The degradation rule and your biodiversity net gain baseline

If habitat on your site has been cleared since 30 January 2020 without planning permission or one of a short list of other consents, the biodiversity net gain baseline is not what is there now. Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021, requires the pre-development biodiversity value to be taken as the value immediately before those activities were carried out. Clear a meadow, apply the following year, and the 10 per cent gain is calculated against the meadow.

That is the degradation provision, and it has teeth because the burden of proving the earlier value falls on the applicant.

What the provision prevents

The target is straightforward: stripping a site of its biodiversity value before the regime applies to it, so that the baseline reads as close to zero and the 10 per cent gain costs nothing.

Without the provision, the incentive would run entirely the wrong way. A field of species-rich grassland generates a large baseline and therefore a large gain requirement. The same field ploughed and left as bare ground generates almost none. The rule removes the advantage by reaching back to the value before the clearance.

Two points of scope are worth holding on to. The provision bites on activities that reduced the biodiversity value of the on-site habitat, so it covers habitat removal, ploughing, scrub clearance, hedgerow removal, drainage of wet habitat and the loss of a watercourse's condition. And it applies whether or not the person who carried out the activities is the person now applying for permission, which makes it a due diligence question for anyone buying land.

The date the baseline reverts to

Two dates matter and they do different jobs.

The relevant date for a normal baseline is the date of the planning application, or another date the applicant and the authority agree, which is how a survey carried out in the previous summer can be used rather than a winter survey in the wrong season.

The cut-off for the degradation provision is 30 January 2020, the date the Environment Bill was introduced to Parliament. Activities before that date do not trigger an earlier baseline. Activities after it can, and the baseline then reverts to the value immediately before those activities, which may be four or five years back.

The practical consequence is a survey of something that no longer exists. You are being asked to establish the habitat type, extent and condition of a parcel as it stood years ago, from evidence rather than from fieldwork.

Evidence an authority will look for

Authorities and their ecologists work through a reasonably standard set of sources, and it pays to assemble them before anyone else does.

1. Historic aerial imagery, including publicly available satellite imagery with date stamps.
2. The priority habitat inventory and other datasets on the MAGIC mapping service, which record what was mapped and when.
3. Local wildlife site citations and local environmental records centre data.
4. Agri-environment scheme agreements and land parcel records, which describe how a parcel was managed and sometimes what habitat it carried.
5. Previous ecological surveys on the site or on neighbouring applications.
6. Tree and hedgerow records, including any hedgerow removal notice and any tree preservation order.
7. Street-level imagery, which is often the only source showing a boundary hedge at eye level.

Add the applicant's own records. Farm diaries, contractor invoices, dated photographs, spray and cutting records, and insurance or safety reports where clearance was carried out for a stated reason. Evidence the applicant produces voluntarily is worth more than evidence an officer finds.

Where the earlier value cannot be established at all, the authority will work from the best available evidence, and best available evidence tends not to favour the applicant.

Historic aerial imagery and its limits

Imagery is the backbone of most degradation assessments and it answers fewer questions than people expect.

It shows extent reliably. A hedgerow that was 180 metres long and is now 120 metres long is visible, measurable and hard to argue with.

It shows broad habitat type with moderate reliability. Woodland, scrub, grassland, arable and hard surface are usually distinguishable. Modified grassland and species-rich grassland are not.

It says almost nothing about condition, which is half the metric calculation. A hay meadow photographed in late August, two weeks after a cut, looks like bare ground. The same parcel in June looks superb. Seasonality cuts both ways, and the date of the image is as important as its content.

Resolution limits what can be seen at the margins. Individual trees in a hedge line can be counted on good imagery and not on poor imagery, and that distinction changes the hedgerow type and therefore its distinctiveness.

So argue about condition on documentary evidence rather than on photographs, and make the seasonality point explicitly where an image is being used against you. An ecologist willing to state in writing what an image can and cannot show is worth more than one willing to state a condition band from it.

Agricultural operations and the boundary of the rule

This is the least settled part of the regime and the honest answer is that it is unresolved at the edges.

The exceptions to the earlier-baseline rule are narrow. Where clearance was carried out in accordance with planning permission, or under one of the other consents the legislation specifies, the current baseline stands.

Routine agricultural operations are not development and need no planning permission, which means they do not fall within that exception simply by being lawful. Ploughing permanent pasture, grubbing out a hedge outside the countryside hedgerow controls, or draining a wet corner can all reduce biodiversity value without anyone breaching anything.

Take advice on specific facts rather than relying on a general principle here, and expect authorities to take different views. What is reasonably clear is that an operation carried out as part of an established agricultural rotation, documented as such over years, is a stronger position than a single clearance six months before an option agreement was signed.

Separate offences run alongside, and they do not depend on the net gain regime at all. Removing a countryside hedgerow without a notice, disturbing nesting birds during the breeding season, or working where protected species are present each carry their own consequences. Nesting bird checks before vegetation clearance sets out the one that catches most people, and Japanese knotweed and the Schedule 9 offence the one most often misunderstood.

Consequences of an adjusted baseline

An adjusted baseline raises the gain requirement in absolute units, and the effect is usually larger than the percentage suggests.

A site whose current baseline is 2 biodiversity units needs 2.2 units afterwards. The same site with an adjusted baseline of 9 units needs 9.9. The 10 per cent is unchanged. The absolute deficit the scheme has to find is more than four times larger, and it has to be found in the same module as the loss.

That often moves a scheme off site. On-site delivery works for small adjustments and runs out quickly, and off-site units carry a spatial risk multiplier that discounts delivery outside the authority area or national character area. Statutory credits remain the last resort and are priced to be unattractive.

The procedural consequence is the one that stops work. The biodiversity gain condition is deemed to be attached to most permissions, and development cannot lawfully begin until the authority has approved the biodiversity gain plan. An authority that disputes the baseline does not need to refuse the application to stop the scheme. It simply declines to approve the plan.

Advice for landowners before any clearance

Six things, in order, and the first is the whole point.

Do nothing to the habitat until you have a dated baseline. A survey in the right season, by a competent person, with a completed metric, costs a fraction of what a disputed baseline costs.

Photograph everything now, with dates, from fixed viewpoints, including the boundaries and the scruffy corners nobody values.

Keep the management records. Cutting dates, grazing regimes, spray records and contractor invoices are what distinguish a rotation from a clearance.

Check whether an exemption applies before you assume the regime bites, because householder development, development below the de minimis threshold for on-site habitat, and self-build within the stated limits sit outside it. Confirm the current exemption list rather than relying on an older summary.

Where clearance is unavoidable for safety or for a statutory duty, record the reason, the advice you acted on and the consent you relied on, at the time.

And if you are buying land, treat habitat history as a due diligence item alongside title and contamination. Order the imagery, check the datasets, and price the risk.

A Preliminary Ecological Appraisal is the document that fixes the baseline, and the metric and the gain plan behind a Biodiversity Net Gain Assessment must be completed by a competent person under the Environment Act 2021 regime. A drafted assessment is a brief for that person rather than a substitute for the survey.

Enter your postcode on PlanScope to see which ecology reports a site is likely to need.

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.