Biodiversity Net Gain Assessment

What is a biodiversity net gain assessment?

A biodiversity net gain (BNG) assessment compares the biodiversity value of existing habitats with the value expected after development. It combines a habitat survey, condition assessment and metric calculations to identify habitat losses, proposed gains and any remaining shortfall.

It answers practical questions: which habitats should be retained, can the layout meet the required gain, and will off-site units be needed?

The assessment informs planning and habitat delivery. It does not guarantee permission or replace other ecological assessments.

For an introduction to the wider requirements, read our guide to biodiversity net gain.

Do you need a BNG assessment?

Delivering at least 10% BNG is a legal requirement for most non-exempt developments in England subject to the statutory framework. The rules began for relevant major planning applications in February 2024 and extended to relevant smaller developments in April 2024.

However, application dates, exemptions and the planning route must be checked. Since August 2026, qualifying sites of 0.2 hectares or less may be exempt where onsite priority habitat is not negatively affected. Special rules apply to changes to existing permissions.

An exemption from mandatory BNG does not remove requirements relating to protected species or other ecological matters. We help establish the appropriate scope before you commission the work.

How our biodiversity net gain assessments work

1. Review your site and proposals

We start with your development site, boundary plan, proposed works and available ecological information. We also consider relevant planning requirements and any previous correspondence with the council.

This establishes the survey scope and whether existing information can be used. We explain what evidence is needed now and what can follow as the design develops.

2. Survey and map existing habitats

The habitat survey identifies and maps the habitat types present, commonly using UK Habitat Classification. Relevant habitats are assessed against the metric’s condition criteria.

Grassland, woodland, hedgerows, trees and watercourses can contribute differently to biodiversity value. Their area or length, condition and other characteristics need accurate recording.

The assessment also identifies limitations, such as restricted access or seasonal uncertainty. A useful baseline should explain the evidence behind the figures.

3. Calculate the habitat baseline

We enter the survey information into the statutory biodiversity metric to calculate the site’s starting biodiversity value.

The metric framework published by the Department for Environment, Food & Rural Affairs (Defra) considers habitat extent, distinctiveness, condition and strategic significance. It assesses area habitats, hedgerows and watercourses separately.

This baseline is used to compare development options. Where previous habitat clearance or degradation is relevant, earlier evidence may need consideration: clearing vegetation before assessment does not necessarily reduce the baseline used for BNG.

4. Test the proposed development

The metric calculates biodiversity net gain by comparing baseline units with the units expected from retained, enhanced and newly created habitats.

We review your layout and landscape proposals to assess the likely outcome. New habitats are subject to factors reflecting establishment time and delivery difficulty, so a proposed replacement does not automatically equal an existing habitat.

If the scheme falls short, we explain which changes could help. A positive overall percentage must also satisfy the metric’s rules for habitat replacement and separate unit categories.

5. Identify a delivery approach

We consider suitable site enhancements, habitat creation and any off-site requirements.

This means checking whether proposals are achievable in the available space and compatible with access, drainage, public use and future maintenance. The outcome should be a workable habitat strategy supported by the calculations.

What will you receive?

The agreed scope can include a baseline habitat plan, habitat condition evidence, completed metric calculations, a proposed habitat plan and a written BNG report.

The report should explain the results, assumptions and recommendations. You should be able to see where losses occur, what gains are proposed and whether more work is required.

We can also discuss support with biodiversity gain plans and habitat management documents. Your quote should identify which stages are included, whether design revisions are covered and which later tasks require separate instruction.

Designing habitats that work for your site

A BNG assessment is most useful while your design remains flexible. Retaining a valuable hedgerow or changing a building position can sometimes reduce replacement requirements more effectively than adding planting afterwards.

Opportunities to enhance habitats must reflect the land’s conditions and intended use. A species-rich grassland proposal, for example, needs suitable soils, establishment methods and mowing arrangements.

We coordinate ecology with landscape design so habitat proposals can be considered alongside the wider scheme. The final measures should be realistic to deliver and maintain.

How do local nature recovery strategies affect BNG?

Local nature recovery strategies (LNRS) identify priorities and opportunities for nature recovery across England. They can inform the strategic significance inputs used in the metric.

Relevant mapped measures and current guidance need checking for the site’s location. A habitat proposal may attract a higher strategic significance value where it meets the applicable criteria; proximity to a mapped area alone is not sufficient.

For developers, this can help identify opportunities to connect habitat improvements with local priorities rather than choosing measures solely to increase the headline percentage.

What if your site cannot deliver enough gain?

Delivery options vary depending on site and off site opportunities, habitat requirements and the applicable biodiversity gain hierarchy.

Off-site gains can be provided on other suitable land or purchased as units from land managers. The unit type, location, legal security and allocation to your development all need checking.

Our habitat banking support can help you explore off-site delivery. Privately purchased biodiversity units are different from statutory biodiversity credits bought from the government, which are a last resort.

Eligible small developments can consider on-site and off-site options together under the rules introduced in August 2026.

How are irreplaceable habitats treated?

Irreplaceable habitats, including ancient woodland and ancient or veteran trees, require separate consideration. They cannot be treated as ordinary habitat losses that a 10% uplift elsewhere automatically resolves.

The assessment needs to identify relevant habitats and explain potential effects. Avoidance, planning policy and bespoke compensation requirements must be addressed where applicable.

BNG calculations do not override the wider protections affecting these habitats.

BNG assessment, planning approval and ongoing management

Local planning authorities need the required biodiversity information when considering non-exempt applications. A biodiversity gain plan must subsequently be approved before development starts where the statutory condition applies.

The assessment and gain plan serve connected purposes: one establishes the evidence and delivery options; the other records how the approved development will meet its obligations.

Off-site gains and significant on-site enhancements generally need legal security and management for at least 30 years. A habitat management and monitoring plan sets out establishment, maintenance, checks and corrective action.

We help you consider these responsibilities alongside ownership and the future use of the land.

Frequently asked questions

How much does a biodiversity net gain assessment cost?

Cost depends on site size, habitat complexity, survey requirements and the amount of design work needed. Send your boundary plan and proposals for a quote that explains the scope. Habitat works, off-site units, legal agreements and ongoing management may involve additional costs.

How long does a BNG assessment take?

Timing depends on access, survey suitability, available information and the stage of your design. Seasonal limitations or changes to the layout can affect completion. Tell us your submission date so we can discuss a realistic programme.

Is a PEA the same as a BNG assessment?

No. A preliminary ecological appraisal identifies wider ecological constraints and survey requirements. BNG work measures habitat value and tests gains. They can share survey information, but one does not automatically include the other.

Book your biodiversity net gain assessment

Collington Winter helps development projects establish their habitat baseline, understand the calculations and plan how to achieve biodiversity net gain.

Contact our team with your site location, plans and target dates to request a quote.

Call 01204 939 608 or email info@collingtonwinter.co.uk.