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ARBTECH: The Biodiversity Net Gain Experts

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Your complete guide to Biodiversity Net Gain: Metrics, Plans and Costs

Biodiversity Net Gain is set to become a mandatory consideration for all development in England, with very few exceptions. This will have a significant impact on how local authorities make decisions to grant or refuse planning permissions. And, therefore, demonstrating biodiversity net gain will be an integral part of almost all planning applications moving forwards.

What is Biodiversity Net Gain?

Briefly, the UK government has stated that new developments must measurably increase biodiversity on or near the site (Partnership for Biodiversity in Planning, N.D.). There are some exemptions, and these include householder applications and brown-field sites that meet certain criteria (Lodders, 2020), as well as permitted development.

This will be achieved by the developer providing a Net Gain Plan either pre-determination or as a condition of planning consent.

Whilst this policy is yet to come into force, local authorities across England are looking ahead and are making biodiversity net gain a matter of policy (McClean, 2020). Informed by the National Planning Policy Framework requirement to identify and act on opportunities to increase biodiversity (Ministry of Housing, Communities, & Local Government 2019), many local authorities are already applying a 10% biodiversity net gain requirement for all new developments. An example is Cornwall Council, who have stated that effective March 2020, they would be requiring a BNG of 10% from all major applications, a year or more ahead of time (Biodiversity Net Gain – Cornwall Council, 2021).

As local authorities begin to stipulate biodiversity net gain as a condition of granting planning consent in ever greater numbers, it’s sensible to consider this requirement now. If you don’t, you run the risk of your initial application being refused and a costly delay to your development schedule. 

Why was Biodiversity Net Gain brought in?

Although it gets a lot of attention from architects, planning consultants and developers, the biodiversity net gain requirement is only a small part of the Environment Bill 2020. This piece of legislation came into existence as a consequence of the UK’s decision to leave the European Union. As an independent country, the UK is not obliged to follow European environmental regulations. Therefore, a new legal framework was needed to ensure that the UK protects, preserves, and enhances the natural environment (Department for Environment, Food & Rural Affairs, 2020). 

Though only a small part of a larger strategy, biodiversity is a vital contributor to all of the key aims of the legislation. Flourishing ecosystems mean cleaner air and water, and well-tended green spaces not only contribute significantly to mitigating climate change (Department for Environment, Food & Rural Affairs and Villiers 2019) but also add significant value to the economy, both locally and nationally. 

Healthy natural environments are almost universally desirable places to live and work, so it’s likely that the fiscal value of land (and the developments thereon) will increase in tandem with the biodiversity of the area. So, whilst it’s understandable that you might see this renewed push for environmental protections and enhancements as yet another hurdle to jump in the pursuit of a planning consent, it’s probably better to view the 10% biodiversity net gain requirement as an investment.

The government don’t want to stand in the way of development. Indeed, they recognise that there is a critical housing shortage; and understand the challenges faced by the construction sector (Homes England, 2018). That said, nature has been in decline for decades. In fact, a report from 2019 found that 41% of UK species had decreased in abundance, while only 26% saw increases (National Biodiversity Network, 2019). Therefore, the arrival of a strategy that actively promotes developments that improves the natural environment and stops developments that are ambivalent to it (or actively harm it) was inevitable. In many ways, our view is that unifying the goals of various stakeholders in conservancy is a great concept. The devil as they say, is in the detail…

Biodiversity net gain metrics, calculations and assessments

Ultimately, what you really want to know is how this is going to affect your planning application. 

Under this new legislation, you’re going to have to submit and receive approval for a “biodiversity gain plan“. Once the Environment Bill 2020 is passed into law, you’ll need one of these plans to secure planning permission for your development. No plan, no planning permission

The legislation also stipulates that planning permission should only be granted on the condition that your development enhances the biodiversity of your site – specifically, “in a measurably better state than it was beforehand” (Cornwall Council, 2020). Now, in some ways, this has always been the case, at least for the last few decades. In fact, the present National Planning Policy Framework paved the way for this back in 2012! However, the crucial difference here is the word “measurable”. 

Which, of course, begs the question of how exactly the local planning case officer will make that measurement.

Achieving Net Gain Units using the DEFRA 2.0 Calculation

How will an ecologist assess the biodiversity of my site?

Biodiversity Net Gain
One of our ecologists inspecting a site for biodiversity net gain metrics

You’ll be reassured to learn that the process for measuring biodiversity net gain is not based on an arbitrary decision by your planning Case Officer. There are several ways to measure biodiversity, but the primary tool that you’ll currently encounter is the Defra Biodiversity Metric 2.0 (Natural England, 2019). This tool is specifically designed to measure biodiversity gains and losses that result from the development of land and allows Planning Officers to determine the net gain (or loss) of any proposed development. 

Three broad areas are assessed under the Defra Biodiversity Metric:

  • Distinctiveness- the value of any given habitat the local wildlife
  • Condition- the quality of the habitat
  • Extent- the location and size of the habitat

These assessments draw from a variety of information from relevant habitat surveys undertaken before development. These data are then used to convert the distinctiveness, condition, and extent of habitats into a score, measured in biodiversity units Ministry of Housing, Communities, & Local Government, 2019).

How to measure and calculate biodiversity net gain on and off site

This assessment will have to take place at the point planning permission is applied for, and recent, deliberate harm to the biodiversity of the site will be considered. This means that it won’t be possible to harm local flora and fauna to artificially reduce the biological “value” of land to make the 10% net gain requirement easier to achieve (Easton and Bell, 2020). If this is found to be the case, and the work or other activity on the site took place without permission, the consequences of those works won’t be taken into consideration when assessing the biodiversity of the site. 

At the outset, then, you’ll probably need a preliminary ecological appraisal, which is a baseline walkover ecological assessment and as part of it, habitats will ned to be classified in accordance with the prevailing Biodiversity Metric (currently; 2.0). Once development is complete, further surveys will be performed, and your site will be scored again. A “net gain” in biodiversity will be achieved if the score is higher after development than it was before. Under the Environment Act 2020, your site will have to achieve a post-development score that is 10% higher (Ministry of Housing, Communities, & Local Government, 2019).

Biodiversity Net Gain Plan

To give a dramatically simplified example: if your site had a pre-development score of 100 biodiversity units, to satisfy your obligations under the Environment Act 2020, you would need a post-development score of at least 110.

How to deliver biodiversity net gain of 10% on your site

You have a number of options here. But regardless of which route you choose to take, the habitats you create or enhance as part of biodiversity net gain must be maintained for 30 years. Local authorities will also be permitted to stipulate protection over and above this (Partnership for Biodiversity in Planning, N.D). There’s also the option for a “conservation covenant”. These are new devices that work as an agreement between the developer and a responsible body. These are legally binding agreements that are potentially indefinite; even after the original landowner sells or otherwise parts with the land, the new owner becomes bound by the covenant. In practical terms, this will make further development of the land rather difficult (Thompson, 2020).

There are broadly two value approaches to the assessment, hedgerow and habitat. They aren’t mutually exclusive, so you can expect to have to meet both. Each biodiversity net gain unit is assigned a value, so that there can be a direct comparison between ecologically rich habitat, and habitat of a relatively poor diversity.

What about offsetting and credits for BNG?

Ideally, local authorities will be looking for you to demonstrate biodiversity net gain “on-site” (Leeds City Council, N.D.). The exact mechanisms you’ll be able to use to achieve this will, obviously, depend on all sorts of things, including (but in no way limited to): location, condition, past and present habitats, etc. But, generally speaking, things like planting or seeding with native tree and shrub species, allowing wildflowers to flower, and managing eutrophication (an over-abundance of nutrients – common on many urban sites) will go a long way towards enhancing biodiversity (Forest Research, N.D.).

However, significantly enhancing the biodiversity of a site isn’t always possible. This isn’t necessarily the end of the road for your development as it won’t always result in you planning permission being refused. In these cases, the local authority will take a very close look at your planning application and may still grant planning consent if you’re able to deliver sufficient off-site enhancements. It’s worth noting that the local planning authority does not have to deliver this off-site net gain with their land. As the developer, the net-gain is technically your responsibility, even if it requires the purchase of more land to facilitate your compensatory or mitigating measures (Leeds City Council, N.D). Here’s an example of a net loss biodiversity net gain assessment we did for a client in Essex, that resulted in significant off-site enhancements and compensation.

You may also have the option to “purchase” biodiversity units from DEFRA, by funding schemes that will generate the equivalent number of units elsewhere (British Ecological Society, 2020). However, concerns have been raised here, as there is potential for total habitat destruction where before at least some of it would have to be preserved (Wood, 2020). 

It’s worth considering that by involving an ecological consultant at the design stage, the likelihood of you needing to deliver off-site enhancements will be significantly reduced, if not eliminated entirely.

Do I need to deliver biodiversity net gain?

Unfortunately, there isn’t a simple “yes” or “no” answer to this question. 


And, as with most sweeping changes to a long-established system, there probably won’t be for years to come. 

The Environment Bill is yet to pass into law. As of the 26th January 2021, the Bill is in the Report stage in the House of Commons (UK Parliament, 2021) and a two-year “transition period” is envisaged after the Bill achieves Royal Assent and passes into law (Environment Analyst, 2019) probably in the autumn of 2021. As a result, implementation will likely be patchy (rather like the great crested newt district licencing scheme) with each local authority taking a slightly different approach to this new mandate as they use the transition period to create and refine a system that’s suitable for the region that they cover. 

How much does a Biodiversity Net Gain plan cost?

Our biodiversity net gain assessments cost from £469+VAT, but clearly the cost varies with the scale and intensity of your site and development.

The price for the purchase of units and credits as part of an offsetting scheme will be in part determined at the local level, and as with most things in life, demand and supply will probably need some time to reach an equilibrium where it is cost effective for developers to uptake the various schemes.

Net gain: the key exemptions

Then one has to consider the list of exemptions and features that are not covered by the 10% biodiversity net gain target. The former is, of course, subject to change at any point before the bill passes into law. The latter (ancient woodland, conservation areas, for example) are governed by existing legislation that is, in many ways, more stringent (Dunn, 2019). The Bill also appears to offer significant latitude to local planning authorities when it comes to agreeing what constitutes an acceptable biodiversity gain plan in terms of mitigation and compensation. Because, although the 10% biodiversity net gain requirement is non-negotiable, the degree and form of acceptable off-site biodiversity enhancement appear to be up to the local authorities to determine with the developer on a case-by-case basis. Finally, thinks like site of special scientific interest are also excluded from the assessment system.

What should you do?

Therefore, the best thing to do if you’re unsure of how the biodiversity net gain mandate will affect your planning application is to contact your local authority as soon as possible.

Then, if they indicate that your development will be impacted by the changes, it’s sensible to involve an expert Ecologist, early.  

When to engage an ecologist if you think that off site BNG is going to be necessary:

Even though the Environment Bill is still some months away from passing into law, it’s prudent to start thinking about what it means for your development now. If you’re at RIBA plan of work stage 0-1 now (feasibility), then by the time you get to stage 3-4 (submitting a planning application); we may well be already in the transition period where some councils will undoubtedly look for strict application of the scheme.

In some cases, even now, the local authority will ask you to demonstrate biodiversity net gain anyway. At least four local authorities have already adopted net gain-like policies ahead of the national rollout (Tunbridge Wells Borough Council, Leeds City Council, Vale of White Horse District Council and South Oxfordshire District Council) (British Ecological Society 2020) and it’s likely that more will soon follow suit.

Biodiversity Surveys, Plans and Reports

The involvement of a specialist Ecologist like Arbtech from the design stage will lend credibility to your application, reduce the cost of compensatory measures by maximising the existing biodiversity on your site, and minimise the overall risk of your planning application being refused. We recently worked for the Royal Borough of Windsor and Maidenhead to prepare a biodiversity net gain assessment and plan for their property team.

Our team has been helping people meet their ecological obligations to secure planning consent for 16 years now. It’s all we do, so you can be sure that the ecologist that comes to your site will have the exact same focus that you do – getting your application through planning.  

Every single one of our ecologists is licenced and educated to Bachelors or Masters level. They know their stuff, and our demanding in-house training programs are designed to keep their knowledge up to date. So, when something like the Environment Bill 2020 causes the goalposts to shift, they take it in their stride.

The majority of our 30-strong team are home working. So, no matter where you are in the country, your Arbtech biodiversity net gain specialist will possess that crucial knowledge of the policies and procedures your local planning department uses to make planning decisions. This means you get the advice you need to secure planning permission, first time, fast – or your money back.

Need an assessment to demonstrate biodiversity net gain (or anything else)? Call us today and talk to a professional ecologist.

References (external links)

British Ecological Society. 2020. Mandatory BNG may not deliver the desired outcomes for nature. [Online]. Available from: (Accessed 8th February 2021)

Cornwall Council. 2020. BNG. [Online]. Available from: 8th February 2021) 2021. BNG – Cornwall Council. [online] Available at: 9 February 2021).

Department for Environment, Food & Rural Affairs. 2019. Net gain: Summary of responses and government response. [Online]. Available from: (Accessed 8th February 2021)

Department for Environment, Food & Rural Affairs. 2020. Policy paper 30 January 2020: Environment Bill 2020 policy statement. [Online]. Available from: (Accessed 8th February 2021)

Department for Environment, Food & Rural Affairs, The Rt Hon Theresa Villiers MP. 2019. Government introduces ground-breaking Environment Bill. [Online]. Available from: (Accessed 8th February 2021)

Dunn, L. 2019. BNG – what are the key features? [Online]. Available from: 8th February 2021)

Easton, J., Bell, C. 2020. The no-nonsense guide to BNG. [Online]. Available from: (Accessed 8th February 2021)

Environment Analyst. 2019. Defra firms up Net Gain requirements. [Online]. Available from: (Accessed 8th February 2021)

Forest Research. N.D. Biodiversity. [Online]. Available from: (Accessed 8th February 2021)

Homes England. 2018. Homes England strategic plan 2018 to 2023. [Online]. Available from: (Accessed 8th February 2021)

Lodders. 2020. Biodiversity net gain – FAQs. [Online]. Available from: (Accessed 8th February 2021)

McClean, K. 2020. BNG: Barrier or opportunity?. [Online]. Available from: (Accessed 8th February 2021)

Ministry of Housing, Communities, & Local Government. 2019. National Planning Policy Framework. [Online]. Available from: 8th February 2021)

Ministry of Housing, Communities, & Local Government. 2019. Natural Environment. [Online]. Available from: (Accessed 8th February 2021)

National Biodiversity Network. 2019. State of Nature 2019. [Online]. Available from: (Accessed 8th February 2021)

Natural England. 2019. The Biodiversity Metric 2.0. [Online]. Available from: (Accessed 8th February 2021)

Partnership for Biodiversity in Planning. N.D. Biodiversity Net Gain: What’s it all about?. [Online]. Available from:  (Accessed 8th February 2021)

Thompson, A. 2020. The Environment Bill and BNG. [Online]. Available from: (Accessed 8th February 2021)

UK Parliament. Environment Bill 2019-2021. [Online]. Available from: (Accessed 8th February 2021)

Wood, N. 2020. Explaining net gain. [Online]. Available from: (Accessed 8th February 2021)

Common Questions

The Agriculture Bill has been in part designed to allow farmers to manage areas of their land for environmental improvement, in exchange for payment.This dovetails neatly with the need for developers to be able to pay for off-site habitat creation and restoration as part of achieving habitat units for biodiversity net gain.
Normally, 30 years.
Yes, household planning applications and brownfield sites will be exempt.
Not really, no. You will need to procure a preliminary ecological appraisal in any event, so the incremental work to assess a site using the DEFRA Biodiversity Metric 2.0 is not much in relative terms. A few hundred pounds perhaps, in most cases.

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