Biodiversity Net Gain and Home Extensions in London

The short answer

Biodiversity Net Gain (BNG) is a planning requirement, in force in England since 2024, for development to leave the natural environment measurably better than before — a minimum 10% net gain in biodiversity value, secured and maintained for at least 30 years. It sounds daunting for a home project, but most home extensions do not have to provide it: householder applications are specifically exempt. The detail matters, though, because not every home project is a householder application.

When BNG started

Mandatory BNG came into force on 12 February 2024 for most (major) developments, and on 2 April 2024 for small sites. It is delivered through the planning system: where it applies, a biodiversity gain plan has to be approved before development can lawfully begin. The 10% gain can be achieved on site, off site, or — as a last resort — by buying statutory biodiversity credits.

Why most extensions are exempt

The regulations exempt householder development — the small projects homeowners carry out at their own home, expressly including home extensions, conservatories and loft conversions. A householder application is the specific application type for alterations or extensions to a single house (not flats). If your extension or loft conversion goes in as a householder application, BNG does not apply to it.

There is also a separate de minimis exemption, but it is narrower than it is often quoted: development qualifies only where it affects less than 25 square metres — a 5m by 5m area — of on-site habitat AND less than 5 metres of on-site linear habitat such as a hedgerow. Both limits have to be met, not either one. Importantly, the de minimis exemption falls away if the works affect priority habitat, in which case BNG must be met regardless of the small area involved.

When BNG can still apply to a home project

The exemptions turn on the type and scale of the application, not the word residential. Before 6 August 2026, BNG could be engaged whenever a project was not a householder application — creating a new self-contained dwelling, converting a house into flats, or a larger scheme submitted as a minor or major application. Since 6 August 2026 there is a further, area-based exemption: the Biodiversity Gain (Town and Country Planning) (Amendments and Transitional Provisions) (England) Regulations 2026 (SI 2026 No. 790) remove mandatory BNG from any application site of 0.2 hectares (about 2,000 square metres) or less, unless on-site priority habitat would be harmed. That threshold covers almost every London house plot, so a house-to-flats conversion or a small infill dwelling applied for on or after that date will normally fall outside BNG too. Applications submitted before 6 August 2026 continue under the earlier rules, and the new exemption is also disapplied for a section 73 permission where the original application was made, or the original permission granted, before that date.

BNG therefore now mainly bites on larger sites and on any plot where on-site priority habitat would be impacted — the statutory test is the effect on the habitat, not merely its presence. If you are unsure which application route your project takes, that is worth confirming early, because the route and the site area together determine whether BNG is in play at all.

What it means in practice

For the typical London homeowner extending, converting a loft or adding a conservatory, BNG is very unlikely to be a hurdle. Where it does apply, it is best identified at feasibility, because the biodiversity gain plan and any habitat assessment need to be built into the design and the programme rather than bolted on at the end. As with all planning matters, the position depends on your specific property and proposal, and should be confirmed with your local planning authority.