BNG for Housing and Residential Developments

Biodiversity net gain for housing developments

Housing is the sector most affected by mandatory biodiversity net gain in England. Since 12 February 2024, major residential applications have needed to show at least a 10% biodiversity net gain, and from 2 April 2024 the requirement was extended to small sites. For volume housebuilders, regional developers, land promoters and registered providers, BNG housing development requirements now shape land value, site layouts and programme from the very first appraisal.

This guide focuses on the issues that matter most for residential development biodiversity net gain: the land take trade off, outline planning BNG, phased development BNG and the choice between on-site and off-site delivery.

Start BNG at land appraisal, not at submission

The cost of BNG on a housing site is largely fixed by the baseline. An arable field and a species rich meadow of the same size can produce very different unit requirements. A desk study and early habitat survey before land is bought or optioned lets the developer price BNG into the land deal, rather than discovering a six figure shortfall after the layout is fixed.

Early survey also identifies higher distinctiveness habitats, hedgerows and watercourses that are best retained. Under the mitigation hierarchy, avoiding valuable habitats is both a policy expectation and usually the cheapest route to net gain.

The masterplanning trade off: green space versus plots

Every square metre of on-site habitat is a square metre that is not sold. Housebuilders therefore have to balance three things: the number of saleable plots, the open space and drainage already required by policy, and the habitat needed to meet BNG.

Good practice is to make existing requirements work harder. Public open space, SuDS basins, swales, landscape buffers and street trees are needed anyway. Designing them as genuine habitats, such as species rich grassland in open space, wetland in attenuation basins and native hedgerow along boundaries, can deliver a large share of the gain without losing plots. Gardens also contribute, although they are recorded at a modest value. Areas that will be heavily used for play or formal recreation should be recorded realistically, as their condition will be limited.

Where on-site measures cannot reasonably reach 10% without harming the scheme, the remaining deficit can be met with off-site biodiversity units. Comparing the cost per unit of on-site creation, including 30 years of management, against the price of off-site units is a key part of the viability exercise.

Outline planning BNG

For outline applications, the developer usually does not yet have a detailed layout. The local planning authority still needs confidence that the biodiversity gain objective can be met. An outline BNG submission therefore normally includes a full baseline, a parameter plan or illustrative masterplan, a metric showing a realistic post-development scenario and a strategy for any off-site provision.

The detailed biodiversity gain plan is then approved before development starts. On outline schemes, a good approach is to fix the green infrastructure parameters at outline stage, so that each reserved matters application can be tested against them. This avoids a later reserved matters layout eroding the habitats relied upon in the outline metric.

Phased development BNG

Large housing sites are usually built out over many years, in phases. The regulations allow for phased development, with an overall biodiversity gain plan for the whole permission and phase specific plans submitted before each phase begins. Each phase plan shows how that phase contributes to the overall objective.

Phased development BNG needs careful tracking. Habitats created in early phases can bank gains for later phases, but a developer who front loads housing and leaves the open space until the end can find the final phase carrying an unrealistic burden. A live BNG tracker, updated at each phase, keeps the strategy on course and makes condition discharge straightforward.

Securing on-site and off-site gains

Significant on-site gains must be secured for at least 30 years, usually through a section 106 agreement or conservation covenant, supported by a habitat management and monitoring plan. On housing estates, long term management is often passed to a management company or a public body, so the HMMP must be clear about who is responsible, what management happens each year and when monitoring reports are due.

Off-site units must be allocated from a site on the biodiversity gain sites register before the biodiversity gain plan is approved. Units located in the same local planning authority area or national character area score better under the spatial risk multiplier. Statutory biodiversity credits are a last resort and are priced to discourage their use. Our guide to BNG units covers purchasing in more detail.

Common pitfalls on residential schemes

The problems we see most often include recording site clearance after January 2020 as the baseline, overestimating the condition of public open space, forgetting hedgerow losses for visibility splays and access points, counting the same SuDS area twice, and failing to align the BNG strategy with the landscape and drainage strategies. Each of these can cause delays at validation or at discharge of the biodiversity gain condition.

Reserved matters and design changes

Housing layouts change frequently between outline consent, reserved matters and construction, for example when house types are swapped, roads are realigned or drainage is redesigned. Each change can alter the habitats delivered. Rerunning the metric at reserved matters, and again before the biodiversity gain plan is submitted, confirms the objective is still met. Small changes, such as moving a play area into a wildflower open space or replacing a mown verge with native hedgerow, can recover lost units without affecting plot numbers.

Affordable housing, registered providers and land promoters

Registered providers and land promoters face the same BNG requirements as private housebuilders. Land promoters in particular benefit from preparing a robust BNG strategy before marketing a site, because buyers will price the off-site unit requirement into their bids. A clear baseline, an agreed metric and evidence of available off-site units make a consented site easier to sell. For registered providers, it is worth confirming early who will take on long term habitat management, as this affects service charges and the transfer of open space.

Working with the planning authority

Pre-application advice is a valuable opportunity to agree the baseline date, the treatment of open space and SuDS, and the approach to off-site provision. Some authorities also have local nature recovery strategy priorities that increase the strategic significance score of particular habitats, which can make on-site delivery more efficient.

How Collington Winter helps housebuilders

Collington Winter supports housebuilders, land promoters and planning consultants throughout England, from land appraisal through to discharge of conditions. We provide baseline habitat surveys, statutory metric calculations, design advice to protect plot numbers, outline and phased BNG strategies, habitat management and monitoring plans and sourcing of off-site units. Speak to our BNG consultant team, read our guide to BNG planning or get a quote.