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What biodiversity net gain will mean for architects

Jenni Reid, Alex Leishman and Tom Butterworth

The biodiversity net gain rules are now in force for major sites, meaning architects need to design with nature, leaving a site’s biodiversity in a better state than at the start of the development. We outline what you need to consider

High value grassland habitat.
High value grassland habitat. Credit: Burton Reid Associates

Biodiversity net gain or BNG is the biggest change to the planning system for nature in a generation. The launch date of mandatory BNG has been confirmed by Defra as today (12 February 2024) for major development and 2 April for small sites. But what does this mean for architects?

BNG is not only going to affect ecologists and planning authorities, it will be a major change for architects, landscape architects, designers and developers. Why? Because it means that we need to design with nature, leaving biodiversity in a better state at the end of the development. Architects will need to work with suitably experienced ecologists to inform the design process from the outset so that we avoid the best places for nature and enhance the poorest. In fact ‘an approach to development that leaves biodiversity in a measurably better state than before development took place” is the definition of BNG.

The measurable improvement – or ‘net gain’ – gets a lot of airtime, and it is the government’s metric that provides clarity over the area and quality of the habitat that we need to create or enhance in order to deliver BNG. The mandatory requirement in England is to achieve a minimum 10 per cent BNG increase, and this applies to nearly all development proposals requiring permission under the Town and Country Planning Act 1990 (with a few exceptions).

Constraints and Opportunities Plan for delivery of BNG (Burton Reid Associates, 2023).
Constraints and Opportunities Plan for delivery of BNG (Burton Reid Associates, 2023).

The best way of achieving 10 per cent BNG is to minimise the loss of higher-value habitats. This can be achieved through careful selection of development sites. It is highly recommended that architects work with suitably experienced ecologists and development partners to guide site selection, ideally with a desk study or an ecological baseline survey (UKHAB) completed at RIBA Plan of Work Stage 0-1 to confirm higher and lower values of sites. This screening exercise can save significant time and money, as developing a site with a high-value biodiversity baseline is likely to require very large offsets. 

At RIBA Plan of Work Stage 2 ,an initial BNG assessment of the masterplan options will be an informative tool for those designing a development. Although outline masterplans do not provide the level of detail required to accurately determine the precise extents of proposed habitats or BNGs, a broadly indicative assessment of the unit gains and losses can be useful in determining preferred options. For instance, if a site is grazed pasture classified as Other Neutral Grassland (a medium distinctiveness habitat) then it can be determined at a very early stage that if the site is too heavily developed, it is not going to achieve 10 per cent BNG within the site boundary.   

A further consideration is that a BNG assessment only considers impacts, mitigation and compensation relating to area and linear habitats. It does not include enhancements such as bat or bird boxes. Area habitats, hedgerows and watercourses are all dealt with separately in the metric tool, so architects should not assume they can compensate for loss of scrub habitat (an area habitat) by, for example, planting hedgerows (a linear habitat). There are also trading rules within the metric which intend to maintain ecosystem function and landscape character by requiring loss of habitats at a site to be offset through the creation or enhancement of the same habitat type or a habitat of higher distinctiveness. This ensures that ecological functionality is maintained.

Stacking (doubling-up) BNGs with other project mitigation such as environmental impact assessment compensation or suitable alternative natural greenspaces (SANGs) also has an impact on the net gain assessment. For example, you can use the habitats created via SANG provision within the calculations but you will only be able to achieve a no net loss (where the post-development score in the metric is equal to the baseline score) within the metric as its primary role is in protecting a special protection area (SPA). This means that you will need to deliver the additional 10 per cent BNG elsewhere – on or off-site.

Delivering 10 per cent BNGs within rural and urban settings can vary greatly. Rural sites often have higher-value baseline habitats but more room to deliver BNGs on or adjacent to the site. Urban sites, on the other hand, may have lower-value baseline habitats but very little room to deliver the required net gains. However, it is not always the case that urban sites have a lower value. There is the chance that there would be areas of open mosaic habitat (high distinctiveness) if it is brownfield, particularly where built infrastructure has been removed and the land has been long abandoned.

Plymouth Argyle Football Club before (2021) and after (2023) delivery of urban BNGs.
Plymouth Argyle Football Club before (2021) and after (2023) delivery of urban BNGs. Credit: Jenni Reid

There can often be a need to increase the greenspace provision to achieve 10 per cent BNG compared with the existing site. It can therefore be more complicated to develop a brownfield site than, for example, an arable field with a low baseline value where there would be much better scope to provide habitats of higher value within proposed open space. This does not alter the fact that if the site is a wildlife-rich grassland field or meadow in a rural location, it will be very challenging to achieve a 10% BNG without finding a suitable offsite location for the creation of a similar habitat.

As can be seen from the above examples, delivery of 10 per cent BNG can be complex. However, a simple set of guidelines for masterplan design could include:

  • Development should seek to retain areas of higher habitat value as loss of these is more difficult to compensate.
  • Retain and enhance habitats to provide higher unit gains compared with habitat loss and recreation.
  • Avoid loss of habitats with long creation times (for instance, woodlands).
  • First avoid impacts on high-quality habitats, then prioritise habitat restoration, creation and enhancement on site.

Finally, the delivery of 10 per cent BNG must also follow the ‘biodiversity gain hierarchy’. For instance, if the project cannot deliver BNG onsite (within the redline boundary) then you will need to enhance or create habitat through a combination of on-site and off-site measures – or entirely off-site – or in some circumstances, through statutory biodiversity credits.

The biodiversity net gain hierarchy is as follows:

  1. Enhance and restore biodiversity on-site (within the red-line boundary of a development site). 
  2. If you can only achieve part of your BNG on-site, you can deliver through a mixture of on-site and off-site. You can either make off-site biodiversity gains on your own land outside the development site or buy off-site biodiversity units on the market.
  3. If you cannot achieve on-site or off-site BNG, you must buy statutory biodiversity credits from the government. This must be a last resort. The government will use the revenue to invest in habitat creation in England.

All three options can be combined, but you must follow the steps in order.

Clearly, the BNG process is not straightforward. Assessments and delivery of 10 per cent BNG is often a complex process. It requires input from specialist ecological consultants with a breadth of experience across a range of sites and habitats. Early engagement is crucial and interdisciplinary collaboration will support the best outcomes for the development and biodiversity. 

Jenni Reid is founder and director of Burton Reid Associates; Alex Leishman is senior ecologist at Burton Reid Associates; and Tom Butterworth is Arup Nature lead and director of the UK Business & Biodiversity Forum. All three are members of the Chartered Institute of Ecology and Environmental Management




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