Biodiversity Net Gain Small Sites Metric

An Explanation of the Small Sites BNG Metric

An approach to development that focuses on the potentially detrimental effects of planning projects on the natural environment, biodiversity net gain (BNG) actively guarantees that the condition of the site will be retained and improved upon to a measurably better state by a minimum of 10% and last for at least 30 years after the planning process has concluded.

With the mandate in place, the Department for Environment, Food and Rural Affairs (DEFRA) created a statutory biodiversity metric for combining all ecological features over the course of a BNG assessment and establishing the current biodiversity value of a development site. Different versions of the metric were updated and released, and ecologists now use DEFRA biodiversity metric 4.0.

After several delays and setbacks, biodiversity net gain became mandatory for all affected developments on 12 February 2024. It was then followed by the introduction of a secondary type of metric for small sites that also apply to BNG. The small sites metric (SSM) isn’t necessarily as frequently used as the traditional metric, but as it could affect your development plans, it would be worthwhile to learn exactly what it is, why it exists and how it differs as an alternative option.

Biodiversity Net Gain for Small Sites

Offering a simplified version for use on small-scale projects, the small sites metric was released on 2 April 2024, enabling ecological consultants, local planning authorities and small developers to streamline the development process whenever possible, as dictated by the UK government through the Town and Country Planning (Development Management Procedure) (England) Order 2015.

Not only that but using the SSM tool is also quicker and cheaper than the traditional method, meaning that opting for the standard BNG metric could cause unnecessary time and money wasted. It also ensures that developments on small sites contribute to habitat enhancement, retain ecological value and insist on new habitat delivery, so the project will achieve BNG and avoid issues with planning applications.

Comparison with the Traditional Biodiversity Net Gain Metric

Between the small sites metric and the current statutory biodiversity metric (biodiversity metric 4.0), there are core differences, such as:

	Biodiversity Metric 4.0	Small Sites Metric (SSM)
Application	Major Developments	Minor Developments
Complexity	Comprehensive	Simplified
Ease of Use	Strictly Requires Ecological Expertise	Applies to Non-Specialists and Small Developers
Habitat Condition Assessment	Involves an Assessment	Involves Pre-Set Conditions Based on Habitat Type
Output	Slower but with More Detail	Quicker but with Less Detail
Purpose	For Medium Developments and Large Developments	For Small Developments
Strategic Significance	Detailed Evaluation Factoring In Local Plans and Strategies	Streamlined Process for Evaluating Habitat Importance

Criteria for the Biodiversity Net Gain Small Sites Metric

Under the Town and Country Planning (Development Management Procedure) (England) Order 2015, ‘small sites’ are defined as projects and sites that carry certain characteristics unlike major developments. For example, common aspects that apply to major development but don’t apply to minor development include projects that:

• Impact Irreplaceable Habitats Present
• Form Part of Nationally Significant Infrastructure Projects
• Play a Role in the Winning and Working of Minerals or the Use of Land for Mineral-Working Deposits
• Relate to a Waste Development Scheme

As for more specific development types affected by the small site metrics, details are provided below:

Commercial Developments and Other Land-Use Developments

• If the site has less than 1,000 square metres of floor space created
• If the total site area is less than 1 hectare

Residential Developments

• If the site includes no more than 9 dwellings
• If the total site area is less than 0.5 hectares and the number of dwellings is unknown

Active Small Site Exemptions

Depending on the circumstances, a BNG small sites exemption may be able to prevent developers from needing to adhere to mandatory BNG. That said, instead of making the development entirely exempt, it could also mean that the developer needs to refer to the full biodiversity metric used in the majority of developments affected by the planning condition.

In the section below, we detail the different types of exemptions and what they mean for your small sites development:

Completely Exempt from Mandatory Biodiversity Net Gain

• Custom-Build Development Projects
• De Minimis Development Projects
• Development Projects Only Affecting an Irreplaceable Habitat
• Householder Applications
• Self-Build Development Projects
• Small-Scale Infrastructure Projects

Referred to the Full Biodiversity Metric 4.0

• Large Developments
• Major Commercial Projects
• Projects Involving Complex Impacts on Biodiversity
• Projects Requiring Detailed Ecology Surveys

Exempt from the Local Policy and/or Local Council

• The Full Biodiversity Metric may be Needed if Sensitive Habitats are Impacted
• Local Policies may Override National Exemptions

Benefits of the BNG Small Sites Metric

Multiple factors point toward why the small sites metric for biodiversity net gain can be advantageous for local authorities and developers. It also strengthens the level of safeguarding over the European protected species, priority habitat and statutory protected sites that appear across the country.

Advantages of the small sites BNG metric include:

Backing On-Site Biodiversity Gains

Although ecological consultants will always favour seeing the mandatory requirements achieved on-site during the process of working to deliver BNG on an applicable development site, the small sites metric prioritises this outcome. It supports habitat restoration, green infrastructure and sustainable development.

Through on-site habitat creation projects and incorporating hedgerow units and watercourse units, onsite BNG is a more likely possibility. As such, it minimises the chances of needing to use the last resort option and buy off-site biodiversity units as land owned by someone else or purchase statutory biodiversity credits.

Complying Correctly with the BNG Requirement

Various legal agreements, planning policies and other considerations feature in how a developer successfully accomplishes the BNG mandate. A matter of regulation from organisations such as DEFRA, Natural England and CIEEM also comes into play, only adding to the responsibilities in the hands of the developer.

Despite not uncovering the further detail that the full biodiversity metric would, the small sites metric manages to cover all legal aspects and ensure that biodiversity commitments are met using a biodiversity net gain plan. Alongside the biodiversity gain plan, other concerns regarding a monitoring plan and long-term habitat management as applicable will also be catered for.

Streamlining the Assessment Process

For developments on small sites where the number of dwellings is unknown, the site area is under 0.5 hectares and potential ecological assets haven’t yet been identified, the small sites metric emerges as a straightforward method for calculating biodiversity units. The further information from a traditional assessment to generate the 10% BNG increase simply isn’t warranted.

Rather than having to conduct extensive habitat condition assessments like you would with biodiversity metric 4.0, the SSM allows for a competent person to gauge pre-development and post-development value, use the biodiversity gain hierarchy, create a biodiversity gain plan, and work out if offsite units or statutory credits are needed in a more efficient manner.

Reach Out for Help with BNG for Small Sites

It may be the case that you are working on major developments, small developments or don’t even know what your project is classed as and whether it applies to the BNG requirement. Whatever the situation, Arbtech can step in to assist, resulting in a biodiversity gain plan to enable your development plans to continue.

Feel free to call us or send us a message using the phone number and quote box at the top of this page, or check out our contact page for other options. We can then plan a date for one of our ecological consultants to attend your site if a BNG survey is necessary and begin supporting planning applications submitted to the local planning authorities.