Achieving Small Site BNG in 2025 | Tunley Environmental

Understanding the BNG Extension to Small Sites

Small sites are defined as:

• Residential developments: Projects with 1 to 9 dwellings, or if the number of dwellings is unknown, a site area of less than 0.5 hectares.
• Commercial developments: Projects where the floor space created is less than 1,000 square metres or the total site area is less than 1 hectare.
• Other developments: Sites that don't involve mineral extraction or waste management.

Some projects qualify for exemptions. These include householder applications, developments that affect less than 25 square metres of habitat (without impacting priority habitats) and self-build or custom housing projects of no more than 9 dwellings on site and no larger than 0.5 hectares.

Implementation Timeline

The BNG requirement became mandatory for small sites on April 2, 2024, following its earlier application to major developments in February 2024.

Achieving 10% Net Gain in Small Sites

Despite initial concerns, the BNG framework offers flexibility in how developers can meet this requirement. First and foremost, developers should try to avoid habitat loss during development work. When planning a small site project, these three primary approaches should be considered:

• On-site solutions: Creating or enhancing biodiversity within the development boundary, often the most cost-effective option and preferred by planning authorities.
• Combined approach: Using both on-site improvements and off-site solutions such as purchasing biodiversity units from the market.
• Statutory credits: Buying statutory biodiversity credits from the government can also be considered as a last measure.

The Impact of BNG One Year On

One year into the policy's application for small sites, several key developments have emerged:

• Mainstreaming Biodiversity Consideration: Biodiversity is now considered a standard part of planning applications and is no longer perceived as an optional or peripheral issue.
• Upskilling Within Industry: Increased demand for ecological surveys and biodiversity assessments has enhanced collaboration between developers and environmental consultants.
• Use of the Small Sites Metric (SSM): The Department for Environment, Food, and Rural Affairs (DEFRA) simplified Small Sites Metric has proven vital in enabling developers to calculate their biodiversity baselines and proposed gains.
• Habitat Banking Growth: The emergence of local habitat banks provides off-site biodiversity units, helping developers achieve compliance when on-site gains are not feasible.

Common Misconceptions About Small Site BNG

1. "BNG only applies to large developments."
   False. As of 2 April 2024, BNG requirements apply to all major and minor developments, excluding exempted categories (such as householder applications or permitted development).
2. "If I can't do it on-site, I'm non-compliant."
   Not true. Developers can purchase off-site biodiversity units or contribute to habitat banks if on-site provision is not possible. These must be registered on the national Biodiversity Gain Site Register.
3. "BNG is just about planting trees."
   Incorrect. BNG includes various habitat types (e.g. grasslands, ponds, hedgerows) and is measured using habitat condition, distinctiveness and spatial risk. Simply planting trees without ecological planning may not meet the 10% net gain threshold and planting trees on small residential sites typically does not contribute to BNG.
4. "Using the Small Sites Metric means I don't need a consultant."
   The metric simplifies calculations but does not replace ecological expertise. Professional advice from Biodiversity specialists ensures compliance, feasibility and integration with planning conditions.

Step-by-Step Guide to Achieving Compliance in 2025

The Small Sites Metric (SSM) is a practical tool that helps small site developers meet their biodiversity obligations without extensive ecological expertise. Introduced in April 2024, this streamlined tool simplifies biodiversity calculations while maintaining compliance with mandatory requirements.

Step 1: Understand the Regulatory Requirements

Before submitting a planning application, it’s important to familiarise yourself with the Environment Act 2021 and its BNG requirements. Afterwards, developers should consult DEFRA's Small Sites Metric and BNG guidance documents here.

Step 2: Conduct a Baseline Assessment

The next step is to commission a qualified ecologist to carry out a habitat baseline survey of the development site then use the SSM to quantify existing biodiversity units. An assessment on whether the development will result in habitat loss or degradation should follow the survey.

Step 3: Design Your BNG Strategy

Once the baseline assessment has been conducted, developers will need to develop a net gain strategy to improve biodiversity on the site. Several approaches can be used such as:

On-site enhancement options:

• Create or improve habitats (e.g. wildflower meadows, ponds).
• Incorporate green roofs or green walls.
• Add native hedgerows and sustainable drainage features.

Off-site options:

• Partner with a habitat bank to purchase off-site biodiversity units.
• Contribute to strategic nature recovery schemes if locally available.

The objective is to ensure that your strategy achieves at least a 10% uplift in biodiversity units, using the metric as your guide.

Step 4: Prepare and Submit a Biodiversity Gain Plan

Your Biodiversity Gain Plan should include:

• A clear baseline assessment
• Proposed habitat enhancements (with locations and methods)
• Maintenance and monitoring plans for at least 30 years
• Evidence of any off-site unit purchases or conservation covenants

The plan should be submitted to the Local Planning Authority (LPA) along with the planning application or as a post-content condition.

Step 5: Implementation and Monitoring

After the plan is approved, biodiversity enhancement works can begin either during or immediately after construction. Ensure evidence of the implementation such as photos and ecological inspection records is recorded. The habitat should be regularly monitored as agreed with the LPA.

 

The Role of Local Planning Authorities (LPAs)

LPAs play a pivotal role in the BNG process including:

Approval of Biodiversity Gain Plans: LPAs must assess and approve developers' biodiversity gain plans within an 8-week timeframe.

Monitoring Compliance: They are responsible for ensuring that developers adhere to their BNG commitments and for taking enforcement action if necessary.

Facilitating Off-Site Gains: LPAs can assist in identifying local areas suitable for habitat creation and enhancement, supporting developers in finding off-site solutions.

Using the Small Sites Metric tool effectively

The SSM converts habitats into measurable biodiversity units through three specific modules: area habitat units, hedgerow units and watercourse units. This calculation allows comparison between pre-development and post-development biodiversity values.

To use the tool effectively:

• Input data about existing habitats including type and area
• Enter post-development plan information, including habitat retention, creation or enhancement
• Allow the tool to automatically calculate biodiversity unit changes
• Review whether your plan achieves the mandatory 10% net gain

Unlike the full metric, condition assessments aren't required with the SSM as habitat values are fixed. Additionally, you're not required to appoint an ecologist specifically for BNG purposes. Nevertheless, you must be competent in identifying habitats and understanding management needs for created/enhanced habitats.

The Bottom Line

Achieving small site BNG in 2025 requires a thorough understanding of regulatory requirements, proactive planning and collaboration with LPAs. Developers can contribute meaningfully to biodiversity enhancement by following a structured approach and utilising the SSM. Throughout 2025, additional guidance and tools will likely continue to evolve, making compliance progressively more straightforward for small site developers. Learn more about the latest biodiversity requirements and how it affects you by visiting the Tunley Environmental Biodiversity services page.