Biodiversity Net Gain: reality bites reproduced from Save Lea Marshes


Biodiversity Net Gain at the Ice Centre?

Last autumn the Lee Valley Regional Park Authority submitted a planning application to the London Borough of Waltham Forest to double the size of the Ice Centre on the Lea Bridge Road.  This site is Metropolitan Open Land (MOL), so the Authority needed to have very compelling arguments amounting to ‘Very Special Circumstances’ to support the application.  Foremost among these arguments was the claim that the development would produce a large “biodiversity net gain” (BNG).

Paragraph 170 of the National Planning Policy Framework (NPPF, 2019) states that

Planning policies and decisions should contribute to and enhance the natural and local environment by … providing net gains for biodiversity.

This is interpreted to mean that any development should aim to produce a Biodiversity Net Gain of at least 10%.  To enable developers to estimate net gain, the Department of Agriculture Food and Rural Affairs (DEFRA) has provided a calculation tool.  It is actually an Excel workbook, and may be downloaded from Natural England’s website at

The Lee Valley Regional Park Authority used the calculation tool to demonstrate a Biodiversity Net Gain of 30%.  This is 3 times more than the minimum expected.  Such an impressive result must have had an important influence on the Planning Committee’s decision to grant permission for the development. Were they misled by the data?

In order to assess this let’s look at how the calculation tool works. In general terms, it is very straightforward: 

  1. Calculate the “baseline” biodiversity score – that is the site’s current biodiversity value, before the development has taken place.
    1. Divide up the site into parcels of different habitat types (e.g. woodland, tarmac, buildings, short grass, rough grass, open water, etc.), and input their characteristics into the tool.  The tool assigns a biodiversity “rating” to each type, in biodiversity units per hectare.  (How it does that is quite technical, and we needn’t go into it here.)
    2. Measure the area of each parcel, and input the areas into the tool.  The tool multiplies the area and the biodiversity rating of each parcel to get its biodiversity score.
    3. The baseline biodiversity score is the sum of the scores of all of the parcels.
  2. Do exactly the same thing for the “post-development” biodiversity score.  In other words, divide up the site into parcels according to the habitat types that the site is expected to consist of after the development has taken place, and input their characteristics and areas into the tool.  The tool calculates the score of each parcel, and the sum of these scores is the post-development score.
  3. The Biodiversity Net Gain (BNG) is simply the difference between the baseline (pre-development) and post-development scores.

The results that the LVRPA obtained for the ice centre development are:

baseline score = 15.58;
post-development score = 20.39;
BNG = 4.81 (= 20.39 – 15.58) or 30.92% (= 4.81 ÷ 15.58).

An important feature of the development is that the built-up area will increase from 0.39 to 0.7029 ha.  Buildings have a biodiversity rating of zero.  So how is it possible to achieve such a large BNG, when so much habitat is being lost?  The answer lies in the areas coloured yellow in this map.

The habitat type of the yellow areas is “Urban – amenity grassland”.  It has a very low biodiversity rating, just 2 habitat units per hectare.  The yellow area at the north-east end of the building will disappear under the new building, but the much larger area at the front of the site (0.769 ha) will be enhanced to become “Grassland – other neutral grassland” and “Heathland and shrub – mixed scrub”.  These have much higher biodiversity ratings, of 7.86 and 9.79 habitat units per hectare respectively.

So in effect the LVRPA is proposing to achieve a gain in biodiversity by making changes to parts of the site that are actually irrelevant to the development.  The urban amenity grassland has such a low biodiversity value because it has been kept short by regular mowing.  Why is this happening?  We have repeatedly requested that the mowing regime should be relaxed to benefit biodiversity.  Surely the LVRPA should be in the business of promoting biodiversity, not impeding it.  Previously it has claimed that it was essential that the area be managed as closely mown ‘amenity grassland’, but without explaining why.

In front of the Lee Valley Ice Centre, mown last week of May 2021

If the LVRPA were genuinely interested in promoting biodiversity, it would have enhanced the urban amenity grassland many years ago.  And it is possible to use the calculation tool to work out what the consequence of doing so would be.  If all of the “Urban amenity grassland” were enhanced to the same combination of “Grassland – other neutral grassland” and “Heathland and shrub – mixed scrub”, but without any of the rest of the development, this would result in a score of 23.22, and hence a BNG of 7.64 (= 23.22 – 15.58) or 49.07% (= 7.64 ÷ 15.58).  This is much better than the 30.92% that will be achieved by the development.  Indeed the consequence of imposing the development on the site after this enhancement to the grassland would be a reduction in the biodiversity score to 20.39 – in other words a biodiversity loss of 2.83 (= 23.22 – 20.39) or 12.17% (= 2.83 ÷ 23.22).

Here is a view of the same area, unmown.

So there is a simple lesson to be drawn from this exercise.  If you want to get planning permission for a development that will cause a loss of biodiversity:

  • degrade the surroundings of the site as much as possible beforehand, so as to minimize its biodiversity;
  • include in your plans for the development details of how you will restore the biodiversity of the surroundings once planning permission has been granted.

As it happens, this is something that Defra has anticipated, as shown in its consultation on Biodiversity Net Gain (

If net gain were made mandatory, there could be a stronger incentive for some developers and landowners to degrade their land in advance of seeking permission to develop it. There are reported cases of suspected pre-consent habitat degradation under the current planning system, although it is not known whether this is a regular occurrence. These include cases of vegetation clearance and the disturbance of protected species. Landowners may be incentivised to degrade their land to reduce environmental obligations long in advance of its sale for development. In a mandatory net gain policy this risk could be mitigated by […] clear guidance for developers and planning authorities on the relevant assessment baseline including how to take account of recent or even historic habitat states where there is evidence of deliberate habitat degradation.

If grassland is frequently mown this is surely a clear example of “vegetation clearance” leading to “deliberate habitat degradation”.

It is the Local Authority, Waltham Forest Council, who made the decision to grant permission for the new ice centre. The poor state of the development site was referred to by the Chair of the Planning Committee, J. Grey, who stated: “It’s a pretty scrubby, desperate bit of Metropolitan Open Land, it’s not like it’s a beautiful green meadow.” 

 More and more empiracle  research  supporting the lack of evidence that Net Gain is working and there is a the  for enforcement on development sites:

"gains fall within a governance gap whereby they risk being unenforceable; a challenge which is shared with other net outcome-type policies implemented internationally." see authors below*


Net outcome-type biodiversity policies are proliferating globally as perceived mechanisms to reconcile economic development and conservation objectives. The UK government’s Environment Bill will mandate that most new developments in England demonstrate they deliver a biodiversity net gain (BNG) to receive planning permission, representing the most wide-ranging net outcome-type policy globally. However, as with many nascent net-outcome policies, the likely outcomes of mandatory BNG have not been explored empirically. We assemble all BNG assessments (accounting for ~1% of England’s annual housebuilding and other infrastructure) submitted from January-November 2020 in four early-adopter councils who are implementing mandatory No Net Loss or BNG requirements in advance of the national adoption of mandatory BNG, and analyse the aggregate habitat changes proposed. Our sample is associated with a 21% reduction in the area of non-urban habitats, compensated by commitments to deliver smaller areas of higher-quality habitats years later in the development project cycle. Eighty-seven percent of biodiversity units delivered in our sample come from habitats within or adjacent to the development footprint managed by the developers. However, we find that these gains fall within a governance gap whereby they risk being unenforceable; a challenge which is shared with other net outcome-type policies implemented internationally


Exploring the ecological outcomes of mandatory Biodiversity Net Gain using evidence from early-adopter jurisdictions in England
Sophus O.S.E. zu Ermgassen Sally Marsh, Kate Ryland, Edward Church, Richard Marsh Joseph Bull Institute of Conservation and Ecology, School of Anthropology and Conservation, University of Kent, Canterbury.High Weald AONB Partnership, Flimwell, East Sussex.Dolphin Ecological Surveys, Edgedown, Kammond Avenue, Seaford, East Sussex BN25 3JL. Deparment of Planning, South Oxfordshire and Vale of White Horse District Councils.
Leeds City Council, Strategic Planning, Environment & Design Group, Merrion House, 110 Merrion
Centre, Leeds, West Yorkshire LS2 8BB.This work has been submitted for publication but has not yet been peer-reviewed.



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