Mandatory Biodiversity Net Gain (BNG) in the town and country planning (TCPA) system in England is now two years’ old. More or less on its second birthday, the Government confirmed its intention to take forward a number of changes to the BNG regime during the course of this year.
This article explores the Government’s proposed changes, including a new consultation relating to residential brownfield development, and considers what these tell us about how the mandatory BNG regime has performed so far and its direction of travel.
Background
Introduced via the Environment Act 2021 and implemented in two phases in February and April 2024, the statutory BNG regime requires most planning applications submitted in England to deliver at least a 10% biodiversity net gain.
This gain is based on the site’s pre-development biodiversity value and is calculated using a statutory metric tool. The requirement is met by either providing, in accordance with the BNG hierarchy, one or a combination of onsite BNG, offsite BNG (known as biodiversity units) or statutory credits as set out in the development’s biodiversity gain plan secured by a mandatory pre-commencement condition on each new planning permission.
In May 2025, the Government consulted on improving BNG for minor, medium and brownfield development. It has now published its consultation outcome response setting out a number of changes to be taken forward during the course of 2026, as well as launching a further consultation on another potential change to the BNG regime concerning residential brownfield sites. We consider these in turn below.
At the same time the Government has published its consultation outcome response for the tailored version of the BNG regime to be introduced for Nationally Significant Infrastructure Projects (NSIPs) and is now busy putting both the legislative and policy infrastructure in place to implement this. We will consider BNG for NSIPs further in a separate article, although for immediate purposes it should be noted that the implementation date for this has been pushed back to November 2026. This is not without significance for the existing BNG regime for TCPA applications in England for the reasons touched upon below.
The Government’s Consultation Outcome Response
The headline changes to the current BNG regime for TCPA applications in the consultation outcome response:
1) Changes to the BNG exemptions
A number of new exemptions are to be introduced later this year, the most significant of which is a new area based exemption for all red line sites up to 0.2 hectares. This will be for all types of development and will be in addition to the current ‘de minimis’ exemption (which is focused on the amount of affected habitat within an application site, not the size of the site itself). This new exemption is intended to strike a balance between improving biodiversity and simplifying the planning process to ease the burdens on developers and local planning authorities.
The other new exemptions are for more specific circumstances as follows:
- Development where the primary objective is biodiversity conservation or enhancement;
- Temporary permissions granted for up to 5 years; and
- Development enhancing parks, playing fields and public gardens.
Additionally, the existing exemption for small scale self-build and custom build development will be removed. The rationale for this is that small scale single dwellings will be covered by the new area based exemption going forward.
2) A change to the BNG hierarchy for minor development
For minor development, offsite biodiversity gains are to be placed on the same level within the BNG hierarchy as onsite gains, i.e. minor development will no longer need to prioritise onside BNG over offsite BNG. This is in recognition that onsite BNG can be more difficult to deliver for minor development and so should give such development greater flexibility going forward.
3) Changes to the BNG Metric
The most notable change to the BNG Metric concerns the spatial risk multiplier (SRM), which is the mechanism within the BNG Metric that prioritises ‘local’ offsite BNG over offsite BNG that is more distant from the application site by reducing the value of the latter within the BNG Metric. Specifically the Government is to take forward a proposal to base the SRM on local nature recovery areas, which are larger than the local authority and National Character Areas used at present. This is intended to both better align with future local government reorganisation and increase flexibility for applicants by increasing what is defined as the ‘local’ area for the purposes of the SRM.
Separately, the Government is to consider changes to water BNG assessment methods for minor development, although as this requires further work with stake-holders, this is likely to be taken forward in a future update of the BNG Metric.
4) Changes relating to open mosaic habitat
Recognising the challenges that BNG poses for brownfield development with open mosaic habitat (OMH), the Government intends to update the definitions, guidance and condition assessments for OMH, including by introducing a new urban habitat category to provide more accurate representation of urban habitats within the BNG Metric.
The Government’s Further Consultation: Brownfield Residential Land
The Government’s further BNG-related consultation concerns a new targeted exemption for residential brownfield land. This potential exemption is in recognition of brownfield land being a long-term priority in national planning policy and the multiple challenges that brownfield residential developers can often face.
In this context the consultation seeks the views on the following aspects:
- The definition of brownfield development: Taking the National Planning Policy Framework’s definition of previously developed land as its starting point, the Government proposes defining ‘brownfield development’ by reference to the amount of the red line site is previously developed. A 75% threshold is proposed for this.
- The threshold for the targeted BNG exemption: To ensure that the exemption “is tightly focused on the types of previously developed, space limited sites where BNG requirements are most likely to create disproportionate challenges” the Government is considering the appropriate site area threshold to which the exemption should apply. In this regard, the consultation considers the range of options from 0.5 hectares to 2.5 hectares.
The consultation runs until 10 June 2026.
Mandatory BNG in England Two Years On
So what does the above tell us about mandatory BNG two years on?
Whilst it is tempting to read the need for these changes as a sign of some kind of early failure in the BNG regime as implemented, it is important to recognise that the system was always designed to be flexible and, given the nature, complexity and practical uncertainties with the new regime, tweaks were always likely.
Having said that, it would be wrong to dismiss the above changes as mere “tweaks” as they go to three of the fundamentals underpinning the mandatory BNG regime: The BNG exemptions; the BNG hierarchy; and the BNG Metric.
Plainly any change to the BNG exemptions will always be potentially significant, particularly as the exemptions were deliberately limited when BNG was introduced two years ago. Here the Government is not only proposing to introduce new exemptions but a general area-based exemption. This will take a notable number of small development sites outside the BNG regime – the Government expects “around 50% of residential planning permissions that were required to do BNG up to now”. The targeted exemption for residential brownfield land the subject of the latest consultation will potentially be even more significant, depending upon the thresholds that are adopted.
Turning to the change to the BNG hierarchy for minor development, this should give these developers more flexibility as to how any requisite BNG can be delivered. This is in fact the same adjustment to the BNG hierarchy that is being made for NSIPs and so there will be much more onus on the biodiversity units market going forward. Whether that market will be able to cope with the future demand when mandatory BNG for NISPs goes live in November remains to be seen, but there is a risk that the efficacy of this change for minor development could be undermined in practice simply because of the increased competition for biodiversity units from NSIPs.
As for the BNG Metric, the change to the SRM should reduce the extent to which the multiplier applies going forward, which will help developers with the cost of meeting BNG requirements. Given the general thrust of the Government’s changes, and in particular the changes to the BNG hierarchy and implications for the biodiversity units market referred to above, is there a case for the Government to go further and perhaps remove the SRM altogether? That would have been a more radical change that would benefit both the developer community and the biodiversity units market alike since it would remove the concept of ‘local’ markets and so help simplify the regime.
As to whether any of the proposed changes signal a change of policy direction by the Government, the consultation response paper repeatedly emphasises the need for striking an appropriate balance between improving biodiversity and the burdens on different types of development. This balance is the main driver for these changes and the fact that mandatory BNG is to be extended to NSIPs later this year is confirmation of the Government’s commitment to the concept, i.e. this is not a retreat from or watering down of mandatory BNG. The Government’s intention for some of these changes to be in force as early as July (including the new area based exemption) can be read as further sign of its commitment.
If you would like to know more about BNG or otherwise require any planning advice or assistance, please do not hesitate to contact Michael Dempsey or other members of the Planning, Infrastructure & Public Law Team at Broadfield.
