The last blog in our Natural Capital series summarised the biodiversity net gain (BNG) framework in the Environment Bill (the Bill)– particularly, the ways to deliver BNG. That blog also noted that the requirement for a 10% BNG would take the form of a condition restricting commencement until a biodiversity gain plan (BGP) is approved. How do these plans demonstrate that a development achieves a net gain?

All about the Metric

An effective BNG requirement depends on a robust, standard approach to valuing biodiversity. The Bill requires the Secretary of State to produce a biodiversity metric for this purpose[1]. Natural England has been developing a biodiversity metric. NE published a “test” Biodiversity Metric 2.0 in 2019 and concluded a consultation on it in February this year.

The Metric calculates biodiversity values by using defined habitat types as a proxy for biodiversity. Biodiversity value is calculated as “biodiversity units” for each existing or proposed habitat type by:

  1. measuring the size (area) of the habitat type;
  2. multiplying that by “quality scores” for distinctiveness, condition, strategic significance and connectivity.

Biodiversity values are calculated for pre- and post-development habitats to determine gains/losses. The value of newly created habitat is further adjusted by factors representing the difficulty in restoring/creating the relevant habitat, the time it takes to establish new habitat (“time to target”), and the distance of compensatory habitat from the relevant losses.

Consultees raised concern that:

  • the metric generates unduly low scores for some habitat types (i.e. woodland habitats);
  • the “time to target” factor
    • for some habitats, reflects periods that are longer than it actually takes to establish those habitats;
    • fails to reflect that compensatory habitat may, at the time of calculation, already have been created/established (i.e. where biodiversity value in a habitat bank is allocated to a development);
    • fails to reflect phased development, where commencement of parts of development are delayed.

NE is seeking to address these concerns and intends to publish an updated metric in December.

Biodiversity Gain Plans

A BGP must identify following biodiversity values (paragraph 14[2]), to be measured using the Defra metric (paragraphs 3 and 4):

  1. Base: the pre-development biodiversity value of on-site habitat on the date of the planning application or such earlier date as may be agreed with the planning authority (paragraph 5); (where unconsented activities diminish the pre-development biodiversity value of the site, the pre-development value of the site is its biodiversity value immediately before those activities were undertaken (paragraph 6));
  2. Development: the biodiversity value of the proposed development (paragraph 2(2)). This is the total of:
    • the post-development biodiversity value – the projected biodiversity of on site habitat as at the date of the completion of the development as compared with A above (para. 8); (proposed on site habitat enhancements may only be taken into account if they are maintained for at least 30 years pursuant to a condition, planning obligation or conservation covenant (paragraph 9));
    • the biodiversity value of any registered off site biodiversity gain allocated to the development (paragraph 10); and
    • the biodiversity value of any biodiversity credits purchased for the development (paragraph 11).

The planning authority must approve the plan if (and only if) it is satisfied that the matters stated in it are correct and that the BGO is met – i.e. that B exceeds A by at least 10% (paragraph 15).


The default position under the Bill is that the biodiversity condition has wide application: only permissions granted by development order (e.g. the GPDO) are excluded[3]. The application of the condition could ultimately be more limited, though, since the Bill would give the Secretary of State powers to:

  • exclude other types of development (paragraph 17);
  • modify the application of the condition in relation to outline permissions/phased development (Paragraph 19).


The use of a planning condition makes the BNG requirement easy for planning authorities to enforce. Non-compliance with enforcement action can amount to an offence.

This condition is unlikely to be enough by itself, however. The post-development biodiversity value specified in a BGP is just a prediction. Planning authorities will need to ensure that the plan is implemented and that a 10% gain is actually achieved. Beyond that, implementation and maintenance (i.e. for 30 years) of habitat measures need to be monitored. If the planning authority lacks capacity, how else would these requirements be secured and how should failures be remedied?

These are not just concerns for planning authorities: biodiversity losses in large/phased developments may be changeable, or uncertain at the point of first commencement. Developers may want a mechanism for adjusting the level and type of biodiversity gain delivered as the project specification evolves.

Some of the issues above may be resolved by regulations, but developers and planning authorities are likely to need to work together in finding solutions to make the BNG regime work well.