Phased schemes and biodiversity gain plans per phase
Phased permissions use a modified biodiversity gain condition: the baseline is fixed once for the whole site, then each phase needs its own approved gain plan. Here is how to allocate the units.
On a phased development the biodiversity gain plan is not one document. The regulations made under the Environment Act 2021 allow a modified version of the biodiversity gain condition for phased permissions, under which an overall plan for the whole site is approved first and then a separate phased development biodiversity gain plan is approved for each phase before that phase begins. The baseline is fixed once, for the whole site. The delivery is then allocated phase by phase, and that allocation is where schemes come unstuck three years later.
How the condition applies to a phased permission
Every relevant planning permission in England carries the deemed biodiversity gain condition, which prevents development beginning until a biodiversity gain plan has been approved by the local planning authority. On an unphased scheme that is a single gate. On a phased scheme, applying one gate to the whole site would mean the entire habitat strategy had to be fixed before the first spade, which for a ten-year build out is unrealistic.
The modified condition splits the obligation. There is an overall plan covering the site, and then a plan for each phase. Development of a phase cannot begin until the plan for that phase is approved, and each phase has to deliver its share of the gain.
Outline permissions with reserved matters, and permissions expressly granted subject to a phasing condition, are the usual candidates. Whether a permission qualifies turns on its terms rather than on the applicant's intentions, so the phasing has to be built into the permission itself.
Approving a phasing plan first
The practical first step is to get the phasing agreed and documented. That means a drawing showing the extent of each phase with areas, a schedule of what each phase contains, and the sequence.
Two traps appear here. A phase boundary drawn through a habitat parcel splits that parcel's units between phases and forces an arithmetic reconciliation nobody wants to do. And a phase that contains all the development and none of the habitat creation loads the gain onto later phases, which is precisely the risk the authority will push back on.
Draw the phase boundaries around whole habitat parcels where the site allows it. Where it does not, record the split areas in the metric from the outset so the later phase plans reconcile against a figure that already exists.
Calculating the baseline once for the whole site
The pre-development biodiversity value is assessed for the whole site, using the statutory metric, as at the relevant date set by the legislation rather than as at the date each phase starts. That single point matters more than anything else in this area.
It means habitat lost between the relevant date and the start of a later phase does not reduce the baseline. Clearing scrub in year four does not help the arithmetic. It also means that degradation of habitat after the relevant date, whether deliberate or through neglect, is assessed against the original value, so there is no advantage in letting a grassland parcel deteriorate while the first phase is built.
The baseline survey therefore has to be done properly once, by a competent person, with habitat parcels mapped, condition assessed against the metric criteria and the whole site covered rather than just the first phase. A Biodiversity Net Gain Assessment based on a survey of phase one only will not support the later phase plans. Where the site has not been surveyed at all, a Preliminary Ecological Appraisal comes first.
Allocating units between phases
The overall plan has to show how the whole site reaches at least the required percentage gain, and how that total is divided. There is no single correct method, and three approaches are in common use.
| Approach | How it works | Where it fits |
|---|---|---|
| Pro rata by area | Each phase carries gain in proportion to its land area | Uniform sites with similar habitat throughout |
| By habitat loss | Each phase delivers against the units it removes | Sites where habitat value is unevenly distributed |
| Front-loaded delivery | Early phases create more than their share | Sites with a large retained or restorable parcel available early |
Front-loading is the one authorities like, because it reduces the risk of a shortfall at the end. It is also the one that costs the developer money early. Expect to be pushed towards it where the later phases depend on land that is currently in agricultural use.
Whichever method is used, state it in the overall plan and carry the running total forward in each phase plan. A table showing units created to date, units committed and units outstanding is the single most useful page in the document.
Risk if later phases cannot deliver
The uncomfortable scenario is a scheme where phases one to three are built, the gain is in deficit, and phase four turns out to be undeliverable or is sold to someone else. The condition still bites on phase four, so it cannot start without an approved plan, but the units already lost are gone.
Mitigation routes exist and none is painless. Off-site units can be bought and registered, at a cost that has no ceiling. Statutory biodiversity credits can be purchased from the government as a last resort, deliberately priced above the market. Or the scheme can be redesigned to create more on site, which usually means losing plots.
The honest position is that nobody yet knows how authorities will handle a serious end-of-scheme shortfall at scale, because the regime is still young and the thirty-year management periods have barely started. Build the headroom in now rather than relying on a solution emerging.
Securing the whole-site position
Three instruments do the securing, and a well-drafted scheme uses all three. The planning condition controls the start of each phase. A section 106 agreement, or a conservation covenant with a responsible body, secures the habitat for thirty years with the management and monitoring obligations attached. And the land itself has to be available, which means checking title, options and any agricultural tenancies over the parcels earmarked for habitat creation.
Draft the obligation so that the significant on-site enhancements are tied to the phases that can actually deliver them, with a mechanism for substituting equivalent provision if a parcel becomes unavailable. Habitat creation on stripped ground needs the soils handled correctly, as our note on soil translocation and habitat creation explains, and hedgerows are counted in their own module with a separate regulatory overlay covered in our note on hedgerow units and the Hedgerows Regulations.
One drafting point repays attention. Tie the monitoring reports to calendar dates from habitat creation rather than to phase completion, because phase completion slips and monitoring obligations that float with it become unenforceable in practice.
The statutory metric must be completed by a competent person, and the biodiversity gain plan is approved by the local planning authority rather than certified by the applicant. An AI-assisted draft can structure the plan, set out the phasing logic and assemble the policy and legislative framework, but the metric calculation and the ecological judgements behind it need the ecologist. Browse the report types to see how the ecology documents fit together across a phased application.
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.
Frequently asked questions
Does every phase of a development need its own biodiversity gain plan?
On a permission that qualifies as phased development, yes. An overall plan for the whole site is approved first, and then a plan for each phase must be approved before that phase begins. The phase plan shows what that phase delivers and reconciles it against the whole-site target.
Is the baseline recalculated for each phase?
No. The pre-development biodiversity value is assessed for the whole site at the relevant date set by the legislation, not at the start of each phase. Habitat lost or degraded after that date is still counted in the baseline, so there is no arithmetic advantage in clearing land between phases.
How should units be allocated between phases?
There is no prescribed method. Common approaches are pro rata by area, allocation against the habitat each phase removes, or front-loading delivery into early phases. Authorities generally prefer front-loading because it reduces end-of-scheme risk. Whichever is used, state the method in the overall plan and carry a running total through each phase plan.
What happens if the last phase cannot deliver the gain?
The condition still prevents that phase starting without an approved plan, but units already lost cannot be recovered. The routes are buying registered off-site units, purchasing statutory biodiversity credits, which are priced above the market deliberately, or redesigning to create more on site. Build headroom into the early phases instead.
Can a phase boundary cut through a habitat parcel?
It can, but it creates work. The parcel's units then have to be split between phases and reconciled in every later plan. Where the layout allows, draw phase boundaries around whole habitat parcels, and where it does not, record the split areas in the metric from the start.
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.