Biodiversity metric baseline for a grazed pasture field

A baseline figure in the statutory biodiversity metric is the product of four things: the area of each habitat parcel, the distinctiveness score attached to its habitat type, the condition score from the assessment sheet, and a strategic significance multiplier. For a grazed pasture field the first and third of those do most of the work, and the habitat type you record decides everything downstream.

The worked figures below are illustrative. They show the method rather than an answer for any real site, and the version of the metric in force on the date of your application is the one that governs, so check the current tools and guides on gov.uk before you rely on any multiplier quoted here.

Identifying the habitat type

Grassland is where most baseline disputes start, because the metric distinguishes several types with very different distinctiveness scores.

A field under regular agricultural management, grazed hard, dominated by a handful of grass species with rye-grass or similar prominent, with few herbs and no notable indicator species, is usually recorded as modified grassland. That sits in the low distinctiveness band.

Move one step up and you are into other neutral grassland at medium distinctiveness, which requires a more varied sward and a greater herb component. Higher still are the priority types, lowland meadows and the calcareous, acid and marshy grasslands, all in the high band and all carrying trading rules that make them expensive to lose.

Getting this wrong in either direction is costly. Recording a species-diverse field as modified grassland understates the baseline and will be challenged by the council's ecologist. Recording a hard-grazed rye-grass ley as other neutral grassland inflates the baseline and raises your gain requirement for no reason.

The way to settle it is a botanical survey in the growing season, ideally between April and September, with a species list and quadrat data rather than an impression from a winter walkover. A preliminary ecological appraisal is the usual route to that survey and will also pick up the protected species questions a pasture field can raise.

Assessing condition against the criteria

Each habitat type in the metric has its own condition assessment sheet with a short list of criteria, and the habitat is scored by how many of them it passes. Condition then feeds the calculation as a multiplier, running from poor at the bottom through fairly poor, moderate and fairly good to good at the top.

For grassland the criteria look at things like the variation in sward height across the parcel, the proportion of the sward made up of herbs rather than grasses, the number of species present, the cover of undesirable or competitive species, the extent of bare ground, and the degree of scrub encroachment. Work from the sheet in the current version of the metric rather than from memory, and record the result criterion by criterion.

A continuously grazed field is often poor or fairly poor. Uniform short sward, very few herbs, no structural variation, and one or two dominant grasses. That is not a failure of the surveyor; it is what intensive grazing produces, and the metric is designed to reflect it.

Write down the evidence for each criterion as you assess it. Photographs with a scale object, the quadrat data, the date and the surveyor's name. A condition score asserted without that record is the first thing an authority's ecologist will query.

Measuring the area properly

Measure the parcel, not the red line. The metric works on habitat parcels, so a field containing a track, a hardstanding, a pond and a line of scrub is several parcels rather than one.

Take the areas from a topographic survey or from a geographic information system using the mapped habitat boundaries. Do not scale off a PDF. State the method and the source in the metric file, and give areas in hectares to two decimal places.

Two common errors. Including the ditch or hedgerow area in the grassland parcel, when watercourses and hedgerows are assessed in their own separate modules with their own units. And measuring to the centre line of a boundary feature that is not in your ownership.

The illustrative baseline calculation

Take a field of 1.20 hectares, surveyed in June, recorded as modified grassland, assessed as poor condition against the sheet, and not identified in any local nature recovery strategy or equivalent local plan.

Multiply them: 1.20 multiplied by 2, by 1, by 1.0, giving 2.40 baseline biodiversity units for that parcel.

Now change one input. If the same field had been assessed as moderate condition, the condition score rises and the baseline rises with it in direct proportion. If it had been recorded as other neutral grassland at medium distinctiveness, the distinctiveness score doubles and so does the baseline. That sensitivity is why the survey work matters more than the arithmetic.

These figures are illustrative only. Run your own parcel through the current metric tool.

Strategic significance and what it adds

The strategic significance multiplier rewards habitat in the right place. The highest multiplier applies where the area is formally identified in a local nature recovery strategy or an equivalent local plan or policy document. A middle multiplier applies where the location is ecologically desirable but not formally identified. Everywhere else takes the neutral multiplier of 1.0.

Check the position rather than defaulting to 1.0. Local nature recovery strategies are being published across England, and a field inside a mapped priority area attracts the higher multiplier on both the baseline and the post-development side. That cuts both ways: it raises the units you lose and the units you create.

State in the report which document you checked, its date and which mapped area the site falls in or outside.

What the figure means for the gain requirement

The statutory requirement is a minimum ten per cent gain over the baseline, secured for at least 30 years. On the illustrative figure above, 2.40 baseline units means 2.64 units have to be delivered.

That is the headline, and it is not the whole test. Trading rules sit underneath it: losses of medium distinctiveness habitat or higher must be replaced with the same habitat type or something better, so a scheme cannot simply buy low-cost units to make up a shortfall in a habitat that matters. Off-site units carry a spatial risk multiplier that reduces their value the further they are from the development, and habitat that takes years to reach target condition carries a temporal multiplier that reduces its value too.

For a small residential scheme on poor modified grassland, a gain of this size is often achievable on site through wildflower grassland creation, native hedgerow planting and tree planting, set out in a landscape strategy and secured by condition. The arithmetic is easier when the baseline is low, which is one honest consequence of a poor baseline.

Self-builders should check the exemptions before doing any of this. Development consisting exclusively of self-build or custom housebuilding, of no more than nine dwellings, on a site of no more than 0.5 hectares, is exempt from the biodiversity gain condition. Verify the current wording and claim it on the application form.

Who must complete the metric

The Environment Act 2021 regime expects the statutory metric to be completed by a competent person, and the biodiversity gain plan to be approved by the local planning authority after permission is granted.

There is no single national register of metric assessors, so competence is shown by relevant professional membership, botanical survey experience and current familiarity with the metric version in use. For a small site there is a simplified small sites metric, which reduces the input burden but does not reduce the need for a proper habitat survey behind it.

A biodiversity net gain assessment drafted in advance gives that person a structured starting point, with missing site facts flagged rather than invented.

Evidence the authority will check

Submit the completed metric file itself, in its native format, not a screenshot of the summary tab. Alongside it:

• a habitat plan showing each parcel, its habitat type and its area, keyed to the metric rows
• the completed condition assessment sheets with the criteria scored individually
• the survey date, the surveyor's name and qualifications, and the survey method
• photographs of each parcel with a scale object
• the source of the area measurements
• the local nature recovery strategy or policy document checked for strategic significance, with its date

Ecologists reviewing for the council go to the condition sheets first. An assessment with complete sheets and dated photographs is rarely argued with. One without them is usually returned.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and browse the full list of report types to see what else a greenfield site normally requires.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. The statutory biodiversity metric must be completed by a competent person, and an AI-assisted draft is a starting point for that person rather than a substitute.