What Is the BNG Small Sites Metric?

Biodiversity net gain (BNG) became a mandatory condition of planning for most development projects in England on 12 February 2024.¹ The same day, two tools that are key to determining whether developers have met the minimum 10% uplift in biodiversity were published. They were the statutory biodiversity metric² and the small sites metric (SSM).³

The former covered all major developments subject to planning permission and, from 2 November 2026, would extend to NSIPs.⁴ But right from the start, the SSM offered a way for projects too big to be exempt (they are listed here) but too small to be a "major" development, to achieve biodiversity net gain in a more streamlined way.

This guide breaks down what the BNG small sites metric is, and how it differs from the statutory biodiversity metric. We'll examine which specific developments can use it, what must be done to complete it correctly, and how the SSM fits within the wider biodiversity net gain process.

What Is the Small Sites Metric?

The small sites metric (SSM) applies the same underlying biodiversity net gain principles as the statutory biodiversity metric. Both help developers achieve the minimum 10% net gain in biodiversity value, as laid down in the Town and Country Planning Act 1990, inserted by the Environment Act 2021.⁵

Where the full biodiversity metric requires very granular, detailed information about habitat potentially or actually impacted by a development, the SSM streamlines the process - while still producing a robust, defensible net gain calculation that developers and local planning authorities can trust.

Which Developments Can Use the Metric?

Only one tool should be used on a development, but not all small sites automatically qualify to use the metric. Projects fulfilling the following criteria can use the SSM:⁶

• A residential development where the number of dwellings is between 1 and 9 on a site of an area 1 hectare or less, or if the number of dwellings is unknown, the site area is less than 0.5 hectares
• A commercial development where floor space created is less than 1,000 square metres or total site area is less than 1 hectare
• A development that is not the winning and working of minerals or the use of land for mineral-working deposits
• A development that is not waste development

Even if a site meets the above requirements, the SSM must not be applied where the site contains or directly affects priority habitats, European protected species, statutory protected sites, or irreplaceable habitats.

In such situations, only the statutory biodiversity metric tool should be used. It's worth noting that more detailed ecological information may be needed to properly complete the assessment.

Who Can Use the Small Sites Metric Tool?

The small sites metric calculation starts with a baseline survey of the development site. This must be carried out by a competent person, as laid out in the British Standard BS 8683:2021, though unlike for the statutory biodiversity metric tool, it does not necessarily have to be done by an ecologist.⁷

The competent person should, however, know which habitats are present on site before any development work has been done, and understand the post-development management requirements for any created or enhanced habitats within the landscape design.

This allows developers to follow the mitigation hierarchy: avoid any environmental harm in the first place, before minimising, restoring, and offsetting a project's impact.

How the Small Sites Metric Tool Works

Baseline data, including the habitat's type, size, distinctiveness, condition and strategic significance is entered into the SSM by the competent person, to generate a pre-development biodiversity value for the site. The small sites metric uses simplified habitat categories and has fewer parameters, making it faster and more accessible for developers with smaller projects.

Next, information about the biodiversity units is inputted. This covers the same criteria as the existing habitat, but also takes into account two risk multipliers: how difficult the required biodiversity units are to create or enhance, and how long they will take to achieve their environmental targets.⁸

It's worth pointing out the SSM requires landscape plans and habitat photographs to be submitted alongside the metric calculation. These give local planning authorities a clearer picture of what existing habitats look like on-site, and what the proposed post-development habitats will involve. This documentation also supports the biodiversity gain plan, which is required for planning permission.

When all this data has been added, the SSM calculates the biodiversity net gain figure. If the result is less than the minimum BNG 10% improvement, the developer must apply the biodiversity gain hierarchy.

They can either reduce the on-site environmental impact of the design, or, if on-site enhancements don't achieve the mandatory, minimum 10% improvement, off-site solutions must be found. In another major difference to the statutory biodiversity metric tool, the SSM does not include a dedicated off-site section. Details of any off-site BNG units needed must be included in the biodiversity gain plan.⁹

In cases where on- and off-site units cannot meet the threshold, statutory credits can be used as a last resort to make up the BNG shortfall.

Ideally, the SSM should be used multiple times so developers refine their designs to achieve their mandatory BNG threshold.

BNG Trading Rules

Both the statutory biodiversity metric tool¹⁰ and the SSM have strict trading rules designed to prevent developers from simply swapping high-value habitat for a lower quality one.

They also ensure that any created or enhanced habitat is of the same or better quality than what was damaged or lost, to a level of no net loss.¹¹

Private Gardens and the Small Sites Metric Calculation

Private gardens inside a development's red line boundary that have important features, such as mature trees or hedgerows, can be included in the baseline habitat assessment using the small sites metric.¹²

Developers can record these features, which can make a difference to a site's BNG, as "retained" in the baseline survey. However, because they cannot legally be secured for the mandatory minimum 30-year period, gains in private gardens can only be assessed post-development as either urban vegetated or unvegetated garden, irrespective of any habitat changes or enhancements.¹³

What Are the Benefits of the Small Sites Metric?

The SSM tool offers sometimes significant benefits for developers who might find the full statutory biodiversity metric difficult or expensive to navigate.

The simplified version is faster than the full statutory biodiversity metric, and reduces the time and cost of experts carrying out habitat assessments for BNG. For developers with small residential developments and tight margins, the SSM can directly impact their viability.

The small sites metric also supports the BNG process by providing clearer, more accessible guidance on boosting biodiversity on-site. For many minor development schemes, the metric's simplified habitat creation requirements mean that relatively modest changes can be enough to meet the minimum 10% uplift on-site.

The SSM plays a crucial role in the wider UK government objective of restoring nature while maintaining housing delivery. It allows smaller projects to contribute to biodiversity net gain without the BNG process becoming a barrier to entry. On their own, each individual development may have a small biodiversity impact, but collectively and over time, they can make a real ecological difference.

How the Small Sites Metric and the Wider BNG Process

For developers using the SSM, the biodiversity net gain process follows the same route as major developments.

A competent person assesses the baseline biodiversity value of the development site, calculates the post development net gain figure using the metric tool, and prepares the biodiversity gain plan for submission to the local planning authority, who will then approve or challenge it within 8 weeks.¹⁴

Like all biodiversity net gain schemes, habitats created or enhanced through the small sites metric are required to be monitored and maintained for at least 30 years.¹⁵

Legal agreements such as a planning condition or, for off-site units, a section 106 agreement or conservation covenant, will confirm how that will happen. Local authorities retain enforcement powers throughout the lifetime of the commitment.

We Can Help at Every Stage of the BNG Process

If you are a developer and want guidance on whether you can use the small sites metric, or you need to source off-site biodiversity units to achieve your biodiversity gains target, call us.

We can connect you with experienced ecological consultants and offer expert advice at every stage of the BNG process.

If required, you can browse our national network of habitat banks; high-quality, off-site BNG units, listed on the biodiversity gain sites register.

Email us at sales@greenshank-environmental.com or fill in this contact form and we'll be in touch as soon as possible.