The Impact of Biodiversity Net Gain on Small Sites
by Joe Bradley
At Norton Mayfield Architects, we have been lucky enough to have dealt with a number of projects requiring biodiversity protection and net gain ahead of the implementation of the new national BNG policy. Below I share the benefit of our experience with a delve into the intricacies of the new rules and how might developers look to navigate them on small sites.
What is Biodiversity Net Gain?
Biodiversity Net Gain (BNG) is a new policy in the NPPF developed to ensure that England’s current biodiversity levels are conserved and enhanced. Decades of relatively ungoverned removal of important habitats for our wildlife to make way for new developments has had a devastating impact on the biodiversity in our country.
Since 2012, DEFRA and Natural England have been developing a Metric to measure a development site’s biodiversity values, before and after intervention.
All new planning applications (with few exceptions, such as householder applications) will be required to demonstrate that a site’s baseline biodiversity value is increased by 10% by the planning proposals. It is expected that the 10% enhancement is to be conditioned in approvals and that the provision should be maintained for a minimum of 30 years. There are questions over how the 30 year continuation of this biodiversity contribution can be enforced, however.
The Policy has been enacted for planning applications submitted for major development sites from 12th February 2024 and will come in for ‘small sites’ from 2nd April 2024. Small sites are considered to be residential developments of fewer than 10 dwellings, or commercial developments of less than 1,000sqm GIA.
How is BNG calculated?
The tool used to calculate the biodiversity value of a site is a fairly complex, free-to-access Excel spreadsheet. A competent person, capable of identifying habitat types and their quality (usually the Project Ecologist), is required to complete the spreadsheet following their site survey. The biodiversity value of a site is broken down into three types of biodiversity unit: area units, hedgerow units and watercourse units. Trading between biodiversity units in order to achieve the 10% required biodiversity enhancement is prohibited - outlined in the Metric’s ‘trading rules’. For example, proposals cannot remove all existing watercourses from a site and replace them with lots of wildflower meadow to achieve a cumulative 10% net gain - effectively there are three, separate 10% net gains that are to be achieved in the Metric.
Within a biodiversity unit are the recognised habitat types, as described in UK Habitat Classification (i.e. within area units are: cropland, marshland, modified grassland). These habitat types are graded in their distinctiveness and carry a score from very high (lowland meadow, scoring 8) to very low (e.g. artificial grass, scoring 0). The habitat type’s distinctiveness, multiplied by its area, condition and strategic significance make its biodiversity value.
The difference between the cumulative biodiversity value in each tranche (area, hedgerow and watercourse) calculated before and after the intervention is the Biodiversity Net Gain (or Loss!).
What if a 10% Net Gain cannot be achieved?
It must be. However, there are mechanisms in place to provide off-site biodiversity value; should all avenues of on-site provision have been exhausted. As a last resort, biodiversity units may be purchased from companies such as Habitat Vault for a one-time fee that will plant/grow/manage the required habitat type on land they own and absorb the liability for upkeep of that biodiversity provision for the required minimum 30 years. Most LPAs are now offering a similar system, including Sheffield City Council who offer developers the chance to purchase off-site units for £25,000 each, at present. Biodiversity land banks are located in remote places of the country and therefore do not contribute to local biodiversity value. As a way of discouraging use of these remote banks, the Metric downgrades the value of remote biodiversity provision by a half (e.g. development in Camden, with biodiversity provision made in a land bank in the Yorkshire Moors), meaning an effective 20% net gain is required to conform to the policy. However, on sites where profit is extremely high, the proportion of monies lost to purchase of expensive off-site units may not be a substantial enough penalty in order to preserve local biodiversity.
Will BNG increase project costs?
Most likely, yes. The trading rules that prohibit exchange of biodiversity value between the different biodiversity units effectively make this policy a ‘conserve and enhance’ policy, rather than just ‘enhance’. Prohibiting trade of biodiversity units places a mini conservation order on every potential development site in England. As built environment professionals, we regularly encounter conservation in relation to a building’s heritage - rarely do most of us encounter significant conservation in relation to the natural environment in urban settings (with some exceptions, such as TPOs). Whilst there is no valid argument against increasing the country’s biodiversity value; the prescriptive nature of the Metric means that there is very little wiggle room for developers and architects to provide innovative solutions to on-site biodiversity enhancement. Without the forward thinking described in the following text, it is my opinion that the Metric will make it very difficult on most semi-urban sites to provide on-site biodiversity enhancement whilst ensuring the development itself remains financially viable.
As such, off-site biodiversity provision will be a necessity on many sites. Purchase of the off-site land units will make the BNG a quasi-‘green tax’ that developers must pay in order to press ahead with development as intended. There are two competing aspects of this that are a cause for concern. The first is the potential to just remove locally valuable biodiversity for cash and the second is the effect on the viability of projects in less affluent areas and the impact on delivering much needed housing.
What should be done differently from now on in response to BNG?
The first recommendation is that the Project Ecologist is brought on board from the outset. The project team must properly understand the baseline biodiversity value of the site before proposals are developed in detail so that abortive work is not incurred.
The architect should have an understanding of the site’s existing biodiversity value and how the Metric will be used by the qualified person prior to the planning application submission - even if this a qualitative understanding of the type of biodiversity units on site, the relative scoring of a habitat’s distinctiveness and the conditions of the BNG trading rules.
Developers should be prepared for an iterative design process in relation to biodiversity. It will be important to test how proposals impact on the end-biodiversity value of a site. If considered properly, BNG will improve the development sites for all parties: developers; local residents and the flora and fauna.
