Summary
BNG used to be something only large housebuilders and their ecologists worried about. Since the small sites extension took effect on 2 April 2024, a builder doing a four-house infill development or a single large barn conversion with a sizeable curtilage can be caught by the same mandatory 10% net gain requirement as a 500-unit scheme — just with a simplified metric tool to make the calculation less onerous.
The mechanism is a biodiversity gain plan: before development can lawfully commence, the local planning authority must approve a plan showing the pre-development biodiversity value of the site (using the statutory biodiversity metric), the post-development value, and how at least a 10% net increase is achieved and secured for a minimum of 30 years, either on-site, off-site (buying biodiversity units from a registered off-site gain site), or through statutory biodiversity credits bought from the government as a last resort (deliberately priced high to discourage reliance on them).
For tradespeople and small developers this is a planning-condition-level issue rather than a trade-specific regulation, but it changes what's biddable on site clearance, landscaping and groundworks packages: hedgerows, trees and habitat features that used to be routinely stripped out to simplify a build now have a measurable value that the client needs preserved, replaced, or compensated for elsewhere — and getting this wrong can hold up commencement on an otherwise-approved planning permission.
Key Facts
- Legal basis — Environment Act 2021, Schedule 7A to the Town and Country Planning Act 1990 (as inserted)
- Mandatory net gain requirement — minimum 10% biodiversity net gain, measured using the statutory biodiversity metric (small sites metric for qualifying small sites)
- Major sites commencement — 12 February 2024
- Small sites commencement — 2 April 2024
- Small site definition (residential) — 1-9 dwellings on a site under 1 hectare (or, where the number of dwellings is unknown, a site area under 0.5 hectares)
- Small site definition (non-residential) — site area under 1 hectare where floorspace created is under 1,000m²
- Securing period — biodiversity net gain must be secured for a minimum of 30 years via planning obligation (s.106 agreement) or conservation covenant
- Exemptions — householder applications, biodiversity gain sites themselves, small self-build and custom housebuilding schemes below a threshold, and certain permitted development rights are excluded
- De minimis exemption — development impacting less than 25m² of onsite habitat (or 5m of linear habitat such as hedgerow) can, in some cases, be exempt from the full metric process
- Metric tool — Natural England's small sites metric (simplified) for qualifying small sites, or the full statutory biodiversity metric for larger schemes
- Off-site units — bought from a registered biodiversity gain site listed on the national biodiversity gain sites register
- Statutory credits — a government-run last-resort mechanism, deliberately priced above market rate for off-site units to disincentivise use
- Habitat baseline survey — normally a UK Habitat Classification (UKHab) survey and condition assessment is needed to establish the pre-development baseline value
- Planning condition mechanism — biodiversity gain plan must be approved by the local planning authority before development can lawfully commence, typically via a pre-commencement condition
Quick Reference Table
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Try squote free →| Site type | Threshold | Metric used | In force from |
|---|---|---|---|
| Major residential | 10+ dwellings or site ≥1ha | Full statutory biodiversity metric | 12 Feb 2024 |
| Major non-residential | Floorspace ≥1,000m² or site ≥1ha | Full statutory biodiversity metric | 12 Feb 2024 |
| Small residential | <10 dwellings, site <1ha | Small sites metric | 2 Apr 2024 |
| Small non-residential | Floorspace <1,000m², site <1ha | Small sites metric | 2 Apr 2024 |
| Householder extension | N/A | Exempt | N/A |
| De minimis habitat impact | <25m² / <5m linear habitat | Simplified/exempt | 2 Apr 2024 |
Detailed Guidance
Working out if a small job is caught
If you're quoting groundworks, landscaping or site clearance on any development requiring planning permission — even a modest infill plot or barn conversion — ask whether a biodiversity gain plan has been approved, or whether the site was assessed as exempt. Do not assume small automatically means exempt: "small sites" under BNG is a defined category that is still fully in scope of the 10% net gain requirement, just assessed with the lighter-touch small sites metric rather than being let off entirely.
What changes on site
Retained hedgerows, trees, and existing habitat features identified in the approved gain plan become contractual requirements, not nice-to-haves. Damaging a retained hedgerow during groundworks, or clearing more scrub than the plan allows, can put the client in breach of their planning obligation and create real cost and delay — this is worth flagging explicitly in your site clearance method statement and pricing a watching brief or ecologist check-in if the plan requires it.
Sequencing on a typical small residential job
The habitat baseline survey needs to happen before any clearance works — surveying a site after it's been stripped destroys the baseline data needed for the metric calculation and can force a worst-case assumption that inflates the net gain requirement. Advise clients to commission the ecology survey early, ideally at the same time as other pre-application surveys (arboricultural, ecology, drainage).
Frequently Asked Questions
Does a single self-build house need biodiversity net gain?
Self-build and custom housebuilding has specific treatment under the regulations, with a threshold-based exemption in some cases, but this has been subject to consultation and change — confirm current status with the local planning authority for the specific application rather than assuming exemption.
Can biodiversity net gain be delivered entirely off-site?
Yes — where on-site delivery isn't practical, developers can buy biodiversity units from a registered off-site gain site on the national register, provided the overall 10% net gain and 30-year securing requirement is still met.
What happens if the biodiversity gain plan isn't approved before we start work?
Development should not lawfully commence without an approved biodiversity gain plan where BNG applies — starting works without one risks the local planning authority treating the development as unauthorised, with enforcement consequences for the client.
Is BNG the same thing as a planning condition to plant some trees?
No — pre-BNG "landscaping" planning conditions were typically unquantified and not independently measured. BNG requires a numerical calculation using the statutory metric and a legally secured 30-year commitment, which is a materially higher bar than a general landscaping condition.
Regulations & Standards
Environment Act 2021 — primary legislation establishing mandatory biodiversity net gain
Town and Country Planning Act 1990, Schedule 7A (as inserted by the Environment Act 2021) — the statutory biodiversity gain requirement mechanism
The Biodiversity Gain Site Register Regulations 2024 (SI 2024/45) — establishes the national biodiversity gain site register and off-site unit registration process
The Biodiversity Gain Requirements (Exemptions) Regulations 2024 (SI 2024/47) — sets out de minimis, self-build/custom-build and other exemptions from the net gain requirement
Natural England Biodiversity Metric (full and small sites versions) — the statutory calculation tools
UK Habitat Classification (UKHab) — the habitat survey methodology underpinning metric baseline assessments
GOV.UK — Understanding biodiversity net gain — Defra/Natural England overview and small sites guidance
GOV.UK — Biodiversity gain site register — off-site unit registration
Natural England — Biodiversity metric — metric tool downloads and guidance
Legislation.gov.uk — Environment Act 2021 — full statutory text
suds adoption schedule 3 guide — another site-level statutory consent that runs alongside planning permission
permitted development householder — for confirming where householder exemptions apply
building regs exemptions — general context on statutory thresholds and exemptions in development