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Biodiversity net gain: how architects can best use the statutory metric calculation tools for small sites

Neal Morris

With biodiversity net gain soon to be mandatory for all sites, learn more about the tool most likely to be used by architects

The Small Sites Metric works alongside the main metric tool, and smaller practices working on a small-scale new build project from now on will be able to consider self-use of the simpler version.
The Small Sites Metric works alongside the main metric tool, and smaller practices working on a small-scale new build project from now on will be able to consider self-use of the simpler version. Credit: iStock Photo

A crucial part of the push towards a more sustainable and greener future is the protection and enhancement of biodiversity. Thanks to a rapid expansion of cities and urban areas, statistics from the World Economic Forum show that the built environment is responsible for a 30% loss of biodiversity. However, if the sector can protect and boost biodiversity it will have myriad positive impacts on both the world as a whole and the industries and activities within the built environment.

To this end, biodiversity is a key discussion point in RIBA Horizons 2034, a four-part insight-gathering programme that highlights global trends of the near future. Available exclusively to RIBA members, each themed instalment comprises four articles that are written by respected experts and thought leaders. RIBA Horizons 2034 launches today (Thursday 21 March) and takes a closer look at The Environmental Challenge. You can also read how to apply those foresights in practice now. 

In the present day, however, biodiversity net gain becomes mandatory for minor sites on 2 April (2024).

The vast majority of small practices work on projects that do not warrant an external specialist to meet their legal obligations under net gain or existing species and habitat protections. This external specialist – such as an ecologist, for example - would normally be the person most able and appropriate to complete the government’s official biodiversity metric tool. The main metric is a powerful tool in this instance, using a filled metric to inform non-ecologists of the biodiversity needs in architectural design scenarios. Working alongside this, smaller practices working on a small-scale new build project from now on will be able to consider self-use of the simpler version – the small sites metric (SSM). The published metric and small sites metric guidance is best used at this point to assess the appropriate tool in an individual project case.

Is filling in the metric difficult?

Minor residential sites are defined as fewer than 10 dwellings (or less than 0.5 hectares (ha)) if the number of new dwellings is unknown. Commercial development sites must be smaller than 1ha or have floorspace to be created of less than 1,000 sqm.

While there is a requirement for a certified ecologist to use the main metric, or someone able to demonstrate comparable competence, the SSM should be useable by a wider audience on a site-by-site basis, says Jamie Hannah, Metric Specialist at Natural England. If using the SSM you must be able to identify and be sure only habitats available in the SSM are present, as well as a clear plan for their management, he adds.

‘Filling in the metric is a relatively straightforward process,’ he says. ‘Most of the work comes from assessing what you’ve got in the first place from your site walk, desk research and whether there is anything special there that requires the use of the main metric.’

Architects are asked to demonstrate that none of the habitat types are classed above ‘medium distinctiveness’. They should also be confident there are no areas designated as priority habitats or with protected species within their site, which would also automatically require use of the main metric. Natural England strongly recommends the use of the main metric if there are any high-value habitats within 500 metres. Use of either metric does not change existing biodiversity protections, statutory obligations, policy requirements, ecological mitigation hierarchy or any other existing requirements.

Any encroachment on water courses within or adjoining the site also rules out use of the SSM.

For guidance on what habitats you have on site or adjoining, Hannah directs architects to the descriptive guidance in the latest UK Habitat Classification Documents V2.01, which can be freely downloaded.

Find out more about RIBA Academy’s Biodiversity Net Gain Policy Update course

What is needed to fill in the metric tool?

Photographic evidence of a particular habitat assessment is strongly advised. This is not only a nod to the potential lower competence of the assessor but will also serve as a check for planners that an assessment seems reasonable when the metric output is submitted.

Architects should also record individual trees on site – for example, how many will be retained/lost and how many will be planted, if that is the case. Subject to the extent of change, it may also be necessary to consult with an arboriculturist and biodiversity consultant for the project needs.

The SSM produces similar outputs to the main metric but needs a greatly reduced amount of information and has limitations on what it can be used for, such as types of interventions and limited types of habitat, says Hannah.

It is quite intuitive and will prompt for all the key information required, such as site area, the number of planned dwellings or the footprint of a commercial scheme, with red flags whenever input information does not accord with the metric, including notifications when the SSM should not be used.

Users should check whether the local planning authority are asking for the minimum 10% net gain for their particular project under planning policy of course, because there are scenarios where the requirement may be higher and it is possible there is a further requirement by condition of the land purchase contract.

Natural England has a biodiversity net gain enquiries mailbox – basically an email-based helpline – that architects can use. Hannah says this can be used for technical queries, interpretation of guidance, or information on registering a site for off-site biodiversity units.

‘The main advice we give is to engage with this tool as early in the design process as you can. You can even use it at the site selection stage. You will be much more successful in achieving net gain if you use the metric early and use it for design iterations, rather than using it as a later stage check.’

Find out more about RIBA Academy’s Statutory Biodiversity Metric training

What's next for biodiversity?

Hannah points out that some developers and their consultants sometimes have been too quick to approach biodiversity net gain as nothing more than a planning condition and an additional cost, and ignore the new opportunities there might be to create a tradeable biodiversity surplus.

Where a site offers an opportunity to create a high-value habitat, perhaps by extending an adjoining habitat of special interest or enhancing a water course, then there is a corresponding opportunity to generate and sell on the surplus offsite biodiversity units. Adding biodiversity could become part of the developer’s business model, he noted.

High-value habitats will not only add to the amenity value of the development, but the units generated can be offered for sale to other developers or one of the habitat bank operators that are expected to emerge. Switched-on developers may choose to trade biodiversity gains across their own pipeline developments.

All of this is outside of the scope of the SSM, of course, as the national biodiversity gain sites register maintained by Natural England, where all tradeable gain sites must be registered, will only interface with the main metric. The message, says Hannah, is that you don’t have to think small with biodiversity, you can do big things.

Access the RIBA Horizons 2034 programme, this week comprising four articles discussing The Environmental Challenge.

Thanks to Jamie Hannah, Natural England

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RIBA Core curriculum topic: Sustainable architecture.

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