Biodiversity net gain baseline: an illustrative example
The 10 per cent uplift is measured from the baseline, so the baseline decides the scheme. This illustrative example shows how the units are built up on a small plot.
Every biodiversity net gain calculation starts from a number that describes the site as it is today. Get that number wrong and everything downstream is wrong, because the 10 per cent uplift is measured from it. This biodiversity baseline worked example walks through how the units are built up on a small residential plot, using the structure of the statutory metric. The figures below are illustrative. The metric itself has to be run by a competent person working from a site survey.
What the baseline records
The baseline is a map and a table. The map divides the site into habitat parcels with their boundaries drawn. The table gives, for each parcel, its habitat type, its area in hectares, its distinctiveness, its condition and its strategic significance.
Those five fields multiply out into biodiversity units. Area in hectares, multiplied by the distinctiveness score for the habitat type, multiplied by the condition score, multiplied by the strategic significance multiplier.
Linear features are handled separately. Hedgerows are measured in kilometres and watercourses in kilometres, each with their own module in the metric, and a site with a hedge along one boundary produces two baselines rather than one.
Classifying habitats on a small plot
Habitat classification uses the UK Habitat Classification system, which is more granular than most people expect. A domestic garden is not one habitat. A typical suburban plot might break down into vegetated garden, a sealed surface for the drive, a strip of modified grassland at the rear that has been mown but not maintained as a lawn, and an individual tree.
Three classification errors recur on small sites.
- Calling everything garden. If part of the plot has been left unmown for years and has developed a grass sward with wildflowers, it may classify as modified grassland or better, which carries a different distinctiveness.
- Forgetting the sealed surfaces. Drives, patios and the building footprint are habitats in the metric, with very low distinctiveness and effectively no units, and leaving them out inflates the baseline.
- Missing trees. Individual trees are recorded by their canopy area, and a mature oak over a corner of the plot contributes a measurable parcel. The approach to measuring canopy is set out in the canopy cover worked example.
Classification is a field judgement made by someone who can identify the species present. It is not something to do from an aerial photograph.
Assessing condition
Each habitat type in the metric has its own condition sheet with a short list of criteria. The assessor walks the parcel, scores each criterion as passed or failed, and the number passed determines the condition band.
The bands and their multipliers in the metric run from good, through fairly good, moderate and fairly poor, to poor. Some habitat types, including several urban ones, have no condition assessment and are scored accordingly.
The practical point for a small site is that condition is usually the difference between an easy scheme and a hard one. A parcel of modified grassland in poor condition carries few units and is simple to replace. The same parcel in good condition carries several times as many, and replacing them on a plot that is about to have a house built on it may not be possible.
Strategic significance
Strategic significance asks whether the habitat sits in a location identified as important in a published local strategy, such as a local nature recovery strategy or a local plan policy map.
The multiplier is modest, applied as a small uplift where the location is formally identified, a smaller one where it is ecologically desirable, and no uplift otherwise. Check the position with the responsible authority rather than assuming the default, because a site inside a nature recovery network opportunity area carries the uplift on both the baseline and the post-development figure.
An illustrative baseline calculation
The following is illustrative only. It shows the arithmetic, not a real site, and the scores used are assumptions for demonstration.
Take a plot of 0.08 ha being developed for a single dwelling. Suppose the survey divides it as follows.
| Parcel | Area (ha) | Distinctiveness score | Condition score | Strategic multiplier | Units |
|---|---|---|---|---|---|
| Vegetated garden | 0.045 | 2 | 1 | 1.0 | 0.090 |
| Modified grassland | 0.020 | 2 | 2 | 1.0 | 0.080 |
| Sealed surface | 0.015 | 0 | 1 | 1.0 | 0.000 |
| Baseline total | 0.080 | 0.170 |
A baseline of 0.17 habitat units means the post-development position has to reach 0.187 units, which is 10 per cent more. On a plot of this size that is achievable through planting, a small area of species-rich grassland and retention of anything mature, provided the layout leaves space for it.
Now change one assumption. If the modified grassland had scored good condition rather than moderate, its units would rise by half again, the baseline would rise with it, and the required post-development figure would rise too. That single field judgement moves the whole scheme.
The hedgerow module would be calculated alongside this in kilometres, and a hedge that is removed has to be replaced within the hedgerow trading rules rather than offset against area habitats.
Where the 10 per cent is measured from
From the pre-development baseline, calculated on the date of the application or, where habitat has been degraded since 30 January 2020, from the earlier condition. That backstop exists to stop sites being cleared in advance, and authorities do check aerial imagery.
For small sites, meaning one to nine dwellings on a site under one hectare, the small sites metric is available and is a simplified version of the full tool. Some development is exempt altogether, including householder applications, development affecting below the de minimis threshold of on-site habitat, and self-build and custom housebuilding that meets the stated conditions. Confirm your exemption in writing with the authority before relying on it, because the biodiversity gain condition is deemed to be attached to most permissions and discharging it is not optional.
Why a competent person has to run 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 before development begins.
That is not a formality. The condition assessment requires species identification and habitat knowledge, the classification requires familiarity with the UK Habitat Classification, and the post-development figures require judgements about creation difficulty and time to target condition that an inexperienced user will get wrong in both directions.
A Preliminary Ecological Appraisal is usually the first step, because it establishes the habitats present and flags protected species at the same time. The metric then runs off that survey rather than off a separate visit.
PlanScope drafts a Biodiversity Net Gain Assessment that sets out the structure, the policy framing and the gaps where site data is needed, flagged rather than filled in. The metric calculation itself, the condition assessment and the gain plan are the work of a competent person, and the draft is a starting point for them. Browse the 66 report types to see what else a small residential scheme usually needs.
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.
Frequently asked questions
How are biodiversity units calculated?
For area habitats, the parcel size in hectares is multiplied by the distinctiveness score for the habitat type, the condition score from the metric's condition sheet, and the strategic significance multiplier. Hedgerows and watercourses are measured in kilometres and calculated in separate modules, so most sites produce more than one baseline figure.
What date is the baseline taken from?
Normally the pre-development condition at the date of the application. Where habitat has been degraded since 30 January 2020, the baseline is taken from the earlier condition instead. That backstop exists to stop sites being cleared in advance of an application, and authorities do compare historic aerial imagery.
Is there a simpler metric for small sites?
Yes. The small sites metric is available for one to nine dwellings on a site under one hectare, and for smaller non-residential schemes meeting the stated limits. It is a simplified version of the statutory tool, though the habitat classification and condition assessment underneath it still need someone who can identify what is growing on the plot.
Which developments are exempt from biodiversity net gain?
Householder applications, development falling below the de minimis threshold of on-site habitat affected, and self-build and custom housebuilding meeting the stated conditions are among the exemptions. Confirm any exemption in writing with the authority, because the biodiversity gain condition is deemed attached to most permissions and has to be discharged.
Who can run the statutory biodiversity metric?
The Environment Act 2021 regime expects a competent person. Condition assessment needs species identification, classification needs familiarity with the UK Habitat Classification, and the post-development figures need judgements on creation difficulty and time to target condition. An inexperienced user will get those wrong in both directions.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.