You have planning permission — but you still cannot lawfully break ground.
For many developers, biodiversity net gain (BNG) is now the reason why. Since February 2024, BNG has been a mandatory planning requirement for most developments in England. Failure to address it properly can delay projects long after permission has been granted.
This article explains what BNG means for developers, when it applies, and why it needs to be built into schemes far earlier than many expect.
What does “10% net gain” actually mean?
Biodiversity net gain is designed to ensure that a development leaves biodiversity in a measurably better state than before works began, whether this biodiversity is on or off site.
In practice, legislation requires at least a 10% increase in biodiversity value compared with the site’s pre‑development condition. That uplift must be evidenced, legally secured and maintained for a minimum of 30 years
Put simply, if your site is assessed as being worth 50 biodiversity units before development, your completed scheme must deliver at least 55 units — and keep them in place for decades.
BNG is not an aspiration or a planning policy preference. It is a statutory condition attached to planning permissions under the Town and Country Planning Act 1990 that which requires a biodiversity gain plan to be submitted to and approved by the local planning authority before development can lawfully commence, even though the condition does not appear in the planning decision notice.
When does BNG apply?
BNG applies to most planning applications in England, subject to limited statutory exemptions.
The regime came into force on:
- 12 February 2024 for most developments; and
- 2 April 2024 for developments on small sites, which are subject to modified requirements and a simplified biodiversity metric.
Crucially, BNG does not apply retrospectively. Applications made before 12 February 2024 generally fall outside the mandatory regime (subject to transitional rules), even if permission is granted later
Although certain developments are exempt — including householder schemes and developments below a de minimis impact threshold — local planning authorities may still expect biodiversity enhancements through planning policy.
How is biodiversity assessed for planning purposes?
BNG is measured using the statutory biodiversity metric produced by Natural England.
The metric assesses the site by looking at habitats rather than individual species. Habitats are categorised, scored and converted into biodiversity units based on factors including habitat type, condition, size and strategic significance.
That baseline is then compared with the proposed post‑development position, taking account of any approved habitat creation or enhancement.
For developers, the key takeaway is this: your biodiversity position is quantified and assumptions will be tested. Early ecological input helps avoid redesigns, budget shocks and programme slippage.
Where does BNG come from?
From a developer’s perspective, BNG operates within a statutory biodiversity gain hierarchy when delivering BNG:
- Avoid damaging valuable habitats where possible;
- Mitigate impacts that cannot be avoided; and
- Compensate for unavoidable loss.
Compensation is prioritised in the following order:
- enhancement of existing on‑site habitats;
- creation of new on‑site habitats;
- use of registered off‑site biodiversity gains; and
- purchase of statutory biodiversity credits, which are intended to operate as a last resort.
Local planning authorities will scrutinise how this hierarchy has been applied when deciding whether to approve a biodiversity gain plan, and deviations require justification.
The biodiversity gain plan: the real gatekeeper
While the content and approval of biodiversity gain plans sit with planning and ecological specialists; the commercial consequences fall squarely into the development and land arena
BNG is delivered through a biodiversity gain plan, which must be submitted to and approved by the local planning authority before development can lawfully commence. From a transactional perspective, this makes the biodiversity gain plan a critical pre-commencement risk that directly affects acquisition strategy, conditionality and longstop dates. In practice, BNG issues often surface not at planning committee, but during land acquisition, funding or when satisfying conditions precedent
This is where many schemes encounter unexpected delay.
The plan must demonstrate:
- the site’s pre‑development biodiversity value;
- how at least a 10% net gain will be achieved; and
- how any habitats relied upon will be maintained and monitored for at least 30 years.
BNG obligations must be secured through enforceable legal mechanisms, such as planning conditions, section 106 agreements or conservation covenants — all of which have long-term implications for ownership, funding and future disposals.
Until the biodiversity gain plan is approved, development cannot begin — regardless of whether all other pre‑commencement conditions have been discharged.
What this means for developers in practice
BNG has immediate and practical consequences for development strategy:
- Timing risk – leaving BNG until after permission is granted can delay lawful commencement.
- Design pressure – late‑stage ecological constraints can force compromises to layout, density or viability.
- Long‑term commitments – land relied on for BNG enters decades‑long management obligations that bind future owners.
Developers who are engaging ecologists and planning advisers early — alongside transactional legal input on land and risk allocation — are finding the regime easier to manage. Those who treat BNG as a compliance issue to be dealt with later are often paying more — in time, cost and flexibility.
Conclusion
Biodiversity net gain is now part of the development process, whether developers welcome it or not.
Approached early, BNG can be accommodated within design and commercial planning. Left late, it can stall projects at the very point you expect to move on site.
For land buyers and promoters, BNG is now part of the commercial context in which sites are acquired, valued and contracted. Understanding how BNG interacts with ownership, timing and long-term obligations is now increasingly as important as understanding the planning status itself.
This article is part of our developer‑focused series on Biodiversity Net Gain. We do not advise on the content or approval of biodiversity gain plans. Our focus is the commercial and land implications—how BNG affects acquisition strategy, conditionality, timing and long‑term obligations.
