Biodiversity Net Gain for Domestic Projects: What NW3 Homeowners Need to Know

Introduction

Biodiversity net gain (BNG) became a mandatory requirement for most planning applications in England from April 2024, following the implementation of Schedule 14 of the Environment Act 2021. The requirement means that developments must deliver a measurable improvement in biodiversity as a result of the planning permission — leaving nature in a better state than before development. For homeowners in NW3 planning extensions, loft conversions and garden developments, understanding when BNG applies — and when it does not — has become a necessary part of the planning process. This guide explains the current BNG framework as it applies to domestic householder projects in Camden and Barnet. For related guidance, see our tree preservation orders guide, Camden planning guide and pre-application advice guide.

What Is Biodiversity Net Gain?

Biodiversity net gain is the principle that development should result in more and better quality natural habitat than existed before. The mandatory BNG requirement (from February 2024 for large sites, April 2024 for small sites) requires that planning applications deliver at least 10% BNG — measured using the DEFRA biodiversity metric (version 4.0) — compared to the pre-development baseline.

BNG can be delivered:

• On-site: Through habitat creation or enhancement within the development site (planting, green roofs, retained habitats)
• Off-site: Through habitat creation on land away from the development site, often via purchasing biodiversity units from a registered habitat bank
• Biodiversity credits: As a last resort, through purchasing biodiversity credits from the government scheme

Does Mandatory BNG Apply to Householder Planning Applications?

This is the critical question for NW3 homeowners. As of 2026, the mandatory BNG requirement does NOT apply to the following:

• Householder planning applications (extensions, loft conversions, outbuildings on existing dwellings) — explicitly exempt from mandatory BNG
• Permitted development works — no planning application, so no BNG requirement
• Changes of use (in most circumstances)
• Permitted development with prior approval (Class AA larger extensions)

The mandatory BNG exemption for householder applications means that most domestic renovation and extension projects in NW3 are not subject to the 10% BNG requirement. A rear extension, loft conversion, or even a new garden building pursued through a householder planning application does not trigger mandatory BNG.

When BNG Does Become Relevant for Domestic Projects

While mandatory BNG does not apply to householder applications, biodiversity considerations can still arise in specific circumstances:

• Protected species surveys: If your project involves removing trees, hedgerows, or vegetation that could support bats, nesting birds or other protected species, an ecological survey may be required as a planning condition — regardless of the householder application exemption. This is not BNG per se, but it is related. See our tree survey guide.
• Full planning applications on separate plots: If a homeowner is developing a separate plot — a garage plot, a garden for subdivision, a new-build on a cleared site — rather than extending an existing dwelling, this is not a householder application and the mandatory BNG requirement applies.
• Planning conditions: Camden and Barnet may attach planning conditions requiring biodiversity enhancements on some householder applications — particularly in Hampstead Heath buffer zones or in areas with notable biodiversity value. These are not mandatory BNG but can require action. See our Hampstead Heath boundary guide.
• Policy-based BNG: Some local planning authorities have adopted local policies requiring biodiversity improvements even for householder applications — check Camden's and Barnet's current adopted policies for their specific position.

Good Practice: Voluntary BNG for NW3 Projects

Even where mandatory BNG does not apply, incorporating biodiversity enhancements into a residential project is increasingly good practice — and can strengthen a planning application in ecologically sensitive areas around Hampstead Heath. Examples of voluntary measures:

• Green roofs on single-storey extensions — sedum or wildflower roofs provide habitat and reduce surface water runoff
• Integrated bat boxes and swift bricks incorporated into new masonry
• Permeable paving in new courtyard or driveway areas
• Native planting in landscape schemes following garden remodelling
• Retained or enhanced boundary hedgerows and trees

An architect who considers ecology as part of the design process — rather than as a box-ticking exercise — will both strengthen planning applications and contribute to the genuinely important goal of improving urban biodiversity.

Conclusion

Mandatory biodiversity net gain does not currently apply to householder planning applications for extensions and residential alterations in NW3. However, protected species surveys may still be triggered by specific project characteristics, and good ecological practice is increasingly expected and valued in Camden's planning framework. An architect who understands the current BNG framework and its exceptions will advise correctly on what is required for your project. Use our free matching service to find an architect experienced in NW3 planning. For project cost guidance, visit hampsteadrenovationcosts.co.uk.