Drafted biodiversity text against the statutory metric

A biodiversity net gain draft versus metric comparison comes down to one line: the metric produces habitat units and the draft produces sentences, and the biodiversity gain condition is discharged on units. No amount of well-structured narrative about ecological enhancement will satisfy a local planning authority that needs to see a pre-development value of 4.62 habitat units and a post-development value of 5.08.

That is not an argument against drafting the narrative. Roughly two thirds of what a planning authority reads on biodiversity is prose, and the prose is where schemes explain the mitigation hierarchy, the delivery route, the management commitments and the fit with local strategy. The error is treating the prose as the submission.

What the statutory metric actually calculates

The statutory biodiversity metric converts habitat into units. For each parcel on site, pre-development and post-development, it multiplies area by a distinctiveness score for the habitat type, by a condition score assessed against published criteria, and by a strategic significance multiplier reflecting whether the location is identified in local strategy.

Created and enhanced habitat is then discounted. There is a difficulty multiplier reflecting how hard that habitat type is to create or improve, a temporal multiplier reducing the value of habitat that takes years to reach target condition, and a spatial risk multiplier penalising off-site delivery further from the development.

Hedgerows and watercourses are calculated separately, in their own units, and they cannot be traded against area habitats. Nor can area habitats fully substitute for each other: the trading rules require like for like or better for medium and high distinctiveness habitat, which is why losing a patch of species-rich grassland cannot be made good with amenity lawn however large the lawn is.

The outputs are three numbers per unit type, being the baseline, the post-development total and the percentage change, with the requirement set at a 10 per cent uplift under the Environment Act 2021 regime.

None of that can be inferred from a site description. It depends on a habitat survey classified to the UK Habitat Classification and a condition assessment made on the ground.

Why narrative text cannot substitute for habitat units

Four things make the gap unbridgeable.

The habitat classification is a field judgement. Distinguishing modified grassland from other neutral grassland, which changes the distinctiveness score materially, requires a botanical survey by someone who can identify the indicator species.

Condition assessment is scored against published criteria with multiple pass or fail attributes per habitat type, and the result moves the unit count by a large factor. A hedgerow in poor condition and the same hedgerow in good condition are different numbers.

Area measurement has to be spatially explicit. Units are calculated parcel by parcel from a mapped habitat plan tied to the site boundary and the proposed layout, and a layout change of a few metres moves the answer.

And the metric is a published calculation tool with a specific version. The version used has to be recorded, because results are not comparable between versions.

A drafted section that asserts a percentage gain without a completed calculation behind it is worse than one that leaves the figure open, because it looks like evidence and is not. The general pattern here is the one described in what AI drafts get right about structure and wrong about evidence.

Baseline condition assessment and who may do it

The regime expects the metric to be completed by a competent person, and in practice that means an ecologist with the habitat identification and condition assessment competence the published guidance describes. For a Biodiversity Net Gain Assessment a licensed or competent professional is legally required under the Environment Act 2021 framework, and an AI-assisted draft is a starting point for that professional rather than a substitute for them.

Three practical constraints on the baseline survey:

1. Season. Botanical condition assessment needs the growing season, broadly April to September, and a January survey will either understate species richness or have to be repeated.
2. Access. The baseline covers the whole red line area, including land you do not control but have included, and arranging access to a neighbouring field can take weeks.
3. The relevant date. The pre-development biodiversity value is assessed as at the date of the planning application, or an earlier date where one is agreed. Habitat degraded deliberately after the date set in the legislation is valued as it was before the degradation, so clearing a site to reduce the baseline does not work.

Book the survey against the application programme, not against the design programme. A scheme that cannot be surveyed until next April has a submission date in May whatever the drawings say.

The gain plan and the local authority approval step

The biodiversity gain plan is a post-permission document. The deemed biodiversity gain condition prevents development from beginning until the authority has approved it, which makes it a pre-commencement step with real programme consequences.

The plan sets out the pre-development value with the completed metric, the steps taken to minimise adverse effect on habitat following the mitigation hierarchy, the proposed on-site post-development value, any registered off-site gains and any statutory biodiversity credits purchased. Its contents and the approval sequence are covered in what goes in a biodiversity gain plan and when it is approved.

Two things about the approval step catch developers out. The authority has a determination period and can refuse, so the plan is not a notification. And where off-site units are relied on, the gain site has to be registered and the allocation recorded before the plan can be approved, which depends on a third party's timetable rather than yours.

Build the sequence into the programme at the outset. The statutory framework and the commencement dates, including 12 February 2024 for major development and 2 April 2024 for small sites, are set out in mandatory biodiversity net gain: the statutory framework.

Where a draft contributes: policy, strategy and commitments

A drafted biodiversity section earns its place in the parts of the submission that are argument rather than arithmetic.

The mitigation hierarchy narrative, explaining what was avoided, what was minimised, what is being compensated and why, in terms the officer can follow against the layout. The policy compliance section, setting out the development plan biodiversity policies and the national policy position, and dealing with the relationship between the statutory 10 per cent and any higher local requirement. The strategic fit, describing how the proposed habitats relate to the relevant Local Nature Recovery Strategy and any local habitat mapping, which feeds the strategic significance multiplier. The management and monitoring commitments over the 30-year maintenance period, including who holds them and how they are secured. And the delivery route, meaning on-site, off-site or credits, with the reasoning for the choice.

PlanScope inserts a "TO BE CONFIRMED" flag wherever a site-specific fact such as a unit count is missing rather than generating one, and those flags mark exactly the points where the ecologist's output has to be dropped in. Treating them as a checklist is the right use of the draft.

Common errors in drafted biodiversity sections

Six recur often enough to check for every time:

• A stated percentage gain with no metric version, no unit figures and no habitat plan behind it.
• Treating biodiversity net gain as if it discharged protected species obligations. It does not. Species licensing under the habitats regulations and the Wildlife and Countryside Act 1981 is a separate regime with separate survey requirements.
• Assuming off-site units are available locally. Supply varies by area and by habitat type, and the trading rules may require a habitat type that no nearby gain site offers.
• Ignoring the 30-year management obligation, which has to be secured by planning obligation or conservation covenant and has a cost attached.
• Claiming an exemption without working through the criteria. The de minimis exemption applies where development affects less than 25 square metres of on-site habitat and less than five metres of linear habitat, and householder and certain self-build and custom build development are also outside the regime, but each exemption has conditions.
• Describing irreplaceable habitat as if the metric applied to it. Ancient woodland and other irreplaceable habitats sit outside the normal calculation and need bespoke compensation agreed with the authority.

Sequencing the ecologist and the drafting

The order that works on a real programme:

1. Commission a preliminary ecological appraisal and a UK Habitat Classification survey with condition assessment, in season, covering the whole application boundary.
2. Run the baseline metric before the layout is fixed, so the high-distinctiveness parcels are known to the design team.
3. Iterate the layout against the metric. Retaining a hedgerow in situ is usually worth more than creating new habitat elsewhere, because the temporal and difficulty multipliers bite on creation.
4. Produce the post-development metric with the ecologist, and only then draft the narrative around the confirmed numbers.
5. Secure the delivery route, including any off-site allocation, before submission so the gain plan is deliverable.
6. Submit the application with the metric, the habitat plans and the narrative, and prepare the gain plan to follow permission.

Drafting at step four rather than step one is the whole point. A narrative written around real unit figures reads as evidence, and the professional reviewing it has something to check rather than something to rewrite. If you need a reviewer, PlanScope's professional directory is the route for requesting review and sign-off of a draft.

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. For specialist technical reports, qualified professional input may be required.