Completions & Planning · Mandatory · Pre-Construction

Biodiversity Net Gain Metric and Habitat Management Plan

The mandatory 10% biodiversity net gain assessment and habitat management plan required for all new planning permissions in England from February 2024 (major) and April 2024 (minor). Uses the DEFRA Biodiversity Metric 4.0 to calculate pre- and post-development biodiversity units. The HMMP creates a 30-year legal obligation for habitat management, secured by s.106 agreement or Conservation Covenant and registered on the Natural England BNG Register.

Last reviewed: 29 March 2026 — This guide reflects UK law as of this date. Environment Act 2021 / Schedule 7A TCPA remains current with no amendments enacted as of 29 March 2026. Next scheduled review: 29 March 2027.

Legal basis	Environment Act 2021 / Schedule 7A Town and Country Planning Act 1990
In force	February 2024 (major developments), April 2024 (minor developments)
Minimum gain	10% net gain in biodiversity value using DEFRA Metric 4.0
HMMP	Habitat Management and Monitoring Plan creates a 30-year management obligation
Legal mechanism	Secured by s.106 agreement or Conservation Covenant
Register	Natural England BNG Register tracks compliance — cannot be ‘greenwashed’

1. Biodiversity Net Gain — A New Mandatory Requirement for Nearly All Development

Biodiversity Net Gain (BNG) is the most significant new planning requirement in decades. The Environment Act 2021 inserted Schedule 7A into the Town and Country Planning Act 1990, making it a condition of every planning permission that the biodiversity value of the development exceeds the pre-development biodiversity value by at least 10%. Without a BNG metric calculation and a Habitat Management and Monitoring Plan (HMMP), planning permission cannot be lawfully implemented.

BNG cannot be ‘greenwashed’ — the Natural England BNG Register tracks compliance, and off-site BNG units must be formally registered before they can count towards a development’s net gain obligation. The 30-year management obligation created by the HMMP is a genuine long-term commitment that must be secured by a s.106 agreement or a Conservation Covenant. Developers who treat BNG as a box-ticking exercise will find that enforcement mechanisms are robust and that non-compliance can prevent occupation of completed units.

Early engagement is essential

Engage a BNG assessor at RIBA Stage 1/2 — early habitat baseline surveys and design integration are far more cost-effective than attempting to achieve 10% net gain retrospectively. Baseline habitat surveys must be carried out before any site clearance, and the metric calculation must inform the site layout and landscape design from the outset.

2. BNG Metric & Habitat Management Plan Record

The following table sets out the information required for the DEFRA BNG Metric 4.0 calculation and the Habitat Management and Monitoring Plan.

Element	Detail
DEFRA BNG Metric 4.0 — Baseline	Pre-development habitat types, areas (hectares), condition assessment, distinctiveness, strategic significance. Baseline survey must be carried out before any site clearance. Survey methodology per UK Habitat Classification.
DEFRA BNG Metric 4.0 — Post-development	Proposed habitat types, areas, target condition, creation or enhancement. On-site habitats shown on landscape/ecology drawings. Off-site habitats identified separately with location and legal agreement reference.
DEFRA BNG Metric 4.0 — Net change	Percentage net change in biodiversity units. Must achieve ≥10% net gain. Broken down by habitat type (area habitats, hedgerow, watercourse). If <10%, additional on-site or off-site habitat required, or statutory biodiversity credits purchased from DEFRA (last resort — significantly more expensive).
HMMP — Habitat areas	All habitat areas subject to the 30-year management obligation. Shown on a habitat plan with areas measured and cross-referenced to the metric calculation.
HMMP — Management actions	Specific management prescriptions for each habitat type: mowing regime, planting schedule, invasive species control, grazing (if applicable), water level management. Detailed enough to be implemented by a land manager.
HMMP — Monitoring programme	Monitoring frequency (typically years 1, 2, 3, 5, 10, 15, 20, 25, 30). Monitoring methodology. Condition assessment criteria. Reporting requirements. Trigger thresholds for remedial action.
HMMP — Responsible party	Named organisation responsible for 30-year management and monitoring. May be the landowner, a management company, a wildlife trust, or a specialist habitat management provider.
Legal agreement	s.106 agreement or Conservation Covenant securing the 30-year obligation. Must be executed before planning permission can be lawfully implemented.
BNG Register	Off-site BNG units must be registered on the Natural England BNG Register. Registration reference, number of units, habitat type, and location recorded.
Assessor	Name, qualifications (e.g. CIEEM membership), and organisation of the ecologist who prepared the metric calculation and HMMP.

3. Common Mistakes

Starting construction before BNG conditions are discharged

BNG is typically secured by a pre-commencement planning condition. The BNG metric, HMMP, and legal agreement (s.106 or Conservation Covenant) must all be in place before construction begins. Starting work before these conditions are discharged is a breach of planning control and can result in enforcement action, including a stop notice. The LPA will require evidence that the baseline survey was carried out before any site clearance.

Off-site BNG units not registered on the Natural England register

Where a development relies on off-site BNG units to achieve the 10% net gain, those units must be formally registered on the Natural England BNG Register before they can count towards the development’s obligation. Unregistered off-site units have no legal status and will not be accepted by the LPA. Registration can take several weeks and requires the off-site habitat provider to submit evidence of the legal agreement securing the 30-year management obligation.

4. Frequently Asked Questions

What are the exemptions from mandatory BNG?▾

The following are exempt from mandatory BNG: householder applications (extensions, loft conversions, etc.), development with de minimis biodiversity impact (sites with minimal habitat value), self-build and custom build housing, Crown development, and permitted development that does not require planning permission. Small sites (minor development) were brought into mandatory BNG from April 2024, so the ‘small site’ exemption no longer applies. If in doubt, check with the LPA — exemptions are narrowly defined.

What happens if the 10% net gain cannot be achieved on-site?▾

If the development site cannot deliver 10% net gain on-site, the developer must secure off-site BNG units. These can be purchased from a registered off-site habitat provider (a landowner who has created or enhanced habitat and registered the units on the Natural England BNG Register). As a last resort, statutory biodiversity credits can be purchased from DEFRA, but these are deliberately priced significantly higher than the market rate for off-site units to incentivise genuine habitat creation.

How long does the management obligation last?▾

The HMMP creates a 30-year management and monitoring obligation from the date the habitat is created or enhanced. This applies to both on-site and off-site habitats. The obligation is secured by a s.106 agreement or Conservation Covenant and runs with the land — it binds future landowners, not just the original developer. Failure to manage habitats in accordance with the HMMP is a breach of the legal agreement and can be enforced by the LPA or the responsible body under the Conservation Covenant.

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