What Is Biodiversity Net Gain and Does It Affect My Planning Application?
If you have submitted a planning application recently — or are about to — you may have come across the term Biodiversity Net Gain, or BNG. It sounds like a technical concept for large developers, and for the most part, it is. But if you own a larger plot or are proposing a significant development in London or the surrounding area, it is worth understanding what BNG means, when it applies, and how it might affect your plans.
What Is Biodiversity Net Gain?
Biodiversity Net Gain is an approach to development that requires the natural environment to be left in a measurably better state after construction than it was before. The principle is simple: new buildings and infrastructure take up land, and that land often has ecological value — even in urban areas. BNG requires developers to offset and ideally improve that value rather than simply eliminate it.
The Environment Act 2021 made a mandatory 10% Biodiversity Net Gain requirement a legal obligation in England for most new planning permissions. This came into force for major developments from February 2024, with smaller sites following from April 2024.
How Is Biodiversity Measured?
The planning system uses a standardised Biodiversity Metric — currently Biodiversity Metric 4.0, developed by Natural England — to calculate the ecological value of a site before and after development. The metric assigns biodiversity units to different types of habitat based on their condition, distinctiveness, and strategic location.
A developer must demonstrate that their proposals result in at least 10% more biodiversity units than existed before the development began. This gain can be delivered on-site (within the development itself), off-site (on separate land), or through statutory biodiversity credits purchased through a national BNG register operated by Natural England.
The BNG register provides a formal record of habitat enhancements that have been secured and protected, ensuring the gains are maintained for at least 30 years.
Does BNG Apply to Householder Planning Applications?
This is the key question for most residential clients. The short answer is that standard householder planning applications — such as a single rear extension or loft conversion — are currently exempt from mandatory BNG requirements. The exemption applies to householder developments and developments below a de minimis threshold (where the impact on habitat is negligible).
However, if you are proposing a larger scheme — a significant new dwelling on your land, a garden development creating a new residential unit, or any project that qualifies as a major development under the planning definition (generally ten or more dwellings, or a site area of 0.5 hectares or more) — BNG requirements will apply in full.
Even where BNG is not a legal requirement, local planning authorities are increasingly encouraging applicants to demonstrate ecological consideration as part of their design approach. This may include adding bat boxes, swift bricks, native planting, or green roofs. While these are often advisory rather than mandatory at householder level, they can help smooth an application through committee and demonstrate design quality.
What Should London Homeowners Know?
London's biodiversity context is more complex than many homeowners expect. Many London boroughs have adopted supplementary planning guidance on ecology, and the London Plan includes policies on green infrastructure, urban greening, and biodiversity. The Urban Greening Factor (UGF) applies to major developments in London and requires a specified proportion of green space and planting.
If your project is close to a Site of Importance for Nature Conservation (SINC), a Local Nature Reserve, or a site with mature trees protected by a Tree Preservation Order, additional ecological considerations may apply regardless of BNG status.
It is also worth being aware that ecological surveys — such as bat surveys or great crested newt assessments — may be required as part of any planning application where the site or building type presents potential habitat. These surveys have seasonal windows, and failure to commission them at the right time can delay a programme significantly.
Practical Advice: Build Ecology Into Your Pre-Application Stage
The most practical thing any homeowner or developer can do is raise ecological questions at the pre-application stage, before submitting for planning. Understanding whether BNG applies, what baseline surveys may be needed, and how the design can incorporate ecological enhancements early avoids delay and ensures your application is robust.
At RCB Design & Build, we help clients navigate planning complexity from day one. That includes understanding which statutory and policy requirements apply to your specific site and project type, coordinating with ecological consultants where needed, and ensuring your application is prepared with the right level of information.
If you are at the early stages of a development project and want to understand what planning requirements apply to your site, get in touch with our team for a consultation.