designated site issues arise. If a preliminary ecological appraisal finds a pond with great crested newts, or a strip of lowland meadow, the simplified tool is no longer available and the full metric applies.

Councils will usually accept a metric completed by an experienced ecologist, and the Environment Act regime expects a competent person to complete the statutory metric. A draft you prepare yourself is a starting point for that person rather than a substitute for them.

Getting the baseline right

The pre-development baseline is the habitat on site on the relevant date, which is normally the date of the planning application. Habitat cleared before you apply does not reduce your baseline if the clearance counts as degradation. The rules allow the authority to assess the baseline as it was on an earlier date, going back to 30 January 2020, where habitat has been degraded without consent.

So strimming the wildflower margin the week before you submit achieves nothing except a worse starting position, since the council can assess you on what was there. Photographs, dated survey notes and an ecologist's site walkover are what protect you. Survey in the growing season where you can, because a February walkover on a frozen field will either understate or overstate what is present.

The gain plan and the order of events

The sequence trips people up because the plan comes after permission, not with the application.

1. Submit the application with the metric calculation and habitat plans, since most validation lists now ask for them.
2. Receive permission, which carries the deemed biodiversity gain condition.
3. Prepare the biodiversity gain plan setting out the baseline, the proposed habitat creation and enhancement, and the 30 year management.
4. Submit the plan to the local planning authority and wait for approval.
5. Only then start development.

Onsite delivery is the least costly route on a small site where there is space for a hedgerow, a wildflower area or a pond. Where the site cannot absorb the gain, you buy offsite units from a registered gain site, or statutory credits from the government as a last resort at prices set to be unattractive. Offsite land has to be registered on the biodiversity gain sites register and secured for 30 years by a section 106 agreement or a conservation covenant.

The 30 year commitment is what small developers underestimate. Someone has to manage that hedgerow, and the obligation runs with the land after you sell.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

PlanScope drafts a Biodiversity Net Gain Assessment as a starting point for the ecologist who will complete the statutory metric, and the report is labelled as one where a competent person is legally required. Enter your postcode on PlanScope to see which reports your site needs.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.