Biodiversity net gain on change of use and conversions

Biodiversity net gain on a change of use scheme usually comes down to two questions, and both are answered before any ecologist visits. Does the biodiversity gain condition apply to this permission at all, and if it does, is the scheme exempt because it touches almost no habitat? Get those right and a conversion of a sealed, built site can be discharged on paperwork. Get them wrong, most often by drawing a generous red line, and a scheme with no garden acquires a biodiversity liability.

The second trap is subtler. Not being exempt is not the same as needing to buy units.

Whether the gain condition applies at all

The biodiversity gain condition in Schedule 7A to the Town and Country Planning Act 1990 is deemed to be attached to planning permissions granted on application in England, from 12 February 2024 for major development and 2 April 2024 for small sites.

Permission granted by a development order is a different thing. Permitted development under the GPDO 2015, including the prior approval routes, is not a permission granted on application in that sense, so the gain condition does not attach to it. A Class MA conversion from commercial to residential, or a Class Q barn conversion, proceeds without a biodiversity gain plan.

That creates a real strategic choice on conversion schemes. Where both routes are available, prior approval avoids the regime and a full application does not.

Two cautions. The route has to be properly available, because a prior approval the class never allowed is unauthorised development, not an exemption. And the policy position is capable of change, so check the current guidance before relying on the distinction on a scheme that will be determined months from now.

Protected species law is unaffected either way. Prior approval gives no protection from the offences under the Conservation of Habitats and Species Regulations 2017 or the Wildlife and Countryside Act 1981, and a barn conversion with bats in the roof needs a licence regardless of which planning route was used. The sequencing of that work is set out in protected species and the demolition method statement.

The de minimis exemption and where it stops

For permissions that do carry the condition, the exemption that matters on conversions is the de minimis threshold. It applies where the development does not impact a priority habitat and impacts less than 25 square metres of on-site habitat, or less than five metres of on-site linear habitat such as hedgerow.

Twenty-five square metres is five metres by five metres. It is less than people assume.

• A grass verge inside the red line counts as habitat, even mown amenity grassland.
• A planting bed beside the entrance counts.
• A line of shrubs along a boundary fence counts, and if it is a hedgerow the five metre linear test applies instead.
• Self-sown scrub on an unused corner of yard counts, and may be more valuable than the mown grass.
• Only sealed surface, buildings and bare hardstanding fall outside the habitat accounting.

Priority habitat defeats the exemption entirely regardless of area. Open mosaic habitat on previously developed land is the one that catches urban sites, because a neglected yard colonised by ruderals can qualify, and it is a priority habitat.

The red line is a design decision

On a conversion, the application site boundary is the single most consequential drawing for biodiversity purposes.

A red line drawn tightly around the building and its sealed curtilage keeps the habitat impact at zero and the exemption available. A red line drawn around the whole title, including the grass bank and the shrub border nobody intends to touch, brings that habitat into the baseline and into the calculation.

There are limits to how far this can be pushed. The red line must include everything needed to carry out the development, including access, parking, bin and cycle storage and any area where construction will take place. Excluding land the scheme actually uses invites a validation refusal or, worse, an argument later that the permission does not cover the works.

Where land is excluded, say why in the covering letter. A plan with an explanatory note reads as considered; a plan with an unexplained notch reads as avoidance.

Baseline on a hardstanding or built site

Sealed surfaces and buildings score zero distinctiveness in the statutory metric, and zero distinctiveness produces zero biodiversity units.

The arithmetic consequence surprises people. Where the baseline is zero units, the gain objective is also zero, because the requirement is the baseline uplifted by 10 per cent. A scheme on entirely sealed land with no habitat at all therefore meets the objective with any post-development figure at or above zero.

What it does not avoid is process. Unless the development is exempt, the biodiversity gain condition still applies, which means a biodiversity gain plan has to be submitted and approved before development begins, with the metric calculation supporting it. The plan is short, but it is a pre-commencement requirement and a forgotten one stops the site.

So the two outcomes to aim for are clear. Exempt, with a short statement in the application explaining why. Or not exempt but with a zero or near-zero baseline, with the metric and the gain plan prepared early and submitted as soon as permission is granted.

Where small habitat parcels create liability

The awkward middle case is the site with a little habitat. Thirty square metres of amenity grassland and a short shrub border is enough to lose the exemption while generating a baseline that has to be replaced and bettered.

The middle row is where most conversions sit, and it is manageable. Replacing mown grass with a native-rich planting scheme, adding a green roof to a single-storey element and improving a boundary line will often close a deficit of that size on site.

A qualifying small development may be able to use the small sites metric rather than the full statutory metric, which reduces the survey and calculation effort. The qualifying criteria include limits on dwellings, floorspace and site area, and the absence of priority habitat or protected species, so confirm eligibility rather than assuming it.

Roof and wall features in the metric

Green roofs are included as an urban habitat type in the statutory metric and can generate units, with the value depending on substrate depth and the planting specified. A substrate of 80 millimetres of sedum mat is a different habitat from 150 millimetres of varied substrate with a wildflower mix, and the metric distinguishes them.

Living walls are treated more narrowly, and their place in the calculation should be checked against the current metric and user guide rather than assumed.

Bat boxes, bird bricks, swift boxes and hedgehog gaps generate no units at all. They are ecological mitigation and enhancement, secured by condition under separate policy, and they are frequently confused with net gain by applicants. Both are often required, for different reasons, and the detail of hedgehog gaps and connectivity conditions sits in that second category.

Securing gain without land, and the evidence to submit

Where on-site delivery cannot close the gap, the options are off-site units registered on Natural England's biodiversity gain sites register, or statutory biodiversity credits sold by government as a last resort and priced accordingly.

For a conversion with a small deficit, off-site units are usually the answer, and the spatial risk multiplier means units sourced inside your local planning authority area or National Character Area count in full while distant ones are halved.

The gain plan itself needs the metric calculation file rather than a summary, the pre-development and post-development habitat plans, the condition assessments with survey dates, the red line plan, any off-site allocation evidence, and the management commitment for 30 years where it is relied on.

A Biodiversity Net Gain Assessment drafted on PlanScope will set out the structure, the policy context and the data the metric needs, and will flag missing site facts rather than inventing them. The Environment Act 2021 regime expects the statutory metric to be completed by a competent person and the gain plan to be approved by the local planning authority, so the draft is a starting point for that ecologist rather than a substitute. Where the baseline needs a site survey, a Preliminary Ecological Appraisal is the document that supplies it.

One honest uncertainty: authorities differ in how rigorously they test a claimed de minimis exemption. Some accept a statement, others ask for a habitat plan proving the area. Prepare the plan either way, because producing it on request is faster than arguing about it.

This is general guidance, not legal advice; check the 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.