While Biodiversity Net Gain has laudable goals, the methods used for assessing a `net gain’ for nature have come under scrutiny and scepticism abounds as to what the policy will actually achieve. The UK is one of the world’s most nature depleted countries: according to the State of Nature 2023 report, one in six species is at risk of extinction. In a bid to combat this alarming scenario, Biodiversity Net Gain (BNG) was introduced in February 2024 to restore nature by requiring development to improve the wildlife habitats affected and leave the environment in a measurably better state.

A development site must be able to demonstrate a 10% increase in biodiversity on completion compared to its predevelopment condition – and the condition must be maintained for 30 years. BNG can be achieved by on-site habitat creation or enhancement, creating or enhancing offsite habitat, or by purchasing biodiversity credits. BNG immediately applied to large sites, and in April 2024 small sites were included and given a specific method of assessment, the Small Sites Metric (SSM), and it is the SSM assessment tool which has come in for particular criticism.

The Small Sites Metric

The Defra Small Sites Metric calculation tool is a simpler version of the statutory biodiversity metric calculation tool. While it does not have to be completed by a professional, such as an ecologist, it must be used by a person competent in identifying the habitats present on a site before development takes place, and in dealing with the required habitat creation or enhancement in the post development design. Competency must be in line with the British Standard Process for designing and implementing biodiversity net gain.

The SSM enables a comparison of a site’s biodiversity value pre-development and post-development, by converting habitats into measurable biodiversity units. Through the SSM, iterations can be made to the design of the site to obtain the required amount of BNG uplift.

One criticism levelled at the SSM is that it makes it harder to achieve a net gain for nature on a small site as it does not allow the same level of detail to be inputted as the statutory metric does, which can often be helpful in assessing a site. The SSM produces similar outputs to the statutory metric based on far less information. While it aims to provide developers with a simple method of measuring biodiversity, obviously small sites are restricted in terms of how much net gain can be delivered, leading critics to say the tool has limited use.

And while the SSM shares the main metric’s aims of improving biodiversity, there are problems with its use as different local authorities appear to be adopting different approaches, resulting in inconsistency. There is also still a lack of industry knowledge about BNG, despite its introduction several months ago.

What is defined as a `small site’?

The definition applies to a residential development where the number of dwellings is between 1-9 on a site measuring one hectare or under. Where the number of dwellings is unknown, the `small site’ area definition is less than 0.5 hectares. With commercial developments, the `small site’ definition applies where the floor space created is under 1,000 square metres, or the site area is under one hectare.

The SSM is not relevant to all small sites; it can only be used where the habitats available in the SSM are present. For example, it cannot be used on sites containing priority habitats, protected sites or where European protected species are present; such sites must use the statutory biodiversity metric calculation tool.

The Small Sites Metric calculation

The SSM can help assess the biodiversity unit value of existing habitats on a site, along with habitat enhancement and creation options. It can assess the size of a habitat and its distinctiveness, condition and strategic significance in calculating biodiversity units. The metric includes modules to calculate three types of biodiversity area: habitat units, hedgerow units and watercourse units.

The metric can compare biodiversity options for the land, calculate and forecast biodiversity units and how they may change. It can also advise on design and provide information about how the results can be used in planning decisions. The SSM can be used to make repeated calculations for a site as plans develop.

How the Small Sites Metric could be improved: industry views

Better training and processes

Ian Hambleton, a BNG units broker based in the southeast, commented that while the Small Sites Metric is sound, several improvements could be made to the way it is used. He said:

“Firstly, widespread training is essential. Currently, many architects, planning consultants, and environmental contractors lack the expertise to effectively apply the Small Site Metric. DEFRA could address this by rolling out comprehensive training programs that would equip these professionals with the knowledge needed to confidently use the metric in planning applications. More training will ensure that BNG becomes second nature for smaller developments, seamlessly integrating into routine project workflows.

The process for small site applications is another area for improvement. Presently, the requirements mirror those of large site applications, which can be excessive for smaller projects. A more efficient, streamlined process specifically for small sites – one that maintains accuracy without unnecessary paperwork – would ease the burden on smaller developers and encourage broader compliance with BNG objectives.

Additionally, Local Planning Authorities (LPAs) need to recognise that many offsite biodiversity unit providers do not offer fractional units. This means that developers sometimes face challenges finding providers within a short radius that can meet exact requirements. LPAs should be more flexible in accepting units from slightly further afield, allowing developers to achieve BNG requirements without unnecessary geographical limitations.

Overall, the Small Site Metric and the legislation around it are commendable steps forward in preserving biodiversity. As more suppliers emerge and industry familiarity with BNG grows, these initial challenges should ease. With better training, streamlined processes, and LPA flexibility, the Small Site Metric can achieve its full potential in supporting sustainable development across the UK.”

Phase out the SSM

However, principal ecologist and BNG lead with environmental consultancy Arbtech, Craig Williams commented that the main problem with the small sites’ metric is its lack of purpose. He said that the main statutory metric should be used instead, and explained:

“Small habitat areas can be entered into the main statutory BNG metric (using an appropriate level of accuracy as the main metric uses hectares to the SSM’s square metres) and it will still function fully. The main metric also gives you full control of retention, enhancement and condition parameters.

Small site developers are not more impacted from BNG than large ones, in fact, if a development impacts less than 25m2 of habitat, they are exempt. They can also be exempt when it comes to self-build and custom-build houses, which are easier to achieve on smaller developments. The units needed to achieve 10% are made very clear in the main metric. The Small Sites Metric should just be phased out:  there’s no need for one. Nobody who can fill in the small sites metric would be unable to fill in the main metric as the process is the same until that point.”

And Luke Mills, director of  The Landscape Service said that the SSM needs a complete overhaul. He commented:

“The Small Site Metric is not fit for purpose. It’s a scaled down version of the statutory metric and doesn’t even output the unit shortfall if you are required to purchase off-site. The information must then be transferred to the statutory (large sites) metric to calculate the off-site units, doubling the consultant’s workload. There is also no option to record a tree’s condition. Unlike the statutory metric where you can record a tree’s condition which has a massive effect on the baseline calculation and if you can achieve a net gain on-site. The Small Sites Metric also makes it harder to achieve a net gain on-site compared to the statutory metric as they do not calculate like for like, unfairly punishing a developer on small sites.

The latest guidance (July 2024) does not allow any habitat enhancement or habitat creation within a private garden. We are told by Natural England that the guidance is up to interpretation by the user. Unfortunately, in most cases we are seeing now, any privately owned land within a red line is now being considered a private garden by most LPA’s. This makes it technically impossible to achieve a net gain on-site, pushing developers straight to purchasing off-site units.

We are now being asked to strip sites of all proposed habitats by LPA’s which is creating empty, habitat free landscapes. Before this change in guidance a 10% net gain was easily achievable. Now this avenue has been closed, developers are forced off-site via habitat banks that might not otherwise be required. The Small Sites Metric requires a complete overhaul due to the slowing of the planning process and sites becoming completely unviable which affects housing targets.”

Metrics will evolve alongside our understanding of biodiversity

The potential for the BNG initiative to restore habitats in the UK is impressive and its aims are good. However, better training in using the SSM is needed along with greater understanding of BNG in general. In adapting to it, planning applicants and developers need to consider their options early on in their plans to avoid delays and unforeseen negotiations with local authorities. Getting the correct information inputted into the metric in the first instance will be crucial to the success and timings of schemes, and as CIEEM guidance points out, the SSM does not have to be used, as the statutory metric tool can always be employed instead.

However, while biodiversity assessment tools such as metrics are bound to evolve as our knowledge of biodiversity increases, it seems inevitable that designing with nature at the forefront of developments will increase. Even small sites can contribute to larger ecological networks via habitat connectivity and wildlife corridors, helping the UK to meet its environmental goals. While the focus on the environment appears to be here to stay, the methods of working out how best to do this in a meaningful may need to adapt. If you need help with purchasing off set units or fractional units, contact [email protected]