Achieving Biodiversity Net Gain with External Storage Buildings

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An approach to development, and land management and acquisition that aims to deliver measurable more for nature and habitats than what is lost through development, Biodiversity Net Gain (BNG) is set to come into effect in January 2024, following a delay from the initial commencement date of November 2023.

There are 6 Statutory Instruments (SIs) designed to help deliver the new BNG framework, applying to most new major developments under the Town and Country Planning Act (TCPA), and to small sites from April 2024.

How Will BNG be Measured?

The metric will measure biodiversity in standardised biodiversity units, assessing the values of an ecosystem by considering the relative features of a specific habitat to calculate its biodiversity value. A habitat will contain a number of biodiversity units, considers aspects such as its:

The biodiversity metric measures how many units a habitat contains before development, and how many units are needed to replace the units of habitat lost and to achieve the minimum requirement of 10% BNG. Assessments for BNG are required in the planning permission stage, and this ensures that any recent or deliberate harm to the biodiversity value of a site can be taken into account. This is preceded by a preliminary ecological appraisal, involving a baseline ecological impact assessment that identifies any priority habitats as part of the metric.

The biodiversity metric tool can be used by – ecologists undertaking a biodiversity assessment; developers who have commissioned an assessment; planning authorities interpreting outputs as part of a planning application, in addition to communities assessing the impacts of a local development. Further surveys are conducted upon completion of the development, with BNG successful if the score is higher in post-development than in the planning phase.

Specifying Green Roofs for External Storage

Green roofs as building features are currently some of the most effective solutions for delivering the highest BNG on developments. External storage units can be designed to accommodate both extensive or intensive green roof types; available as sedum only, or sedum and wildflower.

Extensive green roofs are typically more common, with a sedum-based turf with grass types and sedum plant species. They provide a lightweight and low-maintenance solution, and are chosen mostly largely for their ecological benefits. Intensive green roofs tend to require more intense landscaping and are suited to units placed prominently in the built environment.

Specifying a green roof system can additionally help secure BREEAM accreditation or Sustainable Housing Code value without the complexity of a bespoke build. When considering whether green roof types for securing BNG, the metric utilises a specific criteria that assesses the distinctiveness, strategic significance, and condition of a unit.

What Are the Technical Design Considerations?

For a code of best practice, the GRO Code of Best Practice (2021) helps to guide actions relating to green roof design, specification, installation and maintenance. As per National Building Specification (NBS) eurocodes and British Board of Agrement (BBA) certification process, considering technical design elements helps to meet criteria guidelines:

Sedum based green roof