Biodiversity Net Gain Assessment
Biodiversity Net Gain became a mandatory requirement for most planning applications in England from February 2024 for major developments and April 2024 for minor developments. It represents a fundamental shift in how ecology is handled in planning — developments must now leave biodiversity in a measurably better state than before.
Typical Cost
£300 – £5,000+
Turnaround
1 – 6 weeks
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What is a Biodiversity Net Gain Assessment?
A Biodiversity Net Gain (BNG) assessment uses the statutory biodiversity metric (currently Metric 4.0) to calculate the biodiversity value of a site before and after development, measured in 'biodiversity units.' The assessment must demonstrate that the development will achieve a minimum 10% net gain in biodiversity units compared to the pre-development baseline. This gain can be achieved through on-site habitat creation and enhancement, off-site habitat provision, or the purchase of statutory biodiversity credits as a last resort.
When is a Biodiversity Net Gain Assessment required?
BNG is mandatory for all planning applications granted permission under the Town and Country Planning Act 1990 in England, with limited exceptions. Exempt developments include householder applications, self-build and custom housebuilding, developments affecting biodiversity minimally (below a de minimis threshold), and development on irreplaceable habitats where a separate bespoke approach is required. The requirement applies from February 2024 for major developments and from April 2024 for all other developments, as set out in the Environment Act 2021.
What does a Biodiversity Net Gain Assessment include?
A BNG assessment includes a pre-development baseline habitat survey mapping all habitats within the site boundary using UK Habitat Classification, condition assessments for each habitat parcel, application of the statutory biodiversity metric to calculate baseline biodiversity units, a post-development habitat plan showing retained, enhanced, and newly created habitats, a metric calculation demonstrating a minimum 10% net gain, a habitat management and monitoring plan covering a minimum 30-year period, and details of any off-site gains or statutory credit purchases required to achieve the target.
How much does a Biodiversity Net Gain Assessment cost?
A BNG assessment for a small residential site typically costs between £1,000 and £2,500, covering the baseline survey, metric calculations, and reporting. Larger sites requiring detailed habitat condition assessments and complex metric modelling usually cost £2,500 to £6,000. The assessment cost is separate from the cost of actually delivering the biodiversity gain — habitat creation, management, and 30-year monitoring can add significantly to overall project costs. Off-site biodiversity units, where required, are priced by the market and can cost £15,000 to £40,000 or more per unit depending on habitat type and location.
Who can prepare a Biodiversity Net Gain Assessment?
BNG assessments must be prepared by a competent ecologist who understands the statutory biodiversity metric and UK Habitat Classification system. CIEEM has published guidance on competencies required for BNG practitioners. The ecologist conducting the habitat survey must be able to accurately classify habitats and assess their condition against the published condition criteria — errors in classification or condition assessment can significantly affect the metric outcome and may result in challenge by the planning authority.
How long does a Biodiversity Net Gain Assessment take?
The baseline habitat survey can be conducted at any time of year but is best carried out between April and September when vegetation is fully identifiable. The survey typically takes one to two days on site depending on the area. Metric calculations and report preparation take a further 1 to 2 weeks. However, the overall timeline depends on whether sufficient net gain can be achieved on site or whether off-site solutions need to be identified and secured — negotiating off-site agreements can take several months.
Frequently Asked Questions
What is the 10% biodiversity net gain requirement?
The Environment Act 2021 requires that all planning permissions granted in England result in a minimum 10% increase in biodiversity value compared to the pre-development baseline. This is measured using the statutory biodiversity metric, which calculates biodiversity units based on the area, habitat type, condition, and strategic significance of each habitat on the site. The 10% figure is a minimum — local plan policies may set higher targets.
Do I need BNG for a householder application?
No. Householder planning applications — such as extensions, loft conversions, and garden buildings — are exempt from the mandatory BNG requirement. However, some local authorities may still request ecological information for householder applications in sensitive locations, and general obligations to protect biodiversity under the NPPF and Natural Environment and Rural Communities Act 2006 still apply.
What is the statutory biodiversity metric?
The statutory biodiversity metric (currently version 4.0, published by Defra) is a calculation tool that assigns biodiversity units to different habitat types based on their area, ecological distinctiveness, condition, strategic significance, and connectivity. It uses the UK Habitat Classification system to categorise habitats. The metric produces separate unit totals for area habitats, hedgerow habitats, and watercourse habitats, and net gain must be demonstrated for each category.
Can I achieve biodiversity net gain off site?
Yes, but there is a hierarchy. You should first try to avoid impacts and then maximise on-site habitat retention, enhancement, and creation. Where sufficient gain cannot be achieved on site, off-site habitat creation or enhancement can be used. Off-site gains must be registered on the national biodiversity gain sites register and secured for at least 30 years through a conservation covenant or planning obligation. The purchase of statutory biodiversity credits from the government is the option of last resort.
How long must biodiversity gains be maintained?
All biodiversity gains — whether on site, off site, or delivered through statutory credits — must be maintained for a minimum of 30 years. On-site gains are secured through a planning condition requiring a habitat management and monitoring plan. Off-site gains are secured through legal agreements registered on the national biodiversity gain sites register. This 30-year commitment means long-term management costs must be factored into the development.
What habitats count as irreplaceable under BNG?
Irreplaceable habitats are those that cannot be compensated for through the metric because of their ecological significance and the impossibility of recreating them. Examples include ancient woodland, ancient and veteran trees, blanket bog, limestone pavement, and lowland fen. Development affecting irreplaceable habitats must follow a bespoke approach to compensation, agreed with the local planning authority and Natural England, rather than using the standard metric.
What is the small sites metric?
The small sites metric is a simplified version of the statutory biodiversity metric designed for minor developments. It uses fewer data inputs and a simplified habitat classification, making it quicker and cheaper to apply. It is available for developments of fewer than 10 dwellings on sites under 1 hectare, or for non-residential developments below 1,000 square metres on sites under 1 hectare. Larger developments must use the full statutory metric.
Can I carry out a BNG habitat survey in winter?
A winter survey is possible but has limitations. Some habitats — particularly grasslands and wetlands — are much harder to classify accurately when vegetation is dormant. Condition assessments may also be unreliable outside the growing season. If a winter survey is necessary for programme reasons, the ecologist should note any limitations and may recommend a follow-up visit during the growing season to confirm habitat classifications and condition assessments.
What happens if I cannot achieve 10% net gain?
If on-site measures alone cannot deliver 10% net gain, you must explore off-site options. This involves either purchasing off-site biodiversity units from a registered gain site provider or, as a last resort, purchasing statutory biodiversity credits from the government. The statutory credits are deliberately priced high (starting at £42,000 per unit for some habitat types) to incentivise on-site and local off-site delivery. Planning permission cannot be granted unless the 10% gain is demonstrably achievable.
Does BNG apply to permitted development?
No. Biodiversity net gain is only mandatory for development that requires planning permission under the Town and Country Planning Act 1990. Permitted development rights, including those exercised through prior approval processes such as office-to-residential conversions under Class MA, are currently exempt from the BNG requirement. However, permitted development must still comply with other wildlife legislation and avoid harming protected species.