Environmental · Mandatory · Pre-Construction

Ecology and Biodiversity Net Gain Report

A report demonstrating that the development will achieve a minimum 10% biodiversity net gain.

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

Legal basis	Environment Act 2021 (Schedule 14) + Town and Country Planning Act 1990 Schedule 7A
Status	Mandatory — 10% biodiversity net gain required from 12 February 2024
Prepared by	Ecologist
Metric	4.0 statutory biodiversity metric
Management commitment	30-year habitat management commitment

1. Biodiversity Net Gain — The Most Significant Planning Change in Decades

Biodiversity net gain (BNG) is the most far-reaching change to the English planning system in a generation. Since 12 February 2024, all planning permissions granted in England (with limited exemptions) must deliver a minimum 10% biodiversity net gain, measured using the statutory biodiversity metric. This obligation is embedded in primary legislation through the Environment Act 2021, which inserts Schedule 7A into the Town and Country Planning Act 1990. BNG is not guidance and it is not a policy aspiration — it is a binding legal requirement.

The statutory biodiversity metric quantifies the biodiversity value of a site before and after development by assigning habitat units to each parcel of land based on habitat type, condition, and strategic significance. The developer must demonstrate that the post-development habitat value exceeds the pre-development value by at least 10%. This gain can be achieved through on-site habitat creation and enhancement, off-site habitat provision on land secured through a conservation covenant or planning obligation, or the purchase of statutory biodiversity credits from the government as a last resort.

The implications for site design are profound. The baseline habitat survey and metric calculation must be carried out before any site clearance or enabling works, because the pre-development biodiversity value is fixed at the point of application. Habitat lost before the baseline survey is recorded cannot be accounted for in the metric, and the local planning authority may require the developer to use an earlier, higher baseline if habitat degradation is suspected. The BNG requirement drives landscape design, site layout, and long-term management obligations.

Start early

Commission the baseline BNG habitat survey and metric calculation as early as possible — it drives site layout and landscape design.

2. Key Sections of the Report

A compliant ecology and biodiversity net gain report must address each of the following areas. The statutory biodiversity metric calculation sits at the heart of the report, but the supporting ecological assessments and management proposals are equally essential.

Section	What it covers
Phase 1 habitat survey	A baseline ecological survey mapping all habitat types present on the site using standard UK Habitat Classification or Phase 1 habitat survey methodology. Each habitat parcel is mapped, measured, and assigned a condition assessment. This survey provides the raw data for the pre-development metric calculation.
Protected species assessment	An assessment of the potential for protected species to be present on the site or affected by the development. This may require targeted species surveys (bats, great crested newts, reptiles, badgers, nesting birds) carried out at the appropriate time of year. Protected species constraints must be addressed separately from BNG but are reported alongside the habitat assessment.
Pre-development metric	The statutory biodiversity metric calculation for the site in its pre-development condition. Each habitat parcel is entered into the metric spreadsheet with its area, habitat type, condition score, and strategic significance. The metric generates a total number of habitat units and hedgerow units representing the baseline biodiversity value of the site.
Impact assessment	An assessment of the habitats that will be lost, retained, or enhanced as a result of the development. The metric records which habitats are to be removed entirely, which are retained in their current condition, and which are retained but will be enhanced through management. This step quantifies the biodiversity loss attributable to the development.
Post-development proposals	The proposed habitats to be created or enhanced on the development site after construction. Each new habitat parcel is entered into the metric with its target habitat type, target condition, and the time required to reach that condition. The metric applies a temporal multiplier and a difficulty multiplier to reflect the risk that the habitat may not achieve its target condition.
BNG achievement	A summary demonstrating that the post-development biodiversity value exceeds the pre-development value by at least 10% for both habitat units and hedgerow units. The metric output clearly shows whether the 10% threshold has been met. If it has not been met on site alone, off-site provision or statutory credits are required.
Offsite BNG (if needed)	Where the 10% net gain cannot be achieved entirely on site, the report must set out the off-site biodiversity gain proposal. This may involve habitat creation or enhancement on land elsewhere, secured through a conservation covenant or section 106 agreement registered as a local land charge. Off-site gain must be delivered by a registered gain site on the national BNG register.
30-year management	All habitats created or enhanced to deliver BNG must be managed and maintained for a minimum of 30 years. The report must include or reference a habitat management and monitoring plan (HMMP) that sets out the management prescriptions, monitoring schedule, remedial actions, and responsible parties for the entire 30-year period.
Biodiversity Gain Plan reference	The biodiversity gain plan is the document submitted to the local planning authority to discharge the BNG condition. The ecology report must provide all the information required for the gain plan, including the completed metric, habitat management plan, and evidence of any off-site gain or credit purchases. The gain plan condition must be discharged before development commences.

3. Common Mistakes

Not surveying before site clearance

The pre-development biodiversity value is fixed at the point of application. If habitat is cleared before the baseline survey is carried out, that habitat is still counted in the metric — the local planning authority can require the use of an earlier, higher baseline derived from historical aerial photography, OS mapping, or previous ecological records. Clearing habitat before the baseline survey does not reduce the BNG obligation; it increases the amount of new habitat that must be created to achieve the 10% gain. Developers who clear sites before commissioning a baseline survey routinely find themselves with a significantly higher and more costly BNG requirement than they would have had if the habitat had been left in place.

Assuming 10% is achievable on urban brownfield sites

Many urban brownfield sites have surprisingly high biodiversity value under the statutory metric, particularly where open mosaic habitat on previously developed land is present. This habitat type scores highly in the metric because of its ecological importance for invertebrates and plants. Developers who assume that a derelict brownfield site has low biodiversity value and that 10% net gain will be easy to achieve on site are frequently surprised when the metric calculation shows a significant deficit. On constrained urban sites, off-site BNG provision or statutory credit purchase may be the only viable route to compliance.

Insufficient financial provision for 30-year management

The 30-year habitat management commitment is a legally binding obligation, not a planning aspiration. The habitats created or enhanced to deliver BNG must be managed, monitored, and maintained for the full 30-year period, and the management prescriptions set out in the habitat management and monitoring plan must be followed. Developers who do not make adequate financial provision for 30 years of habitat management — including ecological monitoring, remedial planting, invasive species control, and management reporting — risk being unable to meet their obligations. A commuted sum, endowment, or management company must be established to fund the management in perpetuity.

4. Frequently Asked Questions

What is the statutory biodiversity metric and how does it work?▾

The statutory biodiversity metric (currently version 4.0) is the calculation tool mandated by the Environment Act 2021 for measuring biodiversity net gain. It is a spreadsheet-based tool published by Defra and Natural England. The metric assigns biodiversity units to each parcel of habitat on a site based on three factors: the size of the habitat (area in hectares or length in kilometres for hedgerows and watercourses), the distinctiveness of the habitat type (ranging from very low to very high, reflecting its ecological value), and the condition of the habitat (ranging from poor to good, assessed against defined condition criteria). These three factors are multiplied together to produce a number of biodiversity units. Strategic significance multipliers are then applied to reflect whether the habitat aligns with local nature recovery priorities. The metric is applied to both the pre-development and post-development scenarios, and the difference between the two determines whether the 10% net gain threshold has been met.

What is a conservation covenant and when is one needed?▾

A conservation covenant is a legally binding private agreement between a landowner and a designated responsible body (such as a local authority, Wildlife Trust, or Natural England) that places conservation obligations on the land. Conservation covenants were introduced by the Environment Act 2021 specifically to support biodiversity net gain delivery. A conservation covenant is needed when off-site BNG is being delivered on third-party land — the covenant binds the land for the 30-year management period and runs with the title, so it is enforceable against successive owners. The covenant is registered as a local land charge. It is an alternative to a section 106 planning obligation for securing off-site BNG, and it can be used where the off-site land is in a different local planning authority area from the development site. The responsible body named in the covenant has a statutory duty to monitor and enforce compliance with the conservation obligations.

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