Biodiversity net gain on outline and phased applications
Phased schemes need an overall biodiversity gain plan plus a plan for each phase, all measured against a baseline fixed at the outline application date. Here is how to set that up.
An outline permission carries the biodiversity gain condition in a modified form, and the modification is what makes a phased scheme workable. For phased development the condition requires an overall biodiversity gain plan covering the whole site, approved before the first phase starts, and then a separate phase biodiversity gain plan approved before each later phase. The baseline is fixed once, at the outline stage, and every phase measures against it. Get that wrong on a scheme with a ten year build programme and the metric has to be rebuilt.
How the condition attaches to an outline permission
The deemed condition inserted by the Environment Act 2021 applies to outline permissions as it does to full ones. The modification regulations made in 2024 adjust the sequence so that approval of reserved matters and the start of each phase sit sensibly against the gain plan.
In practice the applicant elects whether the development is phased. That election belongs in the application, not in an afterthought once the first spade is in the ground, because it governs how many gain plans the council will be approving and how the section 106 obligations are drafted.
An outline application should therefore carry a metric calculation based on the parameter plans and the illustrative masterplan, together with the habitat survey that underpins it. A Biodiversity Net Gain Assessment drafted at this stage is a scoping document for the ecologist, and the statutory metric itself has to be completed by a competent person under the Environment Act regime.
Fixing the baseline date and living with it
The pre-development baseline is assessed as at the relevant date, which for an outline permission is the date of the outline application. That date then governs every phase.
This cuts both ways. A developer who surveys a site in a wet spring and records species-rich grassland is locked into that baseline even if the sward declines over the following six years. A developer who clears scrub before applying gains nothing, because habitat degraded without consent can be assessed as it stood on an earlier date, back to 30 January 2020.
Two practical consequences follow. Survey properly at outline stage, in the growing season, with the National Vegetation Classification work that supports the habitat calls. And keep the baseline evidence pack alive, because the ecologist preparing phase four's plan in 2032 will be relying on a survey nobody on the team remembers commissioning.
Parameter plans that protect the habitat you are relying on
Habitat retention areas only survive to reserved matters if they are drawn into the parameters. A green infrastructure parameter plan with fixed boundaries for retained hedgerows, woodland buffers and watercourse corridors binds later design in a way that an illustrative layout does not.
The areas worth fixing are the ones the metric depends on most.
- Retained high distinctiveness habitat, since losing it later forces compensation under the trading rules.
- Woodland and hedgerow buffers, including root protection areas.
- Watercourse corridors, which score in the separate watercourse module of the metric.
- Habitat creation areas large enough to reach target condition, rather than residual strips left over from the engineering layout.
Where ancient woodland, veteran trees or other irreplaceable habitat is present, the metric does not deal with it at all. Those habitats sit under separate rules requiring bespoke compensation agreed with the authority, and no amount of habitat units elsewhere substitutes for them. A Preliminary Ecological Appraisal at the outline stage is where that should be identified.
Phase plans, metric updates and reserved matters
Each phase plan reports the position for that phase against the overall plan. The overall plan sets the trajectory for the whole site, the phase plan proves the phase delivers its share.
| Stage | Document | Approved by |
|---|---|---|
| Outline application | Metric based on parameter plans | Considered at determination |
| Before first phase | Overall biodiversity gain plan | Local planning authority |
| Each reserved matters | Updated metric for that phase layout | Considered at reserved matters |
| Before each later phase | Phase biodiversity gain plan | Local planning authority |
Metric versions change over a long programme. Defra reissues the statutory metric periodically, and a scheme consented on one version may be reporting phases under a later one. Agree with the council at the outset which version governs the overall plan and how updates will be handled, and record it in the section 106 rather than in an email.
Reserved matters is also where habitat creation areas get squeezed. An engineer moves an attenuation basin, a housebuilder adds plots, and the meadow area that carried the gain shrinks by a fifth. The overall plan is the document that makes that visible, because it states the units each phase was relied on to deliver.
If a phase falls short, the shortfall has to be made up somewhere. That is either a redesign, an increase in onsite creation elsewhere, or a purchase of offsite units at whatever the market asks on the day. None of those is a decision to leave to the site team.
Securing delivery when phases change hands
Onsite habitat is secured by condition and by the section 106 obligation, which binds successors in title. Offsite units are secured through registration on the biodiversity gain sites register, with the units allocated to the specific permission, and through a section 106 agreement or a conservation covenant running for 30 years.
The weak point is the phase sold to another housebuilder mid-programme. Three things need to be explicit in the agreement.
- Which phase carries responsibility for each habitat parcel, including parcels that straddle a phase boundary.
- Who submits the monitoring reports, and to whom, once the site is in multiple ownerships.
- What happens if a phase is never built, and how the overall plan is corrected.
Management companies inherit the 30 year obligation on most residential schemes. Their budget has to reflect meadow cutting and arisings removal rather than amenity mowing, and that is a service charge conversation nobody wants to have late.
Where protected species are involved, the licensing position runs alongside all of this and on its own timetable. A Protected Species Survey and any mitigation licence require a suitably licensed ecologist, and a European protected species licence for a later phase cannot be applied for until that phase's detail is fixed.
What to settle before the outline application goes in
Decide the phasing, commission the survey in season, fix the parameter plans around the habitat you intend to keep, and agree the metric version. Then write the section 106 so that a purchaser of phase three in eight years time can work out what they have bought.
An illustrative example: a 200 unit scheme with four phases retains a hedgerow network scoring high distinctiveness. Fixing those hedgerows and their buffers on the green infrastructure parameter plan at outline is what stops phase two's road alignment from removing 400 metres of it, and hedgerow units are traded under stricter rules than the grassland that would have replaced them.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
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Frequently asked questions
Does the biodiversity gain condition apply to outline permissions?
Yes. The deemed condition attaches to outline permissions in a modified form. For phased development an overall biodiversity gain plan must be approved before the first phase starts, and a phase plan must be approved before each later phase. The applicant elects whether the development is phased, and that election belongs in the application.
What date is the biodiversity baseline taken from on an outline scheme?
The relevant date is normally the date of the outline application, and it governs every phase that follows. Where habitat has been degraded without consent the authority can assess the baseline as at an earlier date, going back to 30 January 2020. Survey in the growing season and keep the evidence pack.
What happens if the statutory metric is updated mid-build?
Defra reissues the statutory metric from time to time, so a long programme can span versions. Agree at the outset which version governs the overall plan and how later phases will report, and record that agreement in the section 106 rather than in correspondence. Councils differ in how they handle the transition.
How is biodiversity net gain secured when a phase is sold?
Onsite habitat is secured by the gain condition and by section 106 obligations that bind successors in title. Offsite units are registered on the biodiversity gain sites register, allocated to the permission, and secured by agreement or conservation covenant for 30 years. Name the responsible party for each habitat parcel in the agreement.
Does the metric cover ancient woodland on a phased site?
No. Irreplaceable habitats including ancient woodland and veteran trees sit outside the metric and require bespoke compensation agreed with the local planning authority. Habitat units created elsewhere on the site do not substitute for them, so identify these constraints in the preliminary ecological appraisal before parameter plans are fixed.
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.