Biodiversity Net Gain (BNG) has reshaped how London planning authorities assess development proposals since it became mandatory in 2024, and 2026 is proving to be a pivotal year for the regime. With reforms confirmed by Defra in April 2026 and further changes due from 31 July, developers need an up-to-date understanding of how BNG interacts with tree retention, root protection and the wider arboricultural process because trees are frequently the single biggest factor in a site’s biodiversity baseline.
This guide explains what BNG means for London developments, how it connects to your tree survey, and what’s changing this year.
What Is Biodiversity Net Gain?
BNG is a statutory requirement under Schedule 7A of the Town and Country Planning Act 1990, introduced via the Environment Act 2021. It requires most developments in England to deliver a minimum 10% increase in biodiversity value compared with the site’s pre-development condition, measured using Defra’s statutory biodiversity metric and expressed in biodiversity units. Gains must be secured for a minimum of 30 years through a Section 106 agreement or conservation covenant.
Developers must follow a strict delivery hierarchy: create biodiversity gains on-site first, then supplement with off-site gains where on-site creation isn’t sufficient, and only purchase statutory biodiversity credits as a last resort.
Why Trees Are Central to BNG Calculations
Mature trees are typically the highest-value habitat feature on an urban site under the statutory metric. A single retained mature tree can contribute significantly more biodiversity units than an equivalent area of new planting, simply because of its size, age, and habitat complexity.
This means decisions made at the tree survey stage, which trees are retained, removed, or protected can directly shape your biodiversity unit calculation before an ecologist even applies the metric.
According to government guidance on Biodiversity Net Gain, developers must survey existing habitat and use the statutory biodiversity metric to determine how losses, including tree removal, will be compensated before the 10% target is calculated.
How Tree Surveys Feed Into Your BNG Strategy
A BS5837 tree survey doesn’t just inform your Arboricultural Impact Assessment, it’s now a foundational input for your biodiversity metric calculation. Retention categories, canopy spread, and condition data recorded during the survey help your ecologist establish an accurate pre-development baseline, which in turn determines how much on-site or off-site compensation your scheme requires.
Getting this sequencing wrong is a common and costly mistake. Commissioning a tree survey late, or after the layout has already been designed, often forces avoidable tree removal that increases your BNG liability unnecessarily. For a closer look at what your survey needs to include for planning purposes, see our guide on tree surveys for planning permission in London.
Root Protection Areas and On-Site Habitat Creation
Where developers aim to deliver BNG through on-site habitat creation, Root Protection Areas (RPAs) around retained trees often double as protected zones for new planting and soft landscaping. Coordinating your RPA plan with your biodiversity gain plan avoids conflicts between construction methodology and habitat creation areas later in the process. Our guide to Root Protection Areas on London building projects covers how these zones are calculated and protected during construction.
What's Changing for BNG in 2026?
Following a Defra consultation response published on 15 April 2026, several significant changes are due to take effect, primarily from 31 July 2026:
- A new 0.2 hectare exemption: Developments with a red-line site area of 0.2 hectares or below will be exempt from the mandatory 10% BNG requirement, subject to a carve-out where priority habitats are affected.
- Removal of the self-build exemption: The temporary exemption for self-build and custom-build projects is being withdrawn, meaning more individual homeowner schemes above the 0.2ha threshold will need to comply.
- NSIPs brought into scope: Biodiversity Net Gain becomes mandatory for Nationally Significant Infrastructure Projects from 2 November 2026.
- Brownfield consultation: A separate consultation, open until 10 June 2026, is exploring a targeted exemption for brownfield residential development up to 2.5 hectares.
Industry analysis suggests the 0.2 hectare exemption could remove around 50% of residential planning permissions from the mandatory BNG regime, a substantial shift for smaller London infill and householder schemes that previously had to prepare full biodiversity metric assessments.
What This Means for London Developers Right Now
Until the reforms take legal effect, the current rules still apply in full. Government guidance is explicit that developers should continue working to existing BNG requirements rather than assuming the announced changes are already in force. For any live application being prepared now, that means treating BNG as a current design constraint and not a future consideration to revisit once legislation changes.
For sites close to the 0.2 hectare threshold, it’s worth discussing timing with your planning team: submitting before or after 31 July 2026 could materially affect whether your scheme needs to deliver biodiversity units at all.
Practical Steps for Developers
To manage BNG efficiently alongside your tree obligations:
- Commission your BS5837 tree survey early, before finalising site layout, so retention decisions can support your biodiversity baseline rather than undermine it
- Calculate your site area against the 0.2 hectare threshold to understand whether the July 2026 exemption is likely to apply to your scheme
- Align your Tree Protection Plan and RPA boundaries with proposed habitat creation areas to avoid conflicting site constraints
- Engage an ecologist early to run the statutory biodiversity metric alongside your arboricultural assessment, rather than as a separate, later workstream
Get Your Tree Survey Aligned With Your BNG Strategy
Trees are often the most valuable biodiversity asset on a London development site, and the decisions made during your tree survey have a direct knock-on effect on your biodiversity metric calculation. Coordinating these processes from the outset reduces the risk of unplanned tree loss, unnecessary off-site unit purchases, and planning delay.
If you’re preparing a planning application affected by Biodiversity Net Gain requirements, our team can deliver a BS5837 tree survey designed to support your wider biodiversity strategy. Find out more about our BS5837 tree survey and reporting service or get in touch to discuss your site.