Biodiversity Net Gain (BNG) requires developers to understand the biodiversity value of a development site before development and compare it with the value expected after development.
In England, this is calculated using the statutory biodiversity metric, which converts habitat information into standardised biodiversity units. The metric considers factors including habitat size, type, condition and strategic significance, alongside additional risk factors for habitats that will be created or enhanced.
For developments subject to mandatory biodiversity net gain, the calculation must demonstrate the required uplift in biodiversity value. The statutory framework generally requires at least a 10% net gain, although developers must also comply with relevant trading rules and any applicable local planning requirements. The government’s guidance on calculating biodiversity value explains how the statutory biodiversity metric is used for development and land-management proposals.
A robust biodiversity net gain assessment therefore begins with accurate ecological information. The calculation is only as reliable as the habitat data entered into the metric tool.
What Is the DEFRA Biodiversity Metric?
The DEFRA biodiversity metric is the standardised method used to measure the biodiversity value of habitats for statutory BNG in England.
The current Statutory Biodiversity Metric was published by DEFRA in November 2023 and became part of the mandatory BNG framework in February 2024. It replaced the earlier Biodiversity Metric 4.0 for statutory calculations.
The metric produces biodiversity units representing the relative ecological value of habitats. These units enable the pre-development biodiversity value of a site to be compared with its projected post-development biodiversity value.
There are three separate categories of biodiversity units: area habitat units, linear units (such as hedgerows) and watercourse units. They are calculated separately and cannot simply be combined into a single overall figure.
DEFRA publishes the Statutory Biodiversity Metric tools and guidance, including the calculation tool, condition assessments and user guide required to complete an assessment correctly.
Why Biodiversity Net Gain Must Be Measured
Biodiversity net gain means leaving biodiversity in a measurably better state than it was before development.
The word “measurably” is important. Developers cannot simply state that landscaping, tree planting or habitat creation will improve biodiversity. The biodiversity gain must be quantified using the statutory metric where the mandatory BNG framework applies.
The metric establishes the baseline biodiversity value and then calculates the projected value following development. It accounts for existing habitat that will be retained, lost or enhanced, as well as proposed habitats that will be created.
For developments subject to mandatory BNG, the post-development calculation must demonstrate at least a 10% increase in biodiversity units while also complying with the biodiversity gain hierarchy and statutory trading rules.
From 6 August 2026, however, planning applications for developments on sites of 0.2 hectares or below are generally exempt from BNG, provided the relevant conditions are met and no important features, such as priority habitats, are within the application boundary. Other exemptions also continue to apply, so the scope of mandatory BNG should always be checked for the individual development.
BNG Assessment Process
A biodiversity net gain assessment typically follows a sequence beginning with understanding the existing ecological value of the proposed development site.
The baseline habitat is identified and mapped before its size, habitat type, condition and strategic significance are assessed. This information is entered into the statutory biodiversity metric calculation tool to establish the site’s baseline biodiversity units.
The proposed development is then assessed. Habitats being retained, enhanced, lost or created are entered into the metric, together with the relevant factors affecting their future value.
The resulting post-development biodiversity value is compared with the baseline. Where the required net gain cannot be achieved through on-site gains, the assessment can establish how many off-site units may be required.
This makes the biodiversity metric valuable throughout the planning process. Changes to site plans can alter the number and type of biodiversity units required, so repeated calculations can help a project team understand the biodiversity and financial implications of different layouts. Government guidance specifically recommends early and repeated use of the metric to compare different BNG proposals.
Baseline Habitat Surveys
Before you can calculate biodiversity net gain, you need to know what is already present.
A baseline habitat survey identifies and maps the existing habitat across the development site. Depending on the scheme, this may include a range of habitat and features such as modified grassland, woodland, scrub, ponds, hedgerows, watercourses or other wildlife habitats and habitat features.
The habitat assessment establishes the data needed for the pre-development side of the biodiversity metric.
For the main statutory metric, a competent person such as an ecologist should undertake the assessment and use the metric tool. The government notes that a local planning authority may reject calculations where it does not believe they have been completed by a competent person.
Accurate baseline surveys are particularly important because errors at this stage affect every subsequent calculation. If existing habitat is incorrectly classified or its condition is inaccurately assessed, the resulting baseline biodiversity value may also be incorrect.
Habitat Distinctiveness Explained
Distinctiveness reflects the relative ecological importance of a habitat.
Within the statutory metric, habitat types are assigned distinctiveness values rather than developers choosing their own scores. Habitats with greater ecological importance generally have higher distinctiveness and therefore generate a higher number of biodiversity units.
This has significant implications for development.
Losing an area of low-value modified grassland will produce a very different biodiversity unit deficit from losing the same area of a high-distinctiveness habitat.
The metric’s trading rules also restrict how losses can be compensated. In particular, high-distinctiveness habitats cannot simply be replaced with larger areas of lower-distinctiveness habitat. The framework encourages like-for-like or like-for-better compensation, with specific requirements applying to higher-value habitats.
Irreplaceable habitats are treated separately from the standard 10% BNG calculation because their ecological importance means their loss cannot simply be represented through ordinary biodiversity unit trading.
Habitat Condition Assessment
Habitat type alone does not determine biodiversity value. Its condition also matters.
Two parcels of the same habitat type and size can produce different biodiversity unit values if one is in significantly better ecological condition.
Condition assessments use standard criteria relevant to the particular habitat. These assessments consider ecological characteristics that indicate how well the habitat is functioning.
DEFRA provides statutory biodiversity metric condition assessments to create a standardised approach to collecting the habitat condition information entered into the metric.
Condition becomes equally important when assessing proposed habitat enhancement. If a development proposes enhancing existing habitat from a lower condition to a higher condition, the projected improvement must be realistic and supported by appropriate habitat creation, enhancement and management measures.
Strategic Significance and Spatial Risk
Strategic significance considers whether a habitat is located in an area identified as locally important for nature recovery.
This can be influenced by relevant local strategies, including Local Nature Recovery Strategies (LNRSs). Habitats that contribute to identified local ecological priorities may receive greater value through the strategic significance component of the metric.
This helps encourage habitat creation and enhancement in places where it can contribute to wider ecological connectivity rather than creating isolated pockets of habitat.
Spatial risk is particularly important for off-site habitats. The statutory biodiversity metric applies a spatial risk multiplier where off-site provision is located further from the development, encouraging biodiversity gains to be delivered on or close to the development where appropriate.
This means the location of off-site biodiversity units can directly affect how many units are required.
Using the DEFRA Metric for BNG
The statutory biodiversity metric tool brings the ecological information together into a structured calculation.
For existing area habitats, the calculation takes account of the size of the habitat parcel, its distinctiveness, condition and strategic significance to establish its biodiversity unit value.
Proposed habitats introduce further considerations because the metric must account for the uncertainty associated with creating or improving habitat in the future.
Difficulty considers how challenging the target habitat is to create or enhance successfully.
Time to target condition accounts for the delay before a proposed habitat reaches the ecological condition being claimed.
For off-site habitats, spatial risk reflects the relationship between the development site and the location where the biodiversity gain will be delivered.
These factors negatively adjust the value of proposed gains compared with existing habitat. In practical terms, developers cannot assume that destroying an established habitat and recreating the same area elsewhere will produce an equivalent biodiversity outcome.
The metric therefore provides a numerical incentive to retain existing habitats and enhance them where appropriate before relying on replacement habitat elsewhere.
How Biodiversity Units Are Calculated
At a simplified level, baseline area habitat biodiversity units are determined using the habitat’s area alongside its distinctiveness, condition and strategic significance.
The post-development calculation then considers what happens to each habitat parcel.
Existing habitats may be retained. Others may be enhanced. Some may be lost entirely, while new habitats may be created as part of the development.
For created or enhanced habitats, the statutory metric applies the appropriate risk factors to reflect the difficulty and time associated with reaching the proposed condition. Off-site gains can additionally be affected by spatial risk.
The calculation therefore compares the pre-development biodiversity value against the projected post-development value to establish the percentage change.
For example, if a baseline contained 100 relevant biodiversity units, achieving a 10% net gain would require the relevant post-development calculation to reach at least 110 units, subject to the metric’s trading rules and separate treatment of area, hedgerow and watercourse units.
If the required biodiversity gain cannot be achieved on site, compliant off-site units can contribute towards the remaining requirement.
Statutory biodiversity credits are available only as a last resort when the necessary BNG cannot be delivered through on-site or off-site provision. A spatial risk multiplier applies to statutory biodiversity credits, meaning developers generally need to purchase two credits for every one biodiversity unit required. This can make statutory credits a significantly more expensive option than securing suitable off-site biodiversity units.
What a BNG Assessment Includes
A complete biodiversity net gain assessment involves more than a completed spreadsheet.
Baseline Habitat Survey
The baseline habitat survey establishes what habitats are present, their extent and their ecological condition before development.
This provides the evidence required to establish the site’s current biodiversity value.
DEFRA Metric Calculations
The completed metric tool calculation records the baseline and proposed habitat information and determines biodiversity units before and after development.
It also identifies whether the required net gain has been achieved and can reveal deficits in particular habitat categories.
Biodiversity Gain Plan
Where mandatory BNG applies, a Biodiversity Gain Plan explains how the development will meet its biodiversity gain objective.
It draws together the relevant metric calculations and identifies how the required gain will be delivered through the biodiversity gain hierarchy, including on-site habitat measures, off-site provision and, only where necessary, statutory biodiversity credits.
Habitat Management and Monitoring Plan
Where habitats need long-term management, a Habitat Management and Monitoring Plan (HMMP) sets out how the proposed habitat creation or enhancement will be delivered, maintained and monitored.
The management assumptions should correspond with the outcomes claimed in the biodiversity metric. If a habitat is being valued on the basis that it will reach a particular condition, the proposed management must realistically support that outcome.
GIS Mapping
Accurate mapping helps establish the extent and location of habitat parcels and provides spatial evidence supporting the metric calculation.
Metric Outputs and Planning Reports
The final assessment should provide planning officers with a clear evidence trail explaining the baseline, proposed biodiversity enhancements, metric calculations and route to achieving the required net gain.
This allows local planning authorities to understand not only the final percentage but also how that result has been reached.
Why Independent Quality Assurance Matters
Small changes within a biodiversity metric calculation can have substantial commercial implications.
Incorrect habitat classification, condition scoring, parcel areas, strategic significance or proposed habitat assumptions can change the number of biodiversity units required. That can subsequently affect development design, off-site requirements and overall project costs.
Quality assurance should therefore examine the evidence supporting the calculation rather than simply checking whether the spreadsheet produces a 10% figure.
For developers, having a robust completed metric is particularly important before purchasing off-site units. Buying units against an incomplete or changing calculation can create a risk of purchasing the wrong habitat type or quantity.
This is also why dynamic costing can be valuable during the planning process. As ecological assessments and development layouts evolve, updated metric outputs can be used to obtain revised prices for the likely off-site requirement.
How Civity Supports DEFRA Metric Assessments
Civity’s role should be distinguished from that of the ecological consultant undertaking the development site’s surveys and statutory metric assessment.
Civity does not undertake pre-planning ecological surveys, preliminary habitat assessments or the developer’s statutory biodiversity metric calculation. These should be completed by an appropriately competent ecological professional where required.
Instead, Civity’s qualified ecologists support developers once their ecological assessments and metric calculations are sufficiently advanced to identify a potential requirement for off-site biodiversity units.
Reviewing Metric Outputs
Civity can review the completed metric outputs provided by the developer’s ecological team to understand the likely off-site unit requirement.
This provides an additional commercial sense-check around the habitat types and quantities that may need to be sourced.
Dynamic Off-Site Unit Costing
Development layouts frequently change as ecological, planning, architectural and commercial considerations are resolved.
Each change can alter the post-development biodiversity value and therefore the number of off-site units required.
Civity can provide fast quotations against updated metric calculations, allowing developers to understand how changes to the scheme affect the likely cost of off-site BNG provision. This gives project teams dynamic cost information as their plans develop rather than forcing them to wait until the design is completely finalised.
Off-Site Biodiversity Unit Availability
Once the required habitat categories and unit quantities are understood, Civity can advise on the availability and pricing of appropriate BNG units for sale.
This can be particularly valuable where specific habitat units are in limited supply. Where appropriate, early engagement can allow developers to discuss availability and potential reservation arrangements before the planning process reaches its final stages.
Supporting Off-Site BNG Delivery
Where a scheme cannot achieve its entire requirement through on-site gains, Civity helps developers secure compliant off-site provision from suitable gain sites.
For developers, this connects the ecological calculation completed by their consultant with the practical and commercial process of sourcing the units needed to satisfy the remaining BNG requirement.
For landowners and land managers, Civity also supports the process of bringing suitable habitat creation and enhancement opportunities into the BNG market.
Frequently Asked Questions
Biodiversity net gain is calculated by establishing the biodiversity units associated with the pre-development baseline and comparing them with the projected post-development biodiversity value. The statutory metric considers factors including habitat size, type, condition and strategic significance, with additional risk factors applying to created, enhanced and off-site habitats.
A biodiversity unit is the standardised measure used by the statutory biodiversity metric to represent the relative biodiversity value of habitat. Area habitat, hedgerow and watercourse units are calculated and treated separately.
For developments using the main statutory biodiversity metric, government guidance says developers or land managers should use a competent person, such as an ecologist, to complete the metric and advise on its calculations. Different provisions apply where the Small Sites Metric can legitimately be used.
The Small Sites Metric is a simplified statutory biodiversity metric available for qualifying small developments. It should only be used where the development meets the relevant criteria.
From 6 August 2026, planning applications for developments on sites of 0.2 hectares or below are generally exempt from BNG, subject to conditions including rules around priority habitat. This should be considered before deciding whether a Small Sites Metric assessment is required.
No. Although a minimum 10% uplift applies to developments within the mandatory BNG regime, a number of exemptions exist. Current exemptions include certain householder developments and, from 6 August 2026, qualifying developments on sites no larger than 0.2 hectares.
The biodiversity gain hierarchy prioritises appropriate on-site delivery, but developers can use registered off-site biodiversity units where the required gain cannot be achieved entirely within the development site.
Statutory biodiversity credits are available as a last resort where the necessary biodiversity gain cannot be secured through on-site or off-site options.
Yes. The statutory biodiversity metric includes a spatial risk multiplier for off-site provision. This creates an incentive to deliver gains on or close to the development where appropriate and means the location of off-site habitats can affect their effective value within the calculation.
The BNG regime for nationally significant infrastructure projects comes into effect from 2 November 2026. Specific biodiversity gain statements and guidance apply to NSIPs, including requirements relating to their baseline and use of the statutory biodiversity metric.
Speak to Civity About Your BNG Requirements
Understanding how biodiversity net gain is calculated using the DEFRA metric is essential because the final calculation determines whether a development can achieve its required uplift on site or needs additional biodiversity units elsewhere.
The ecological assessment and statutory biodiversity metric should be completed by an appropriately competent professional. Once those calculations begin identifying an off-site requirement, developers can benefit from understanding the likely commercial implications as early as possible.
At Civity, we work alongside developers and their ecological teams by reviewing completed metric outputs, providing fast quotations for off-site biodiversity units and updating those costs as development layouts and biodiversity requirements evolve.
Where off-site provision is required, we can help identify suitable units, advise on availability and provide a clear route towards securing the biodiversity units needed for the development.
Early visibility of off-site costs can help developers make better-informed design and planning decisions while ensuring their BNG strategy remains commercially realistic as the project progresses.