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Small Sites Metric (SSM)

This online resource paper provides information about Natural England's free accounting tool for calculating biodiversity net gain in small developments

Definition and purpose

The Small Sites Metric (SSM), developed by Natural England, provides a method to measure biodiversity and the impact that small development projects may have upon it in a consistent way. Whereas larger developments or conservation projects will use the main Biodiversity Metric 3.0 (see Biodiversity Metric 3.0 resource paper), the SSM is a similar but simplified process that can assist those delivering smaller developments or projects to take biodiversity into account.  

The NPPF 2021 sets out that development should deliver measurable net gains in biodiversity. Paragraph 174 (d) states that developments should minimise impacts on and provide net gains for biodiversity, including by establishing coherent ecological networks that are more resilient to current and future pressures. Craven Local Plan policy ENV4: Biodiversity also sets out that developments should make a positive contribution towards achieving a net gain in biodiversity where possible.

The Small Sites Metric and Biodiversity Metric 3.0 are the standard methods in England for measuring biodiversity change which result from new development and will assist in demonstrating whether or not net gains in biodiversity have been achieved. The metrics are designed to quantify biodiversity to inform and improve planning, design and decision-making. They can support planning applications to calculate the losses and gains in biodiversity from development.

Key points to consider

The SSM can only be used when both of these criteria are met:

1. The development is either:

  • A residential development, where the number of dwellings to be provided is between one and nine inclusive on a site having an area of less than one hectare;
  • Where the number of dwellings to be provided is not known, there is a site area of less than 0.5 hectares;
  • For all other development types where the site area is less than 0.5 hectares or 5,000 square metres.


2. There is no priority habitat within the development area (excluding hedgerows and arable margins).

What this means for spatial planning and development management

Existing levels of protection afforded to trees, protected species and protected habitats are not changed by use of this or any other metric. Statutory obligations and other policy protections must still be satisfied in addition to the calculated biodiversity impacts using the SSM. The SSM must not be used for assessing biodiversity outside the development area. Any habitat creation or enhancement outside the site area must be assessed using Biodiversity Metric 3.0.

If the above criteria are met, the SSM can be used. It is not necessary for the SSM metric to be completed by an ecologist but by someone who is competent to use that metric. This is somebody who has acquired through training, qualifications or experience, or a combination or these, the knowledge and skills enabling that person to accurately complete the SSM. LPAs should verify that the person who has completed the metric fulfils this criterion and is suitably competent to be able to do so, as described in the metric User Guide. If the above criteria are not met, the site should be scored using the Biodiversity Metric 3.0 by a suitably qualified ecologist.

Planning policy (Policy ENV4 in the Craven Local Plan) supports the application of the mitigation hierarchy (see the Mitigation Hierarchy resource paper). When using the SSM, applying this mitigation hierarchy in practice starts with retaining habitats in situ and avoiding habitat damage. Biodiversity gains are easier to achieve when habitat losses are avoided in the first place. It might not be possible to avoid all impacts, but because the SSM measures every loss in area/length, even the smallest habitat patch that is retained can make a difference. The mitigation hierarchy is presented below:

1. Avoid - damage to habitats should be avoided

2. Minimise - damage to habitats should be minimised

3. Rehabilitate/restore - damaged or lost habitats should be restored

4. Offset - as a last resort, damaged or lost habitats should be compensated

Relevant Craven Local Plan policies

Relevant Craven Local Plan policy guidance


March 2022. This webpage provides general information about relevant planning topics and we hope you find it helpful. Please be aware that it is not a statement of Council policy and does not provide formal policy guidance. For those things, please refer to the Craven Local Plan and supplementary planning documents.