One of the most common questions developers ask is, ‘When should you start sourcing off-site BNG units?’ The simple answer is as early as possible.
Under the mandatory biodiversity net gain (BNG) framework introduced by the Environment Act, developers must achieve at least a 10% biodiversity net gain before development can commence. While many schemes can deliver some biodiversity gain on-site, a significant number of development projects require off-site biodiversity units to bridge the gap between the pre-development biodiversity value and the required post-development biodiversity value.
Leaving off-site BNG procurement until the end of the planning process can create delays, increase costs and limit the availability of suitable units. Understanding the correct timeline can help secure planning permission efficiently and meet biodiversity obligations.
Understanding Mandatory Biodiversity Net Gain
Mandatory biodiversity net gain began in England in early 2024. Major developments became subject to mandatory BNG from 12 February 2024, while small sites followed on 2 April 2024. Nationally Significant Infrastructure Projects will be subject to BNG requirements from November 2026.
Under the legislation, developers must demonstrate a minimum 10% net gain in biodiversity value using the statutory biodiversity metric. This information is submitted through a biodiversity gain plan, which must be approved before development starts.
The biodiversity gain plan forms a key part of the wider planning process and demonstrates how biodiversity gains will be delivered through onsite habitat enhancement, habitat creation and, where necessary, offsite biodiversity gains.
Stage 1: Feasibility and Baseline Assessment
The earliest opportunity to consider off-site biodiversity units is during the feasibility stage.
Before finalising any design work, developers should carry out biodiversity assessments to establish the baseline biodiversity of the development site. This includes identifying habitats, assessing existing ecological value and calculating baseline biodiversity units using the statutory biodiversity metric tool.
The assessment establishes the pre-development biodiversity value and acts as an early indicator to identify whether the proposed scheme is likely to achieve biodiversity net gain through on-site delivery alone.
At this stage, developers should already begin considering potential off-site solutions, particularly where significant biodiversity loss is anticipated.
Stage 2: Concept Design and Biodiversity Metric Calculations
As site design evolves, biodiversity metric calculations become increasingly accurate.
The statutory biodiversity metric and associated on-site habitat survey assesses existing habitats, area habitat units, linear habitats and habitat condition to determine biodiversity value. It also calculates the projected post-development biodiversity value based on the building(s) footprint, proposed habitat enhancement works, and habitat creation measures.
Where calculations indicate a shortfall, developers can begin estimating how many biodiversity units may be required from off-site habitats.
This is the point at which many developers first engage with habitat bank operators and registered habitat banks to understand availability and indicative pricing.
Stage 3: Applying the Biodiversity Gain Hierarchy
Before sourcing off-site units, the biodiversity gain hierarchy must be followed.
The hierarchy requires developers to prioritise habitat retention, onsite habitat enhancement and biodiversity improvements before relying on offsite biodiversity units. The mitigation hierarchy remains a fundamental principle of biodiversity net gain implementation.
This means site design should focus on protecting existing habitats, enhancing biodiversity within the red line boundary and maximising site biodiversity units wherever possible.
Only once these opportunities have been explored should developers move towards securing off-site gains.
Stage 4: Pre-Application Planning
The best time to actively pursue off-site biodiversity units is before submitting planning applications.
At this stage, biodiversity metric calculations should be sufficiently developed to identify likely unit requirements. Developers can then engage with providers to check availability, obtain accurate pricing and, where appropriate, secure or reserve suitable units within the relevant local planning authority area. If the required habitat type is in limited supply or competitively priced, placing a deposit or reserving units early can help reduce the risk of delays and future price increases as the project progresses.
Early engagement is important because off-site unit availability varies significantly by location, habitat type and demand. Prices also fluctuate depending on habitat distinctiveness, strategic significance and proximity to the development site.
Units located within the same local planning authority area and aligned with local nature recovery strategy priorities generally achieve the highest ecological value within the metric.
Starting early helps avoid situations where suitable units are unavailable when planning permission is approaching determination.
Stage 5: Planning Submission
At the planning submission stage, developers should have a near-final biodiversity metric calculation and sufficient ecological information to demonstrate how the scheme intends to achieve biodiversity net gain. While a formal Biodiversity Gain Plan is not submitted as part of the planning application, many developers choose to prepare a draft version to support discussions with the local planning authority and demonstrate that the proposed approach is achievable.
Where significant on-site biodiversity gains, habitat creation or habitat enhancement are proposed, it is also good practice to prepare a draft Habitat Management and Monitoring Plan (HMMP). This helps show how new or enhanced habitats will be established, managed and maintained over the required 30-year period.
Providing this information at the planning stage gives local planning authorities greater confidence that biodiversity net gain requirements can be delivered and may help resolve ecological queries earlier in the planning process. The formal Biodiversity Gain Plan is then submitted after planning permission has been granted and must be approved before development can commence.
Stage 6: Securing Off-Site Units Before Development Starts
Although planning permission may be granted before final allocation, developers must secure and allocate off-site biodiversity units before development commences.
All off-site units must come from a registered biodiversity gain site listed on the Biodiversity Gain Site Register maintained by Natural England.
The units must also be legally secured through a conservation covenant or planning obligation and supported by a long-term habitat management and monitoring plan.
These legal agreements guarantee that biodiversity gains are maintained for at least 30 years.
Once biodiversity credits are purchased and allocated, responsibility for delivering the biodiversity gains typically transfers to the habitat provider through the relevant legal framework.
What If Suitable Off-Site Units Cannot Be Found?
In some locations, particularly where demand is high, sourcing suitable off-site BNG units can be challenging.
Where developers cannot secure sufficient units through the private market, statutory biodiversity credits may be available. However, developers can only purchase statutory biodiversity credits as a last resort after demonstrating that all other reasonable options have been explored.
These statutory credits are intentionally priced at a premium to encourage investment in habitat creation and biodiversity gain sites rather than government-backed credits.
Why Early Engagement Matters
The most successful projects treat biodiversity net gain as a core part of site strategy from the outset.
Early engagement allows developers to understand likely biodiversity obligations, estimate costs, secure unit availability and avoid delays during determination. It also improves site design by identifying opportunities for habitat retention and biodiversity enhancement before layouts become fixed.
Waiting until planning permission is granted often reduces flexibility and can create unnecessary pressure on programme timelines.
Expert Support for Off-Site BNG Units
At Civity, we help developers navigate every stage of the BNG framework, from biodiversity assessments and metric calculations through to sourcing and securing off-site biodiversity units.
Our team works with registered habitat banks and biodiversity gain sites across England, helping clients identify suitable units early in the planning process and secure compliant off-site BNG solutions aligned with local planning authority requirements.
For developers looking to source units, our available inventory can be viewed through our BNG units for sale service, where we provide rapid pricing, availability checks and expert guidance tailored to individual development projects.
