Biodiversity Net Gain (BNG): What You Need to Know

This information is our best understanding of the current BNG requirments as of March 2025,. We will endeavour to update this information as new advice is provided. If you have any queries, reach out directly.

Update – June 2026

This article was originally written to explain the Biodiversity Net Gain (BNG) requirements in 2025.

The Government has since announced a number of significant changes to BNG that are expected to take effect from 31 July 2026, including a new exemption for sites up to 0.2 hectares and the removal of the existing self-build exemption.

If you are planning a self-build home, small development, barn conversion or change of use project, we recommend reading our updated guide: https://www.ikonic-architecture.co.uk/post/biodiversity-net-gain-changes-2026-what-the-new-rules-mean-for-self-builders-homeowners-and-small

Biodiversity Net Gain (BNG) is the legal requirement in England that ensures new developments leave nature in a better state than before the development began. This means that any impact on biodiversity caused by a construction project must be balanced by improving habitats on-site or elsewhere, with at least a 10% net increase in biodiversity compared to the original condition, including any replacement of lost habitat.

BNG became mandatory in 2024 and impacts large and small developments, so from one unit to 100+. Planning applicants for market housing must measure biodiversity using a baseline assessment and submit a plan showing how they will achieve the required gain. If they cannot provide the 10% increase on-site, they must secure off-site biodiversity units or buy statutory biodiversity units to compensate.

If there is evidence of habitat removal (degradation), the baseline can be taken back to January 2020, so applicants must ensure they get an assessment completed prior to making any site clearance. Removal of poor habitat will be assessed as good-quality habitat without accurate information to judge it.

Three core groups need to be aware of the responsibilities that impact new development sites;

1. Self-build and Custom Build

2. Homeowners with garden plots

3. Developers looking at development sites

Self-Build and Custom-Build Exemptions

The government recognises that small-scale, one-off developments have a lower impact on biodiversity and wish to encouge a larger proportion of self build properties. Therefore, Self-build and custom-build homes on plots smaller than 0.5 hectares (about the size of a football pitch) are exempt from mandatory BNG requirements. Householder applications, such as home extensions and garden buildings, are also exempt.

What This Means for Self-Builders and Custom Build sites

If you are building your own home on a small plot, you will not need to provide the 10% biodiversity net gain. However, local councils may still encourage biodiversity improvements, such as hedgerow planting or bird boxes, as part of the planning process.

If your self-build site is larger than 0.5 hectares, you may need to comply with BNG rules like any other development, consult your architecture or planning consultant to explore the site requirements of your specific scheme.

If you are developing multiple custom-build plots, the rules may vary, it will need to be reviewed how you plan to develop the site seek out expert constulants to assist.

Housing Projects

1. Garden plots for low-density development: The BNG replacement and uplift is required to be delivered outside of private curtilage; on-site measures will need a 30-year management plan. Off-site delivery of BNG can overcome these challenges and can be more cost effective long term than delivering on-site BNG.

2. Financial Constraints: Implementing BNG can be costly. Developers must invest in ecological assessments and habitat creation, which can strain budgets, especially for smaller projects. This financial burden might impact the overall viability of developments. 

3. Land Availability: On compact sites, allocating space for biodiversity features can reduce the area available for housing, potentially leading to fewer units and affecting project profitability.​

4. Complex Regulations: Navigating BNG requirements adds layers of complexity to the planning process. Developers need to understand and comply with detailed ecological guidelines, which can be challenging without expert assistance.​

5. Long-Term Management: BNG commitments will involve maintaining habitats for 30 years. 

6. Pre-Commencement Condition: BNG is a pre-commencement condition within a planning approval, which crucially requires the discharge of a Biodiversity Gain Plan (BGP) prior to commencing construction onsite. If BNG is being achieved offsite, the offsite units can be purchased after the decision notice is issued, and then referred to in the BGP that's submitted for discharge of conditions (DOC).

On-Site and Off-Site Biodiversity Units

On-Site Units: These are biodiversity improvements made within the boundaries of the development site. For example, planting native trees or creating ponds on the property can enhance local habitats.​ This has to be outside the private curtilage. Typically, BNG requirements focus on enhancing biodiversity within the development site itself. However, if achieving the required 10% net gain within the development site isn't feasible, developers may need to consider off-site options.

Off-Site Units: Developers can invest in biodiversity projects elsewhere if on-site enhancements aren't sufficient to meet BNG requirements. This could involve creating or restoring habitats in different locations to compensate for the impact of their development. 

Where can you get off-site Biodiversity Units?

If achieving BNG on-site isn't feasible, developers can buy off-site biodiversity units from landowners, conservation organisations, or local authorities. These units represent habitat improvements made elsewhere to compensate for biodiversity loss on the development site.

Applicants can;

1. Partner with local landowners who create and manage new habitats.

2. Use Habitat Banks, where organisations restore land for biodiversity and sell the resulting units.

3. Work with local planning authorities if they have established a biodiversity unit scheme.

Developers can also purchase statutory biodiversity units from Natural England if on-site and off-site options are not viable. These units fund government-led conservation projects but are intended as a last resort because they are priced higher to encourage on-site or local off-site enhancements first.

Risks of Not Conducting Proper Assessments Before Site Clearance

Before starting any development, assessing the site's existing biodiversity is crucial.

Clearing a site without this assessment can lead to several issues:​

• Inaccurate Baseline Data: Without knowing the original state of the site's habitats, it's challenging to measure the impact of the development and determine the necessary enhancements to achieve BNG.​

• Loss of Important Habitats: Uninformed clearance might destroy valuable or protected habitats, causing irreversible environmental damage.​

While BNG aims to harmonise development with environmental preservation, it presents real challenges for new residential development. Finding a balance between creating new homes and enhancing nature requires thoughtful planning, adequate resources, and stakeholder collaboration.​

By acknowledging these challenges, homeowners and developers can better prepare and adapt their projects to support biodiversity while achieving their development goals. ]

Additional Links and Information

Local to Norfolk, there is the Langley Abbey Estate, who are restoring nature across their estate to reverse biodiversity loss, conserve peatland and improve water quality in the Yare Valley. To learn more about the work they are doing or the help they can provide with BNG and delivering offsite units take a look at their webpage.

If you wish to learn more about the underlying Goverment regulations visit their website to explore additional information.

If you have any queries or require a site assessment to proceed with a potential housing site in Norfolk or Suffolk, please contact us to discuss this further. Important Update – July 2026 Changes To Biodiversity Net Gain

Since this article was first published, the Government has confirmed several changes to Biodiversity Net Gain that are intended to simplify the process for smaller developments.

Subject to the final legislation being implemented, the key changes include:

• A new exemption for development sites up to 0.2 hectares.

• Removal of the current self-build and custom-build exemption.

• Temporary planning permissions of up to five years becoming exempt.• Greater flexibility for minor developments to deliver biodiversity gains off-site.

These changes are expected to affect a significant proportion of small residential developments, self-build projects and rural development schemes.

Before relying on any exemption, we recommend reviewing the latest guidance and seeking project-specific advice, as requirements may vary depending on site constraints, habitats present and local planning policy.