Biodiversity Net Gain Exemptions — What Developers & Planners Need to Know 

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 to trees and hedgerows 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 — meaning the removal of a single significant veteran tree or a length of ancient hedgerow can generate a substantial biodiversity deficit that the development must compensate for.

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, ensuring that the arboricultural outputs integrate seamlessly with the ecological consultant's biodiversity metric calculation — avoiding the need for separate data collection and reducing overall survey costs.

How BNG is Achieved

BNG can be delivered in three ways — on-site habitat creation or enhancement, off-site habitat creation on registered habitat banks, or as a last resort through the purchase of statutory biodiversity credits from the government. The biodiversity gain hierarchy requires developers to prioritise on-site gains first, then off-site gains, with statutory credits used only where on-site and off-site options are genuinely not available or deliverable.

For minor developments — those that are not exempt but fall below certain thresholds — the hierarchy is being amended from July 2026 to put on-site and off-site gains on an equal footing, removing the requirement for minor development to prioritise on-site delivery. This change is designed to reduce constraints for smaller developers in tight urban environments where on-site habitat creation is often impractical.

Key Changes to BNG in 2026

The BNG framework is undergoing its most significant changes since it was introduced in February 2024. The following updates are expected to come into effect by 31 July 2026, subject to parliamentary scheduling:

• A new area-based exemption will be introduced for developments on sites of 0.2 hectares or below. The existing exemption for small-scale self-build and custom-build development will be removed — it is anticipated that many of these developments will be captured by the new 0.2 hectare exemption. Temporary planning permissions granted for a maximum of five years will be exempt from BNG. The biodiversity gain hierarchy for minor development will be amended to give on-site and off-site gains equal status. The spatial multiplier will be decoupled from LPA boundaries and aligned with local nature recovery area strategies, allowing developers to source off-site units from a wider area.
• BNG for Nationally Significant Infrastructure Projects — major transport, energy, and water projects — will become mandatory from 2 November 2026. A new consultation on a potential exemption for residential development on brownfield land closed on 10 June 2026, with outcomes expected later in 2026.
• Until these changes are formally implemented through secondary legislation, the current BNG requirements remain in place and developers must continue to follow existing guidance.

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, you need to consider BNG from the earliest stage of the project. Urban Tree Management provides BNG-compatible tree and hedgerow baseline data as part of our BS5837 survey service — capturing the data your ecological consultant needs to run the biodiversity metric calculation at the same time as the arboricultural survey, saving time, cost, and the need for a return site visit.

Early engagement with both the arboricultural and ecological baseline is increasingly important as local planning authorities become more experienced in reviewing biodiversity gain plans — and as the consequences of getting the baseline wrong become more significant.