Hedgerow units and the rules you cannot trade around

Hedgerow units in the statutory biodiversity metric run in their own account, separate from area habitats and from watercourses, and the trading rules will not let you swap between them. Lose 300 metres of hedge and plant a wildflower meadow instead, and the biodiversity gain plan fails however many habitat units the meadow generates. The ten per cent gain has to be achieved in hedgerow units, by retaining, improving or creating hedgerow. That single rule shapes site layouts more than any other part of the metric.

Why hedgerows are counted separately

The metric treats habitat in three modules. Area habitats are measured in hectares and produce habitat units. Hedgerows and other linear features are measured in kilometres and produce hedgerow units. Watercourses are measured in kilometres and produce watercourse units.

Each module has its own baseline, its own post-development calculation and its own gain requirement. Where hedgerow is present on the baseline, the plan has to show the required gain in hedgerow units as well as in habitat units.

The reasoning is ecological rather than administrative. A hedge is a connective feature. It links woodlands, gives commuting routes to bats and dormice, supports nesting birds and provides shelter and forage across otherwise open ground, and none of that is replaced by creating an equivalent area of grassland in the corner of a site.

Where a site has no hedgerow at all on the baseline and none is proposed, the module simply does not apply, and the gain plan deals with area habitats alone.

Measuring length and assigning a hedgerow type

Measure along the centre line of the hedge, in metres, converted to kilometres for the calculation. Record the length that lies within the red line and within your control.

Boundary hedges cause the most argument. Where the hedge sits wholly on a neighbour's land you cannot count it as yours to improve, and where ownership is uncertain the title plan and the hedge and ditch presumption matter before the ecology does. Get this right at baseline, because a boundary hedge counted as retained and then found to belong to somebody else creates a shortfall discovered at the gain plan stage.

The metric then asks for a hedgerow type, and the types carry different distinctiveness. A line of trees sits at the bottom. A native species-rich hedgerow sits higher. Hedgerows with trees, and hedgerows associated with a bank or a ditch, sit higher again, and a native hedgerow with trees that is also associated with a bank or ditch sits at the top.

That ranking has a practical consequence worth noticing early. Removing a bank or filling a ditch alongside a hedge you intend to keep can drop its type and cost you units without a single shrub being cut.

Condition criteria and what a gappy hedge scores

Condition is assessed against a set of pass or fail criteria in the published condition sheet, applied to each distinct hedgerow parcel. They cover the height and width of the hedge, the proportion of the length that is gappy and the size of the largest gap, the presence of an undisturbed margin of perennial vegetation at the base, the absence of invasive non-native species, and damage or nutrient enrichment from adjacent land use. Take the exact thresholds from the current condition sheet rather than from memory, because they have been revised.

The number of criteria failed produces a good, moderate or poor score, and the score multiplies the units. A tall, wide, unbroken hedge with a grass margin and no invasive species scores well. A flailed remnant with large gaps, cultivation up to the base and a stand of Japanese knotweed at one end does not.

Two things follow. First, the survey has to be done at a time of year when condition can actually be judged, which means in leaf for most criteria. Second, a poor baseline condition is an opportunity rather than a problem, because lifting an existing hedge from poor to good condition generates units at far lower cost and risk than creating new hedgerow.

Trading rules specific to linear habitats

The rules that constrain hedgerow compensation are straightforward to state and awkward to design around.

The discounting mechanisms bite harder on hedgerows than people expect. Delivery further away loses value, and so does delay, because time to target condition is discounted year on year. Newly planted hedgerow takes years to reach good condition, so a plan that relies on creation carries both a temporal discount and a difficulty multiplier.

How the distance-based reductions work in practice is covered in our note on spatial risk multipliers in the biodiversity metric.

Creating and improving hedgerow to gain units

Improving what is there comes first, creation second. That is the order that produces units most reliably.

The metric calls this enhancement. In practice it means gapping up with native whips, widening the hedge by reducing the cutting profile, establishing or widening an undisturbed margin at the base, removing invasive species, allowing selected stems to grow on as standard trees, and moving to a cutting rotation of every two or three years rather than annually.

Creation means planting a new hedge, usually a double staggered row of native whips with guards, on a line that connects something to something else rather than running to a dead end. Specify the species mix, the planting density, the protection, the establishment watering and the management for thirty years, because the gain has to be secured and maintained for that period.

Layout decisions matter more than planting specifications. Keeping a hedge along an estate road with a service margin either side is frequently impossible once drainage, lighting and visibility splays are drawn, so the hedgerow strategy has to enter the layout at concept stage rather than being retrofitted by the ecologist. Start from the Preliminary Ecological Appraisal and mark the hedgerows that are going to be expensive to lose before the road layout is fixed.

Hedgerow protection under other regimes

Net gain is not the only hedgerow control, and satisfying the metric does not authorise removal.

The Hedgerows Regulations 1997 require notice to the local planning authority before removing most countryside hedgerows, and the authority can prohibit removal where the hedge meets the criteria for importance on wildlife, landscape, historical or archaeological grounds. That is a separate procedure with its own timetable.

Separate management rules apply to hedgerows on agricultural land, including a cutting ban across the bird nesting period and a buffer strip requirement, subject to the exemptions in those rules. Trees within a hedge may be protected by a tree preservation order even where the hedge itself is not. And nesting birds are protected whenever a nest is active, whatever the metric says.

Removing hedgerow before the baseline survey is also pointless. Where habitat has been degraded by activities carried out after 30 January 2020, the pre-development value can be assessed at the earlier state.

Recording hedgerows on the habitat plan

The plan submitted with the Biodiversity Net Gain Assessment should show each hedgerow parcel as a separate line with a reference, its type, its condition score, its strategic significance, its length in metres, and whether it is retained, improved, lost or created. Match every reference to a row in the metric so the authority can audit the calculation without a phone call.

Show the retained hedgerows with their protection during construction, using fenced root protection areas where trees are present, and show new hedgerow with its planting line and margin. Where units are delivered off site, identify the receptor site, the responsible body and the registration position.

Remember that the statutory metric must be completed by a competent person and the biodiversity gain plan approved by the local planning authority. An AI-assisted draft is a starting point for that professional, not a substitute. Where a scheme may fall outside the requirement altogether, our note on biodiversity net gain exemptions and how to claim them sets out the tests, including the small threshold below which on-site linear habitat is disregarded.

To see which ecology documents your site is likely to need, enter your postcode on PlanScope and run the constraint check before the layout is fixed.

This is general guidance, not legal advice. Biodiversity net gain calculations should be prepared by a competent person and reviewed before submission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.