Frequently asked questions.
What is Biodiversity Net Gain and how will this affect UK developers?
Biodiversity Net Gain (BNG) is a mandatory requirement under the Environment Act. It requires English developers to demonstrate a measurable increase in biodiversity resulting from new development. This means undertaking biodiversity assessments, implementing habitat enhancements, and providing evidence of net gain to secure planning permission.
Where a developer cannot deliver BNG wholly on-site, they can meet the requirement by purchasing off-site Biodiversity Units, which provide a flexible and cost-effective route to compliance.
What is the DEFRA Metric? How is it used to calculate BNG Units?
The DEFRA Biodiversity Metric is a habitat-based tool used to calculate BNG in England. It provides a proxy biodiversity value based on habitat type, distinctiveness, condition, and location. The metric is used to assess biodiversity losses and proposed gains for a development site.
You can find official guidance here: DEFRA Biodiversity Metric Guidance
What is the Biodiversity Metric?
The Biodiversity Metric is central to demonstrating BNG. Developed by Natural England, it allows planners, ecologists, and developers to assess the biodiversity value of land before and after development. The latest version, known as the Mandatory Biodiversity Metric, must be used for all BNG calculations.
How many BNG Units will I need?
The number of units required depends on your site’s size, location, and existing ecological value. An ecologist will use the DEFRA Metric to calculate this. You may be able to deliver some or all of your BNG on-site, but any shortfall must be addressed through off-site units or statutory biodiversity credits.
What is on-site vs. off-site BNG?
• On-site includes land within the red-line boundary of a development application.
• Off-site is any land outside of this boundary, regardless of ownership.
Note: Land within the blue-line boundary is not considered on-site.
How long must off-site BNG be maintained?
All off-site biodiversity gains must be legally secured for a minimum of 30 years, and registered before they can be counted toward BNG. This must be done via a planning obligation or a conservation covenant.
What is the Spatial Multiplier in BNG?
The DEFRA Metric includes a Spatial Risk Multiplier, which reduces the value of habitat enhancements made farther from the development site. This encourages delivery of BNG as close to the development as possible.
If on-site delivery is not feasible, developers can secure off-site units that meet the criteria of the national Biodiversity Gain Site Register. If neither of these options are possible, statutory biodiversity credits from the government may be used as a last resort.
What is a Biodiversity Gain Plan?
A Biodiversity Gain Plan (BGP) is submitted after planning permission is granted. It outlines how BNG will be achieved — including baseline biodiversity, post-development enhancements, and off-site purchases (with DEFRA register reference numbers if applicable).
This plan must be approved by the Local Planning Authority before development can begin.
When does a BNG Unit Option or Purchase need to be included in a planning application?
BNG is typically secured via a Section 106 agreement. To obtain planning permission, developers must demonstrate a compliant BNG solution.
Once planning is granted, the developer must submit a Biodiversity Gain Plan. Off-site units are formally retired from the DEFRA Register at this stage, and this must be completed before development begins.
What is the process for buying BNG Units through Biodiversity Units UK?
The buying process is simple and structured:
1. Submit project info – Provide your DEFRA Metric, site address, and planning timeline.
2. Receive BNG options – We’ll search the market and present tailored solutions.
3. Select your preferred option – We’ll issue a Letter of Support and Solution Methodology for your planning application.
4. Reserve your units – Pay a 10% deposit to reserve units for up to 12 months.
5. Complete your purchase – Pay the balance, and your units will be registered on the DEFRA Register.
6. Notify your planning officer – Submit your documents to the Local Planning Authority as evidence of BNG compliance.
What documents will I receive during the BNG Unit purchase process?
From Biodiversity Units UK, you will receive:
✅ Letter of Support
✅ Solution Methodology
✅ Letter of Reservation
✅ Option Agreement (or Purchase Agreement)
✅ Updated DEFRA Metric
✅ Biodiversity Gain Site Registration Number
✅ Supplier’s Habitat Management Plan
✅ Support for Section 7 of your Biodiversity Gain Plan
From your ecological consultant or project team, you should prepare:
✅ Original DEFRA Metric
✅ Ecologist’s Report
✅ Your Biodiversity Gain Plan (including other project information)
✅ On-site Habitat Management and Monitoring Plan (if applicable)
What are the key components of mandatory BNG?
• Minimum 10% net gain in biodiversity
• Calculated using the DEFRA Biodiversity Metric
• Must be secured for 30 years via legal agreement
• Delivered on-site, off-site, or through statutory biodiversity credits
• Biodiversity Gain Plan must be submitted and approved
• Sites delivering BNG must be recorded on the Biodiversity Gain Site Register
• Applies to most planning applications, except certain exemptions
• Retains the mitigation hierarchy: avoid, mitigate, compensate
When did BNG become mandatory? What about small sites?
Mandatory BNG began on 12 February 2024 for most developments. For small sites, the requirement started in April 2024, giving smaller developers and planning authorities additional time to adapt.
Small sites are defined as:
• Residential: 1–9 dwellings on a site under 1 hectare
• Non-residential: Less than 1,000 m² of floor space or a site under 1 hectare
Is BNG mandatory outside of England?
No. BNG under the Environment Act currently applies only in England. The provisions are set out in Schedule 14 of the Act. Other nations, such as Scotland and Wales, are developing their own approaches to biodiversity enhancement.
What is the national biodiversity credits scheme?
This is a government-run scheme where developers can purchase statutory biodiversity credits when BNG cannot be delivered on-site or off-site. These credits are priced higher than market rates to encourage natural market delivery first. Revenue from credit sales is reinvested into habitat creation. Natural England manages the scheme and is developing a user-friendly digital platform for transactions.