Statutory metric
You must use the statutory metric to measure the biodiversity value of your development site. It measures all types of habitat, including grassland, hedgerows, lakes, woodland and watercourses such as rivers and streams.
The metric measures the biodiversity value of habitats by calculating the number of biodiversity units. It calculates:
- how many units a habitat contains before development takes place
- how many units are needed to replace the units of habitat lost and to achieve 10% BNG, through the creation or enhancement of habitat
The formula takes different factors into account, including the habitat’s size, condition, strategic significance and type. For created or enhanced habitats, the formula also takes account of:
- difficulty of creation or enhancement
- the time it takes a habitat to reach its target condition
- distance from the habitat loss
The statutory metric should be completed by a competent person such as an ecologist.
Small sites metric
You can choose to use a simpler version of the metric tool, called the small sites metric, if your development meets the criteria to do so. A qualified ecologist does not need to complete this.
You can find more details on the metric, including a user guide on GOV.UK. You can also find the latest statutory metric.
Strategic significance
Strategic significance is the local significance of the habitat based on its location and habitat type. Where a Local Nature Recovery Strategy (LNRS) has been published this should be used to assign strategic significance.
The Hertfordshire Local Nature Recovery Strategy (LNRS), expected to be completed in 2025, will identify areas for ecological improvement and outline the actions needed to deliver those improvements.
Until the LNRS is published applicants are advised that the Herts Ecological Networks (HEN) Map should be used instead to assess strategic significance.