Understanding Biodiversity Net Gain 

What Changed on 6 August 2026

Four changes came into force for planning applications made on or after 6 August 2026, under the Biodiversity Gain (Town and Country Planning) (Amendments and Transitional Provisions) (England) Regulations 2026.

The changes are not retrospective. Applications made before 6 August continue to be assessed under the previous rules.

A new exemption for sites of 0.2 hectares or less

• Development is exempt from mandatory BNG where the area within the planning application red line boundary is no more than 0.2 hectares — 2,000 square metres.
• Two points are commonly misread. The threshold is based on the development site area, not the area of habitat being removed. And the exemption does not apply where the development would negatively affect an on-site priority habitat, regardless of site size.

A new exemption for temporary development

• Development may be exempt where the whole scheme is temporary and permission is granted for five years or less, with a requirement to remove the development and reinstate the land within that period. Again, this does not apply where on-site priority habitat would be negatively affected.

The self-build and custom-build exemption has been removed

• The separate exemption for self-build and custom-build housing — previously covering schemes of up to nine dwellings on sites of 0.5 hectares or less — no longer applies to applications made on or after 6 August 2026.
• Many single plots will now fall within the new 0.2 hectare exemption instead, so for smaller self-build schemes the practical effect is limited. Larger self-build and custom-build schemes exceeding the new threshold must deliver 10% BNG in the usual way.



The gain hierarchy has been relaxed for minor development

• Minor development that remains subject to BNG now has more flexibility over how the required gain is delivered. On-site habitat creation and registered off-site biodiversity gains can be considered at the same stage, rather than off-site being treated as a fallback once on-site has been ruled out.

Still to Come

Nationally Significant Infrastructure Projects are expected to become subject to mandatory BNG from 2 November 2026, covering major transport, energy and water schemes. Defra intends to apply the same requirements across all NSIP sectors, with flexibility allowing off-site gain to be aggregated and delivered within any one of the local planning areas affected by the project.

A consultation on a potential exemption for residential development on previously developed land closed on 10 June 2026, with the outcome expected in due course.

Exempt Does Not Mean No Tree Survey

Worth stating plainly, because it is the most common misunderstanding following the August changes.

An exemption from mandatory BNG removes the requirement to deliver a 10% biodiversity gain. It does not remove anything else. The National Planning Policy Framework still requires development to avoid harm to biodiversity where possible, local planning authorities may still request ecological information, and where trees are present on or adjacent to the site a BS5837 arboricultural survey remains a validation requirement in its own right.



A great many smaller schemes that are now BNG-exempt still need a tree survey. The two requirements are separate and always have been.

And the priority habitat condition means an exemption cannot be assumed from site area alone. Where habitat of any significance is present, that needs establishing before relying on it.

How Trees and Hedgerows Affect the Biodiversity Metric

Trees and hedgerows are measured as habitat features within the statutory biodiversity metric and contribute biodiversity units to the site baseline. The number of units attributed depends on their species, size, condition, and connectivity.

Mature, native, and well-connected trees and hedgerows in good condition carry significantly more biodiversity units than young, non-native, or isolated specimens. The practical consequence is that removing a single significant veteran tree or a length of ancient hedgerow can generate a substantial biodiversity deficit that the development then has to compensate for — often at a cost well beyond what the removal appeared to save.



For developers and architects, the key implication is that the tree and hedgerow survey data produced as part of the BS5837 arboricultural assessment is directly relevant to the BNG calculation.

Urban Tree Management produces BNG-compatible tree and hedgerow baseline data as part of the BS5837 survey process, so the arboricultural outputs integrate with the ecological consultant's metric calculation — avoiding separate data collection, a second site visit, and the risk of two datasets that disagree with each other.

Time to Target Condition — Why Planting Does Not Count Immediately

One point that catches schemes out at design stage.

Newly created habitat does not deliver its target condition on the day it goes in the ground, and the metric accounts for that with a time-to-target-condition factor. For woodland, that period is measured in decades rather than seasons.

A scheme relying heavily on new tree or woodland planting to meet its net gain therefore carries a significant time-lag penalty, which means more planting is required than a straightforward area-for-area calculation would suggest. Understanding that early affects how much land the scheme has to allocate — and whether on-site delivery is realistic at all.

Where planting forms part of the delivery, see our BNG tree and woodland planting support.

How BNG is Delivered

BNG can be delivered three ways:

• On-site habitat creation or enhancement.
• Off-site habitat creation, on the developer's own land or through registered habitat banks.
• Statutory biodiversity credits purchased from the government — a last resort, priced deliberately high to discourage reliance on it.
• For major development the hierarchy still requires on-site gains to be prioritised first, then off-site, with credits used only where the other routes are genuinely not available. For minor development, following the August 2026 changes, on-site and registered off-site gains now sit on an equal footing.

What This Means for Your Development

If you are a developer, architect, or planning consultant working on a site in England where planning permission is required, BNG needs considering from the earliest stage of the project — not after the layout is fixed.



Urban Tree Management provides BNG-compatible tree and hedgerow baseline data as part of our BS5837 survey service, capturing what your ecological consultant needs for the metric calculation at the same time as the arboricultural survey.

We do not carry out the BNG assessment itself — the statutory metric is properly the work of a qualified ecologist, and we work alongside yours. Where you do not have one appointed, we can point you towards CIEEM-registered consultants we work with across the North West.

Early engagement with both the arboricultural and ecological baseline matters increasingly, as local planning authorities become more experienced at reviewing biodiversity gain plans and the consequences of getting the baseline wrong become more significant.