Biodiversity Net Gain & Development

Updated August 2026

Biodiversity Net Gain (BNG) has been a significant consideration for developers in England since becoming mandatory for most development.

However, the rules have changed from 6 August 2026, and the change is particularly relevant to small development sites.

New 0.2 hectare BNG exemption

From 6 August 2026, development on a site of 0.2 hectares or less can be exempt from mandatory BNG, subject to the relevant conditions.

0.2 hectares is 2,000 square metres.

This could be particularly useful for:

  • Single dwellings;

  • Replacement dwellings;

  • Garden development;

  • Small residential schemes; and

  • Small infill developments.

The exemption is subject to an important exception where the development negatively impacts onsite priority habitat.

The site area is assessed using the development site's red-line boundary.

What does this mean for developers?

Previously, even relatively small developments could be caught by the BNG requirements.

In my experience, the requirements for some small developments were ill thought out and disproportionate. The additional ecological assessment, administration and potential costs could be significant compared with the scale of the development.

I therefore consider the new 0.2 hectare exemption to be a positive change for the development industry.

For a small site, the first question should now be whether the development is exempt before commissioning a biodiversity assessment or metric.

Does a replacement dwelling need BNG?

Not necessarily.

A replacement dwelling is not automatically exempt, but if the development site is 0.2 hectares or less and the development does not negatively affect onsite priority habitat, the new exemption may apply.

This could be particularly relevant to replacement bungalows and houses on established residential plots.

What if my site is larger than 0.2 hectares?

The new exemption will not apply, but other exemptions may be available.

For example, the de minimis exemption can apply where development does not affect onsite priority habitat and affects less than:

  • 25 square metres of onsite habitat; and

  • 5 metres of onsite linear habitat, such as a hedgerow.

If no exemption applies, the development will generally need to achieve at least 10% Biodiversity Net Gain.

Does BNG exemption mean there are no ecological issues?

No. Being exempt from mandatory BNG does not remove other environmental planning requirements.

A development may still need to consider protected species, trees, hedgerows, priority habitats, protected sites and other ecological constraints.

The Local Planning Authority can also consider biodiversity through its normal planning policies.

What about self-build?

The previous exemption for qualifying self-build and custom-build development has been removed for new planning applications submitted from 6 August 2026.

A self-build project should therefore not be assumed to be exempt simply because it is a self-build.

Advice to developers

BNG should be checked at the beginning of a development project.

Before commissioning an ecological assessment, I would establish:

  1. The size of the application site;

  2. Whether it is 0.2 hectares or less;

  3. Whether onsite priority habitat is present; and

  4. Whether another BNG exemption applies.

For many small developments, this initial check may establish that a statutory BNG assessment is not required.

If BNG does apply, it is better to identify this at an early stage because the ecological requirements and potential costs can affect the design and viability of a development.

Need advice on BNG?

If you are considering a new dwelling, replacement dwelling or small residential development and are unsure whether Biodiversity Net Gain applies, I can review the proposal and advise on the planning requirements before you commit to unnecessary consultant costs.

Contact Paul Ashton Architects to discuss your development.