Guide: Biodiversity Net Gain & Development
From April 2024, even small development sites in England became part of one of the most significant environmental shifts in planning for a generation: Biodiversity Net Gain (BNG).
BNG became law under the Environment Act 2021, introduced by the Conservative government led by Boris Johnson. The aim is simple in principle but challenging in practice: every qualifying development must leave the natural environment in a measurably better state than before.
What Biodiversity Net Gain Means
Biodiversity Net Gain requires each development to deliver at least a 10 percent measurable improvement in biodiversity value compared with the pre-development baseline. This is assessed using the Defra Biodiversity Metric, which assigns a numerical “biodiversity unit” score to the habitats that exist on a site before development and then compares it to the value of what will be created or retained afterwards. If the uplift cannot be achieved fully on-site, it can be provided through off-site habitat creation or, as a last resort, by purchasing government biodiversity credits, which are intentionally priced to discourage routine use. The enhancement must be maintained for a minimum of 30 years, usually through a Section 106 agreement or Conservation Covenant.
When It Became Law
BNG was introduced in two phases:
12 February 2024 — for major developments
2 April 2024 — for small sites
For planning purposes, “small sites” are defined as:
Residential — one to nine dwellings on a site under one hectare (or under 0.5 ha if the number of dwellings is unknown)
Non-residential — under 1,000 m² of floor space, or a site under one hectare
From that date, most small architectural projects (even those previously considered minor) came within scope.
What This Means at the Coal Face
From a practical, day-to-day point of view, in our experience removing even one medium-sized tree can have a disproportionate effect on a project’s BNG outcome.
That single tree triggers a baseline biodiversity calculation, and its loss must be offset through measurable habitat gain. While a single tree might not sound significant, the Biodiversity Metric treats habitat value cumulatively. The removal of one tree can tip a site below its baseline score, meaning that several smaller trees, or even habitat areas that would otherwise fall below the threshold, suddenly have to be considered within the calculation.
It is our opinion that small sites are disproportionately affected by BNG. The fixed 10 percent uplift and long-term maintenance requirement weigh more heavily on small developments where space and budget are already constrained. Nevertheless, as this is the current legislation, we deal with it in a professional and pragmatic manner..
In many cases, the compensation required to replace lost biodiversity simply cannot be accommodated on-site, forcing developers to look at off-site options. These may take the form of:
Purchasing government biodiversity credits (priced high to discourage reliance on them), or
Working with a local land bank to deliver new planting or habitat creation elsewhere.
Often this means planting ‘whips’ (young trees typically 30–60 cm high). The initial outlay for planting may be modest compared with the overall project cost, but the real commitment comes from ensuring those trees are maintained for 30 years, and from entering into a Section 106 agreement with the Local Planning Authority to secure that management legally.
Why It Was Introduced
BNG stems from the government’s wider 25-Year Environment Plan, aiming to halt biodiversity decline and embed nature recovery into all development decisions. Rather than simply minimising harm, BNG seeks to make measurable ecological improvement a standard part of the development process. Local Planning Authorities are now responsible for ensuring that this improvement is secured and monitored through Planning Permission and legal agreements.
How It Affects Architects and Small Developers
Early ecological input. An ecological survey is now a critical part of the design process, even on smaller projects. The baseline survey defines what habitats exist, and therefore how much uplift is required.
Design integration. Elements such as native planting, tree corridors, wildflower areas and ponds are now essential contributors to the BNG score, not decorative extras.
Legal and financial implications. Long-term maintenance obligations and off-site compensation can add notable cost and administrative effort. These must be factored into early feasibility and viability discussions.
Planning process. A Biodiversity Gain Plan must be approved before development begins, showing exactly how the uplift will be delivered and managed.
