Worked biodiversity baseline for a grazed paddock
Modified grassland in poor condition is the usual starting point on a small site. This illustrative walk-through covers parcel mapping, condition scoring and how the baseline units are calculated.
A biodiversity baseline is a calculation, not an opinion, and on a small site it usually comes down to three inputs: the area of each habitat parcel in hectares, its habitat type and distinctiveness from the statutory metric, and the condition score from the metric's own assessment sheet. Multiply those together, apply the strategic significance multiplier, and you have your baseline habitat units. Everything that follows in a biodiversity net gain case is measured against that number.
This walk-through uses a grazed paddock behind a house, the most common small-site starting point. All the figures below are illustrative. Your own metric run will differ, and under the Environment Act 2021 regime the metric has to be completed by a competent person.
Mapping the habitat parcels first
Draw the parcels before you think about units. Walk the site with the boundary plan and split it wherever the vegetation changes, because a single field is rarely one habitat.
A typical paddock splits into four: the grazed sward itself, a hedgerow along two boundaries, a strip of scrub and bramble in the corner the mower never reaches, and the hard standing by the gate. Hedgerows and watercourses are counted separately from area habitats in the metric, with their own units, so a hedge is measured in kilometres and never folded into the grassland figure.
Measure areas from a georeferenced plan rather than by pacing. The metric is sensitive to small area differences, and an over-stated baseline is the easiest thing for a council ecologist to knock down.
Assigning habitat type and distinctiveness
The statutory metric works from a fixed habitat list, and each entry carries a distinctiveness band. Modified grassland, which is what most grazed, fertilised or agriculturally improved paddocks are, sits in the low distinctiveness band.
Resist the temptation to record it as something better. If the sward is dominated by rye grass, creeping buttercup and docks, it is modified grassland. If there is a genuine diversity of wildflower species across the parcel, it may be another lowland meadow type, and that changes the distinctiveness band, the trading rules and the cost of replacing it. It also changes who has to sign the assessment, because higher distinctiveness habitats take the scheme out of the simplest small-sites route.
The bare ground by the gate falls into the urban category as developed land or sealed surface, which carries no units at all. That matters, because building on the hard standing rather than the grass reduces the loss you have to make up.
Running the condition assessment honestly
Each habitat type in the metric has a condition sheet with a short list of criteria, and the number of criteria passed sets the condition as good, moderate or poor. For modified grassland the criteria look at things like the range of species present, sward structure, the extent of bare ground and the cover of scrub or undesirable species.
A horse-grazed paddock cut tight, with a uniform sward and heavy poaching around the gate, will almost always score poor condition. That is not a failure. It is the result you want recorded accurately, because it lowers the baseline you have to replace and makes an on-site gain easier to demonstrate.
Take photographs of each parcel from fixed points and record the date. The condition assessment is the part most often challenged, and a dated photographic record is what settles the challenge.
Calculating the baseline units
The area habitat calculation is area in hectares, multiplied by the distinctiveness score, multiplied by the condition score, multiplied by the strategic significance multiplier.
Strategic significance is the one people forget. It is set by whether the parcel is identified in a local plan, a local nature recovery strategy or other published strategy. Most back-garden paddocks are not, so the multiplier sits at the lowest level.
Illustrative worked example, for a paddock of 0.4 hectares of modified grassland in poor condition, not identified in any local strategy:
| Input | Value used |
|---|---|
| Area | 0.4 ha |
| Habitat type | Modified grassland, low distinctiveness |
| Condition | Poor |
| Strategic significance | Lowest multiplier |
| Baseline area habitat units | Area times distinctiveness times condition times multiplier |
Run the same calculation for the hedgerow in kilometres, using the hedgerow distinctiveness and condition scores, and keep the two totals separate. The metric will not let you replace hedgerow units with grassland units, and nor will the council.
Where on-site gain is achievable
On a paddock the arithmetic is usually kind, because a low distinctiveness baseline in poor condition is not hard to beat. The moves that work on small sites are unglamorous.
- Convert a strip of the retained paddock to a species-rich grassland creation area, with an appropriate seed mix and a management plan that says who cuts it and when
- Plant new native hedgerow along a boundary that currently has post and rail fencing
- Enhance the existing hedgerow from poor to moderate condition by laying it and stopping the annual flail
- Retain the scrub corner rather than clearing it for site compound space
Each of those has to be secured for 30 years, so the management plan is part of the evidence rather than an afterthought. Time lags and difficulty multipliers in the metric reduce the value of created habitat, which is why creating a hectare of something does not deliver a hectare's worth of units on day one.
When off-site units become the answer
If the developable footprint takes the whole paddock, on-site gain will not reach the required uplift and you are into off-site units or, as a last resort, statutory credits bought from the government. The hierarchy matters: the regime expects on-site gain first, then off-site, then credits.
Off-site units are secured through a legal agreement or conservation covenant and registered, which adds legal fees and time to the programme. Budget for that at the point you first suspect the site cannot absorb the uplift, not at the point the officer asks.
The biodiversity gain plan has to be approved by the local planning authority before development begins. That is a statutory pre-commencement requirement, and it sits alongside the other evidence a small scheme needs, such as a Drainage Strategy for a minor residential scheme.
What to hand to the council
Submit the metric file itself, not just the summary. Councils increasingly ask for the calculation tool output so their ecologist can check the condition scores and the habitat classifications, and a summary table without the working invites a request for further information.
Alongside it, provide the habitat plan showing parcel boundaries and areas, the dated site photographs, the condition assessment sheets, and the management and monitoring plan for anything created or enhanced. A Preliminary Ecological Appraisal usually supplies the survey context the metric sits on, and a Biodiversity Net Gain Assessment is where the metric outputs are written up for the planning file.
One honest caution. The metric is updated periodically and the trading rules are unforgiving about replacing higher distinctiveness habitat, so a baseline run on an old version of the tool can produce a number the council will not accept. Check the version before you start, and have the assessment done or checked by someone competent. The professional directory is the route for that review.
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, and the statutory biodiversity metric must be completed by a competent person.
Frequently asked questions
Is a horse paddock always modified grassland?
Usually, but not always. A sward dominated by rye grass and a few broadleaved weeds, kept short by grazing, is modified grassland. A paddock with a real diversity of wildflower species may fall into a higher distinctiveness grassland type, which changes the trading rules and the replacement cost. The habitat classification should be made on site by a competent ecologist.
How are baseline habitat units calculated?
For area habitats, the metric multiplies the parcel area in hectares by the distinctiveness score for the habitat type, by the condition score, and by the strategic significance multiplier. Hedgerows and watercourses are calculated separately in kilometres with their own scores and cannot be traded against area habitat units.
Does poor condition help or hurt my application?
An accurate poor condition score lowers the baseline you have to replace, which makes the required uplift easier to achieve on site. Recording condition as worse than it is would be misleading and is likely to be challenged by the council's ecologist, so the answer is to assess it honestly and photograph the evidence.
Can I run the metric myself?
The tool is publicly available, but the Environment Act 2021 regime expects the statutory metric to be completed by a competent person, and the local planning authority has to approve the biodiversity gain plan. Running it yourself to understand the arithmetic is sensible. Submitting it without competent input is not.
What happens if the site cannot deliver the gain on its own?
The regime expects on-site gain first, then off-site units, then statutory credits as a last resort. Off-site units are secured through a legal agreement or conservation covenant and registered, which adds legal cost and time, so identify the shortfall early rather than when the case officer raises it.
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.