Biodiversity Net Gain 2026 Reform: Small Site Exemption, NSIPs and Brownfield Changes Explained

Last updated: June 3, 2026

Quick Answer: The June 2026 Biodiversity Net Gain reform package introduces six significant changes to England's mandatory BNG regime. The most immediate is a new 0.2-hectare area-based exemption expected by 31 July 2026, projected to remove BNG obligations from roughly half of all residential planning permissions. Mandatory 10% BNG for Nationally Significant Infrastructure Projects (NSIPs) follows on 2 November 2026. Developers, ecologists and land managers must act now to screen sites, update baselines and position for the NSIP offset market.

Key Takeaways

• A new 0.2-hectare small site exemption is expected by 31 July 2026, covering sites with no priority habitats and projected to exempt approximately 50% of residential planning permissions from BNG obligations.
• The existing self-build and custom-build exemption will be removed by the same date; most such schemes will fall under the new 0.2ha threshold instead.
• Temporary planning permissions of up to five years will gain their own BNG exemption.
• For minor developments, off-site biodiversity gains will receive equal preference to on-site gains in the revised biodiversity gain hierarchy.
• Mandatory 10% BNG extends to all NSIPs in England from 2 November 2026, with no sector-specific carve-outs.
• A Defra consultation on a potential BNG exemption for brownfield residential sites up to 2.5 hectares closed at 23:59 on 10 June 2026; outcomes are pending.
• Demand for off-site biodiversity units may fall by an estimated 10% as a result of the 0.2ha exemption, affecting offset market liquidity.
• Defra published updated NSIP baseline guidance on 2 June 2026, specifying which habitats to include in pre-development biodiversity value calculations.

Table of Contents

1. What Exactly Is Biodiversity Net Gain and Why Does It Matter?
2. The June 2026 BNG Reform Package: Six Changes at a Glance
3. How Small Does a Site Need to Be to Get the Exemption?
4. What Happens to the Self-Build Exemption and Temporary Permissions?
5. What Counts as a Brownfield Site for Biodiversity Net Gain?
6. Which Infrastructure Projects Are Considered NSIPs — and What Changes on 2 November 2026?
7. How Do You Calculate Biodiversity Units for a Small Brownfield Site?
8. Can Ecological Mitigation Credits Be Bought Instead of On-Site Improvements?
9. What Are the Biggest Mistakes Developers Make with Biodiversity Net Gain?
10. Who Is Responsible for Monitoring Biodiversity Net Gain Compliance?
11. Are Heritage Sites or Conservation Areas Treated Differently?
12. What Happens If You Don't Meet BNG Requirements?
13. Practical Steps for BNG Surveyors, Ecologists and Land Managers Right Now
14. FAQ
15. Conclusion

What Exactly Is Biodiversity Net Gain and Why Does It Matter?

Biodiversity Net Gain (BNG) is a legal requirement under Schedule 7A of the Town and Country Planning Act 1990, as inserted by the Environment Act 2021. It requires that development in England leaves biodiversity measurably better off than before — currently by a minimum of 10% — measured using Defra's statutory biodiversity metric. For a fuller grounding, see this biodiversity net gain explained guide.

BNG matters because England has lost significant wildlife habitat over the past century. The mandatory regime, which became live for major developments in February 2024 and minor developments in April 2024, ensures every planning permission contributes to nature recovery rather than simply avoiding the worst harm. The June 2026 Biodiversity Net Gain 2026 reform package refines who must comply, how they comply, and at what scale.

The June 2026 BNG Reform Package: Six Changes at a Glance

Defra confirmed the reform package in June 2026, with coverage from Pinsent Masons, Bird & Bird, CIEEM, the CLA and Inside Ecology. The six headline changes are:

Sources: [1][3]

How Small Does a Site Need to Be to Get the Exemption?

The new threshold is 0.2 hectares of total site area. Developments on sites at or below this area will be exempt from mandatory BNG requirements, provided no priority habitats are present on the site [1]. This is an area-based test, not a unit-count or dwelling-number test, which makes screening straightforward.

Key criteria to qualify:

• Total site area is 0.2 hectares or less.
• No priority habitats (as listed under Section 41 of the Natural Environment and Rural Communities Act 2006) are present on the site.
• The permission is not a temporary permission of more than five years (which has its own exemption route).

Why it matters at scale: Defra estimates this single change will exempt approximately 50% of residential planning permissions that would otherwise have been caught by BNG obligations [1]. For ecologists, this means a significant portion of current minor-development BNG assessment work will no longer be required after July 2026.

Common mistake: Assuming the exemption applies automatically. Local planning authorities will still expect applicants to demonstrate eligibility. Site-screening tools should be updated to flag priority habitat presence using the most current Magic Map and UKHab data before any exemption claim is made.

For guidance on which projects fall outside BNG obligations entirely, see the exempt projects overview.

What Happens to the Self-Build Exemption and Temporary Permissions?

The existing exemption for small-scale self-build and custom-build housing will be removed by 31 July 2026 [1]. In practice, many self-build schemes involve sites well under 0.2 hectares, so a large proportion will simply fold into the new area-based exemption without needing to meet BNG requirements. Self-builders on larger plots, or those affecting priority habitats, will need to comply with the full 10% BNG requirement.

A new, separate exemption will cover temporary planning permissions of up to five years. This is practically significant for:

• Construction compounds and temporary works areas.
• Short-term agricultural or storage consents.
• Pop-up or meanwhile uses in urban settings.

Applicants for temporary permissions should ensure the consent is clearly worded to fall within the five-year limit and should document the temporary nature of the use at application stage.

What Counts as a Brownfield Site for Biodiversity Net Gain?

For BNG purposes, brownfield land broadly means previously developed land — sites with existing or former built structures, hard standing, or industrial use. The Defra consultation that closed at 23:59 on 10 June 2026 proposed a targeted exemption for residential development on brownfield sites up to 2.5 hectares [2]. The outcome of that consultation is pending and expected later in 2026.

Brownfield sites present a particular challenge for BNG because:

• Baseline biodiversity value can be surprisingly high. Open Mosaic Habitats (OMH) — a priority habitat type — frequently colonise derelict brownfield land and can generate significant biodiversity unit values that development must then offset.
• Updated Defra guidance now includes a proxy habitat for assessments where no OMH units are available in the local offset market [5].
• Developers should not assume brownfield means low-value. A thorough UKHab survey is essential before any exemption or offset strategy is finalised.

If the brownfield residential exemption is confirmed, it would further reduce the pool of sites requiring BNG compliance beyond the 0.2ha threshold. Until the outcome is published, developers with brownfield residential sites between 0.2ha and 2.5ha should prepare a BNG assessment in parallel with monitoring the consultation response.

Which Infrastructure Projects Are Considered NSIPs — and What Changes on 2 November 2026?

Nationally Significant Infrastructure Projects (NSIPs) are large-scale developments consented under the Planning Act 2008 rather than the standard Town and Country Planning Act route. They include:

• Onshore wind farms above 50MW.
• Solar farms above 50MW.
• New or expanded airports and major port facilities.
• National highways and strategic rail projects.
• Large power stations and electricity transmission infrastructure.
• Major water treatment and supply projects.

From 2 November 2026, all NSIPs in England must deliver a minimum 10% biodiversity net gain [1]. This applies to applications submitted on or after that date, with no sector-specific exemptions. Defra published baseline guidance on 2 June 2026 specifying which habitats to include when calculating pre-development biodiversity value for NSIPs [3].

The November cliff edge: Any NSIP promoter who submits an application before 2 November 2026 avoids the mandatory requirement. This creates a strong incentive to accelerate submission timelines. Ecologists and planning consultants advising NSIP promoters should flag this deadline immediately and assess whether baseline surveys, metric calculations and biodiversity gain plans can be completed in time for a pre-November submission — or whether a post-November submission with a robust BNG strategy is the more realistic path.

How Do You Calculate Biodiversity Units for a Small Brownfield Site?

Biodiversity units for any site — including small brownfield plots — are calculated using Defra's statutory biodiversity metric. The metric scores habitats by type, condition and size, then applies multipliers for location, difficulty of creation and time to target condition. For a step-by-step breakdown, see what is in a biodiversity net gain assessment.

For brownfield sites specifically:

1. Commission a UKHab Phase 1 survey to identify all habitat types present, including any OMH.
2. Assess habitat condition using Defra condition assessment guidance.
3. Enter baseline data into the statutory metric (currently Excel-based; Defra plans a digital integrated service in due course) [5].
4. Model the proposed development's impact to calculate the post-development biodiversity value.
5. Identify the shortfall and determine whether on-site, off-site or statutory credit routes are needed.

If the site qualifies for the 0.2ha exemption and has no priority habitats, steps 3 to 5 are not required — but steps 1 and 2 remain necessary to confirm eligibility.

Can Ecological Mitigation Credits Be Bought Instead of On-Site Improvements?

Yes. Where on-site delivery is not possible or sufficient, developers can purchase off-site biodiversity units from registered habitat banks, or — as a last resort — buy statutory biodiversity credits from the government. For a detailed comparison of these routes, see off-site or on-site delivery of biodiversity net gain.

The June 2026 reform makes this more relevant for minor developments by giving off-site gains equal preference to on-site gains in the revised biodiversity gain hierarchy for that tier [1]. Previously, the hierarchy required developers to demonstrate on-site delivery was not possible before moving off-site. For minor development applicants, this removes a procedural hurdle and gives genuine flexibility.

Market liquidity note: The 0.2ha exemption is projected to reduce demand for off-site biodiversity units by approximately 10% [4]. Land managers and habitat bank operators should factor this into pricing and unit release strategies. Conversely, the NSIP mandate from November 2026 will generate significant new demand for large-volume offset packages, particularly for linear infrastructure projects crossing multiple local authority areas. For those looking to sell units, see how to sell biodiversity units.

What Are the Biggest Mistakes Developers Make with Biodiversity Net Gain?

The most common errors — and how to avoid them:

• Assuming exemption without checking for priority habitats. The 0.2ha exemption fails if any priority habitat is present. Always run a Magic Map check and commission a UKHab survey before claiming exemption.
• Starting baseline surveys too late. Seasonal survey windows for protected species and certain habitats are fixed. Missing them delays the metric calculation and can push an application past a critical deadline (particularly the November 2026 NSIP cliff edge).
• Underestimating brownfield baseline values. Open Mosaic Habitat on derelict land can produce a high biodiversity unit score, making the 10% gain harder to achieve than expected.
• Treating the biodiversity gain plan as an afterthought. The gain plan must be submitted with or before the planning application for most consents. Late preparation causes delays.
• Ignoring the revised gain hierarchy for minor development. Post-reform, minor development applicants no longer need to justify why on-site delivery is not feasible before going off-site. Failing to use this flexibility can add unnecessary cost.

For a broader planning checklist, see 8 biodiversity net gain points on planning your project.

Who Is Responsible for Monitoring Biodiversity Net Gain Compliance?

Responsibility is shared. The developer or landowner is legally responsible for delivering and maintaining the biodiversity gain for the required 30-year period. Local planning authorities (LPAs) are responsible for securing compliance through planning conditions and legal agreements. Natural England oversees the statutory credit scheme and the biodiversity gain site register.

For NSIPs, the relevant Secretary of State and the Planning Inspectorate hold the consent, so monitoring obligations will be embedded in the Development Consent Order conditions rather than standard planning conditions.

Ecologists and land managers acting as habitat bank operators must register gain sites on the national register and submit monitoring reports at intervals specified in their conservation covenant or planning obligation.

Are Heritage Sites or Conservation Areas Treated Differently?

BNG applies to the biodiversity value of the land, not to its heritage designation. A development within a conservation area or on a scheduled monument must still meet BNG requirements unless another exemption applies. Heritage designations do not themselves exempt a site from BNG.

However, the types of habitat management compatible with heritage conservation (traditional orchard management, chalk grassland restoration, low-intervention scrub) can generate biodiversity units, so there is often alignment between heritage and BNG objectives rather than conflict.

Edge case: If a heritage site contains a priority habitat — for example, ancient semi-natural woodland within a registered park and garden — the 0.2ha exemption would not apply even if the development footprint is small.

What Happens If You Don't Meet BNG Requirements?

Failure to deliver the required 10% BNG is a breach of planning condition. Consequences include:

• Enforcement action by the LPA, potentially requiring remediation works or cessation of development.
• Inability to discharge relevant planning conditions, blocking practical completion and occupation.
• For habitat bank operators, failure to maintain registered gain sites can result in enforcement and reputational damage affecting future unit sales.

There is no statutory fine structure specifically for BNG non-compliance, but standard planning enforcement powers apply, including stop notices and injunctions in serious cases. For guidance on managing BNG risk, see achieving biodiversity net gain without the risk.

Practical Steps for BNG Surveyors, Ecologists and Land Managers Right Now

The Biodiversity Net Gain 2026 reform package for small site exemptions, NSIPs and brownfield land requires immediate action across several workstreams:

For ecologists and BNG surveyors:

• Update site-screening tools to apply the 0.2ha area test and priority habitat check as the first filter on all incoming minor development instructions.
• Review all active minor development BNG instructions to identify which may qualify for the new exemption and advise clients accordingly.
• Ensure UKHab survey protocols reflect the latest OMH and urban habitat guidance published alongside the reform package [5].
• For any NSIP instruction, apply the Defra NSIP baseline guidance published 2 June 2026 [3] and flag the 2 November 2026 application deadline to clients immediately.

For planning consultants advising NSIP promoters:

• Audit the project programme against the 2 November 2026 mandatory BNG commencement date.
• If submission before that date is feasible, commission baseline surveys and metric calculations now — survey seasons will not wait.
• If post-November submission is unavoidable, begin scoping the biodiversity gain plan and off-site unit procurement strategy early, given the scale of units likely required for major infrastructure.

For land managers and habitat bank operators:

• Model the effect of the 0.2ha exemption on local unit demand. An estimated 10% reduction in demand [4] may affect unit pricing and the pace of unit release from existing banks.
• Position available units to serve the incoming NSIP market, which will require large-volume, spatially flexible packages.
• Monitor the brownfield residential consultation outcome; a confirmed exemption for sites up to 2.5ha would further reduce demand from that development type.

For those looking to enter the offset market as a unit supplier, see how to sell biodiversity units and the guidance for landowners.

FAQ

Q: Does the 0.2ha exemption apply to commercial as well as residential development?
A: The reform package is primarily framed around residential planning permissions, but the area-based threshold applies to the site area regardless of use class, provided no priority habitats are present. Developers of small commercial sites should seek specific advice on whether their application qualifies.

Q: If my self-build site is 0.15ha with no priority habitats, do I still need a BNG assessment after July 2026?
A: No. Once the 0.2ha exemption is in force, a site of 0.15ha with no priority habitats will be exempt from mandatory BNG. However, you should document the site area and priority habitat check in case the LPA requests evidence of exemption eligibility.

Q: Can an NSIP promoter avoid mandatory BNG by submitting before 2 November 2026?
A: Yes, in principle. Applications submitted before 2 November 2026 are not caught by the mandatory NSIP BNG requirement. However, promoters should be aware that voluntary BNG commitments may still be expected by the Planning Inspectorate and that rushing a submission without adequate ecological baseline data carries its own risks.

Q: What is the difference between off-site biodiversity units and statutory biodiversity credits?
A: Off-site biodiversity units are purchased from private habitat banks registered on the national biodiversity gain site register. Statutory biodiversity credits are sold by the government at a fixed (higher) price and are intended as a last resort when off-site units are not available. For a cost comparison, see the cost of biodiversity units and statutory credits guide.

Q: Does the revised gain hierarchy for minor development mean on-site BNG is no longer preferred?
A: For minor development specifically, off-site gains now receive equal preference to on-site gains in the hierarchy [1]. On-site delivery is still encouraged where feasible, but applicants no longer need to demonstrate it is impractical before using off-site units.

Q: When will the brownfield residential consultation outcome be published?
A: The consultation closed at 23:59 on 10 June 2026 [2]. Defra has indicated the outcome will be announced later in 2026. Until then, brownfield residential sites between 0.2ha and 2.5ha remain subject to mandatory BNG and should be assessed accordingly.

Conclusion

The June 2026 Biodiversity Net Gain reform package is the most significant recalibration of the BNG regime since mandatory requirements came into force in 2024. For small-site developers, the 0.2ha exemption will remove BNG obligations from a large share of residential applications — but only where no priority habitats are present, making accurate site screening non-negotiable. For NSIP promoters, the 2 November 2026 deadline is a hard cliff edge that demands immediate programme review and early ecological commissioning.

Actionable next steps:

1. Update screening tools to apply the 0.2ha area and priority habitat test to all incoming minor development instructions before July 2026.
2. Review active self-build instructions and advise clients on whether the new area-based exemption applies.
3. For NSIP projects, apply the June 2026 Defra baseline guidance immediately and assess whether a pre-November submission is achievable.
4. Monitor the brownfield residential consultation outcome and model its effect on your pipeline of 0.2ha–2.5ha brownfield sites.
5. If operating a habitat bank, model the demand-side effect of the exemptions and position unit inventory to serve the incoming NSIP market.
6. Ensure all UKHab surveys and metric calculations reflect the latest guidance on OMH and urban habitats.

The Biodiversity Net Gain 2026 reform small site exemption, NSIPs and brownfield changes collectively reshape who must comply, how flexibility is allocated and where offset demand will flow. Those who act on these changes now — rather than waiting for secondary legislation to be laid — will be best placed to advise clients, manage risk and capitalise on the emerging NSIP offset market.

References

[1] Biodiversity Net Gain: Mandatory BNG for NSIPs Confirmed as Government Unveils Significant Reforms – https://cms.law/en/int/legal-updates/biodiversity-net-gain-mandatory-bng-for-nsips-confirmed-as-government-unveils-significant-reforms

[2] Biodiversity Net Gain: Considering a Targeted Exemption for Residential Brownfield Development – https://consult.defra.gov.uk/defra-biodiversity-net-gain/biodiversity-net-gain-considering-a-targeted-exemp/supporting_documents/biodiversity-net-gain-considering-a-targeted-exemption-for-residential-brownfield-developmentpdf-1

[3] What to Include in Biodiversity Net Gain Baselines for NSIPs – https://www.gov.uk/guidance/what-to-include-in-biodiversity-net-gain-baselines-for-nsips

[4] Biodiversity Net Gain April 2026 Reform: Small Sites and Brownfield Exemptions – https://biodiversitysurveyors.com/blog/biodiversity-net-gain-april-2026-reform-small-sites-brownfield-exemptions

[5] Biodiversity Net Gain – Gaia Company – https://gaiacompany.io/biodiversity-net-gain/