The small sites metric: when it can be used

The small sites biodiversity metric is a simplified version of the statutory metric, built so that minor development does not have to fund a full habitat assessment to demonstrate a 10 per cent gain. For a self-builder putting up one or two houses on a paddock, it is usually the right tool. It is also the tool most often used on sites that do not qualify for it, which produces a rejected biodiversity gain plan and a delay measured in months.

Before reaching for either metric, check whether the gain condition applies to your scheme at all. Several exemptions take small development out of the regime entirely, and a self-build scheme is one of them.

Exemptions from the gain condition

Four exemptions matter most at the small end.

The de minimis exemption applies where the development affects less than 25 square metres of on site habitat and less than five metres of on site linear habitat such as hedgerow. A modest extension or an infill plot within an existing garden of amenity grass will often fall inside it, and a plot that takes out 30 metres of hedgerow will not.

Householder development is exempt, so an extension, a garage or a garden room within the curtilage of an existing dwelling does not engage the condition.

Self-build and custom build development is exempt where the scheme consists exclusively of dwellings of that kind, the number of dwellings is no more than nine, and the site is no larger than 0.5 hectares. Read the definition carefully, because the exemption turns on the dwellings really being self-build or custom build, with the individual commissioning the home to occupy it.

Biodiversity gain sites themselves are exempt, along with certain urgent Crown development and specified infrastructure.

Check the current exemptions before assuming one applies, and record in the application which exemption you rely on and why. Authorities validate this, and an unexplained assertion of exemption is a common reason for an application being returned.

Which sites can use the small sites metric

Where the condition does apply, eligibility for the simplified metric depends on both the scale of the development and what is on the land.

On scale, a small site is broadly residential development of between one and nine dwellings on a site under one hectare, or where the number of dwellings is unknown, a site under half a hectare. For non-residential development the equivalent is floorspace under 1,000 square metres or a site under one hectare.

On the land itself, the simplified metric is not available where the site contains priority habitat, with limited exceptions for hedgerows and arable field margins. Nor is it available for development requiring an environmental impact assessment, or for nationally significant infrastructure.

Check the current user guide published with the metric, because the eligibility criteria are set there rather than in the legislation and they have been refined since the regime began.

Habitats and features that disqualify a site

This is where self-builders get caught, because the disqualifying features are often the ones that look like nothing.

Priority habitats include lowland meadows, other species-rich grasslands, lowland heathland, traditional orchards, ponds meeting certain criteria, wood pasture and parkland, and several woodland types. A field that has not been ploughed or fertilised for decades may be a lowland meadow. An overgrown corner with three old apple trees may be a traditional orchard. Neither looks remarkable in February.

Watercourses bring their own complication, since river units are assessed separately and the simplified metric handles them differently from the full one.

Designations are a separate question. Where the site is within or adjacent to a site of special scientific interest or an internationally designated site, the habitats regulations and the designation's own protections apply on top of the gain requirement, and a simplified metric will not answer those.

The practical answer is to have someone look at the land before deciding which metric to use. A Preliminary Ecological Appraisal with a walkover in the growing season tells you what you have, and that determines everything that follows.

What the simplified metric asks for

The small sites metric asks for less than the full version and still asks for real information.

You identify each habitat parcel on the site and classify it, measure its area, and assess its condition using a simplified scheme rather than the full condition assessment sheets. Hedgerows are measured in metres and classified. You then do the same for the proposed habitats after development, and the tool calculates whether the scheme achieves the required uplift.

Two simplifications are worth knowing about. The condition assessment is coarser, which reduces survey effort and also reduces your ability to demonstrate that retained habitat is in good condition. And the tool applies precautionary assumptions in places, which tends to produce a more conservative answer than the full metric on the same site.

The output feeds the biodiversity gain plan, which has to be submitted and approved by the local planning authority before development can lawfully begin. The plan needs the pre-development and post-development scores, the habitat plans, and details of any off site units or credits relied on.

Who can complete it

The metric was designed to be usable without specialist software, and habitat identification is still a skilled task.

Government guidance expects the habitat classification and condition assessment to be carried out by someone competent to do it, and many local planning authorities say plainly that they will not accept a metric completed by an applicant with no ecological training. Misclassifying a habitat type is not a technicality: it changes the baseline score, and the baseline drives everything.

A sensible split on a small site is an ecologist doing the survey and classification, and the applicant or agent assembling the plan and the drawings around it. That keeps the cost proportionate while producing something the authority will accept.

Where protected species are also in play, that is separate work again. Buildings due for demolition and mature trees both need checking, and a bat survey has seasonal constraints that can delay a scheme by months if left late.

Errors that cause rejection

The same handful of mistakes account for most returned gain plans on small sites.

1. Surveying in winter, so species-rich grassland is recorded as modified grassland and the baseline is wrong in both directions.
2. Using the simplified metric on a site containing priority habitat, which disqualifies it.
3. Omitting hedgerows, or measuring them to the wrong line, since hedgerow units balance separately from area units.
4. Ignoring habitat lost to construction access, storage and haul routes, which counts even though it is temporary.
5. Claiming retention of habitat that will be inside the construction working area with no protection specified.
6. Submitting the metric without the gain plan, or submitting the gain plan after starting work.
7. Assuming an exemption applies without stating which one and why.

The fifth is the one that causes arguments on site. Habitat you intend to retain has to be protected, with fencing shown on a drawing, or it will be re-scored after damage.

When the full metric is the better choice

Counterintuitively, paying for the full metric sometimes costs less overall.

Because the simplified tool is precautionary, a site with good habitat that you intend to retain can score better under the full metric, where condition can be evidenced properly rather than assumed. If the simplified result shows a shortfall that would mean buying off site units, run the numbers on a full assessment before buying anything. The difference in survey cost is often smaller than the cost of the units.

The full metric is also the right choice where the site is close to an eligibility boundary, where the scheme might grow, or where the authority has an ecologist who scrutinises these things closely. And it is the only option where priority habitat is present.

PlanScope drafts a Biodiversity Net Gain Assessment and supporting documents structured to UK planning policy, with clear flags where habitat survey data still has to come from an ecologist. The statutory metric and the biodiversity gain plan must be completed by a competent person under the Environment Act 2021 regime, so a draft is a starting point for that professional rather than a substitute. Reports are AI-assisted drafts that should be reviewed before submission, and we do not guarantee planning approval. This is general guidance, not legal advice: check the current eligibility criteria and exemptions with your local planning authority before relying on them.