Technical Guide to Biodiversity Net Gain (BNG)

We deliver many CPDs on BNG to architects and developers across the country and have found that many are struggling to make sense of the new Biodiversity Net Gain requirements. To help you understand this new area of regulation, we’ve done the heavy reading to bring you all the information in one place. However, please note that this is just a guide and we strongly recommend you appoint an experienced consultant in relation to any projects.

Biodiversity Net Gain (BNG) is a mandatory approach in England requiring developments to enhance biodiversity, leaving it in a measurably better state than before. As of February 2024, most developments must deliver at least a 10% net gain in biodiversity. This is measured using biodiversity units, which are calculated using the Biodiversity Metric and adjusted for habitat type, condition, location, and time.

This blog addresses the key technical questions surrounding BNG, from calculation methods to compliance and enforcement.

BNG is part of the Environment Act 2021 and applies to most new developments in England. It came into force in February 2024 but many of the finer details have only recently been incorporated.

Key requirements include:

• A 10% biodiversity net gain, measured in biodiversity units adjusted for habitat type, condition, location, and time.

• Biodiversity improvements must be maintained for at least 30 years.

• Developers must submit a biodiversity gain plan detailing how they will achieve net gain.

• Net gain can be achieved onsite, offsite, or through BNG credits (as a last resort).

It is important to note that areas under private demise, such as private gardens, do not count towards BNG, as all net gain areas must be maintained and managed for 30 years.

Also, Local Planning Authorities (LPAs) do not have the discretion to waive or reduce BNG obligations due to development viability issues. The 10% net gain is a statutory requirement, and developers must factor it into their project planning.

BNG applies to most planning applications but has some exemptions.

Certain developments are exempt from the mandatory biodiversity net gain (BNG) requirements. These exemptions include:

Existing Planning Applications: Developments with planning applications submitted before 12 February 2024 are exempt from BNG requirements.

Variations of Planning Permission: Transitional arrangements apply for variations to existing planning permissions. More information is available in the planning practice guidance.

De Minimis Developments (Developments Below the Threshold): Developments that do not impact priority habitats and affect less than:

• 25 square metres (e.g., 5m by 5m) of on-site habitat, or

• 5 metres of on-site linear habitats, such as hedgerows.

A development 'impacts' a habitat if it decreases the biodiversity value.

Householder Applications: Applications made by householders, such as home extensions, conservatories, or loft conversions, are exempt.

Self-Build and Custom Build Applications: To encourage small-scale self-build these are exempt. To qualify for this exemption, the development must:

• Consist of no more than 9 dwellings.

• Be on a site no larger than 0.5 hectares.

• Consist exclusively of dwellings that are self-build or custom housebuilding, as defined in section 1(A1) of the Self-build and Custom Housebuilding Act 2015.

Biodiversity Gain Sites: Developments undertaken mainly for the purpose of fulfilling the BNG planning condition for another development are exempt.

High-Speed Rail Transport Network: Developments forming part of, or ancillary to, the high-speed railway transport network, comprising connections specified in section 1(2) of the High Speed Rail (Preparation) Act 2013, are exempt.This is as BNG is handled via separate bespoke agreements for these nationally significant projects. Some other nationally significant infrastructure projects (NSIPs) have their own timeline (BNG for NSIPs is expected to commence by late 2025)

Urgent Crown Development & Permitted Development Emergency works by the Crown) and any development carried out under General Permitted Development Orders are exempt.It is also worth noting that, technically, works undertaken as permitted development (not requiring a planning application) fall outside the BNG regime entirely, as BNG only attaches to grants of planning permission.Developers should always consult with their Local Planning Authority (LPA) to confirm whether their specific project qualifies for an exemption, as interpretations can vary. Whilst LPAs can have their own BNG requirements, the statutory exemptions still apply and LPAs cannot arbitrarily impose the 10% requirement on genuinely exempt projects.

A common misunderstanding with BNG is the assumption that lost habitats can simply be replaced like-for-like. For instance, removing a mature tree and planting a few younger trees in its place. However, the Biodiversity Metric explicitly addresses this misconception. It measures not just habitat quantity but also quality and ecological value. This includes:

Habitat Condition

Mature habitats, such as established woodlands, mature trees, or rich grasslands, have significantly higher biodiversity value than newly planted equivalents. Simply planting new trees won’t automatically replace the biodiversity lost from felling established specimens.

Habitat Distinctiveness

Each habitat type is assigned a distinctiveness score based on its ecological importance. Rare or ecologically valuable habitats - like species-rich grassland or native woodland - cannot be compensated for with lower-value habitats such as amenity grass or ornamental planting.

Trading Rules:

The Metric includes clear rules that limit which habitats can be replaced by others. Typically, you must replace lost habitats with habitats of equal or greater ecological value - meaning a lower-value habitat cannot substitute a high-value one. If high-value habitats are lost, compensation must be proportionate and ecologically appropriate.

For example, removing a 50-year-old oak tree can’t simply be offset by planting several small saplings. Instead, compensation might involve creating or enhancing a substantial area of high-quality woodland or other similarly valuable habitat elsewhere onsite or offsite. This ensures genuine ecological improvements, not merely numerical compliance.

In short, BNG isn’t about simple replacement; it’s about achieving a genuine and measurable uplift in biodiversity value, taking account of ecological quality, distinctiveness, and long-term habitat establishment.

The red line boundary used for your Biodiversity Metric calculation defines the area over which biodiversity losses and gains are measured. Choosing where to draw that boundary is a critical decision, especially in complex or phased developments.

Key points:

• The larger your baseline area, the more existing biodiversity value you may be starting with - which can make achieving a 10% uplift harder.

• Conversely, excluding areas with high-value or irreplaceable habitats (where appropriate and legitimate) from the BNG boundary can make compliance more achievable, provided those areas are unaffected by development.

• Any habitat within the red line that is impacted, removed, or modified by the development must be assessed - so expanding the boundary can increase your required compensation.

Example:

Imagine a proposed housing scheme includes a large area of rough grassland that will remain untouched and unmanaged. If that grassland is inside the red line, it must be assessed, and its condition maintained or enhanced to help achieve net gain. If the same area is outside the red line and not part of the planning application or its impacts, it may not need to be included in the BNG calculation at all.

Important caveats:

• LPAs will scrutinise boundaries closely. You cannot exclude land just to make the metric easier - any land affected by the development (either directly or indirectly) must be included.

• In phased or outline applications, developers often submit an overall site metric and then define sub-boundaries for each phase, allowing more manageable tracking of BNG across the site.

The Biodiversity Metric 4.0, developed by Natural England, is used to assess a site’s biodiversity value. 

Three types of units are calculated: habitat (area) units, hedgerow (linear) units, and river (watercourse) units 

The process involves:

1. Baseline assessment: measuring existing habitat size, type, and condition.

2. Post-development assessment: estimating the future biodiversity value of the site.

3. BNG calculation: comparing the two values to ensure a 10% uplift.

To assist with these calculations, the UK Government provides a full suite of tools and guides:

• The official Statutory Biodiversity Metric Tools and Guides page includes:

  • A user guide for the statutory metric.

  • An Excel calculator tool.

  • A Small Sites Metric and accompanying user guide for minor developments (9 or fewer dwellings on <0.5ha).

  • Condition assessment criteria to help standardise habitat data.

Small Sites Metric (SSM)

For minor developments, Natural England provides a simplified version of the Biodiversity Metric designed to reduce the burden on smaller projects. The SSM uses default habitat condition scores and reasonable assumptions, meaning a full ecological survey isn't always required. It’s suitable for developments of fewer than 10 dwellings on sites under 1 hectare, or non-residential schemes on sites under 1 hectare. However, if offsite units are needed or the site is more ecologically complex, developers may still need to use the full Biodiversity Metric 4.0.

For example, if you're proposing a development of 8 new homes on a 0.4-hectare grassland site with no priority habitats, you could use the Small Sites Metric to complete your BNG assessment - avoiding the need for complex condition scoring or detailed ecological input. However, if the development required offsite biodiversity units or impacted higher-value habitats, the full Biodiversity Metric 4.0 would still apply.These tools allow for consistent and transparent measurement of biodiversity values before and after development.

BNG can be achieved in three ways:

• Onsite BNG: Enhancing biodiversity within the development boundary.

• Offsite BNG: Purchasing biodiversity units from external sites (approved by LPAs).

• BNG Credits: Government-backed credits used only as a last resort.

Offsite units are sold by:

• Habitat banks (organisations restoring large-scale biodiversity).

• Conservation charities and landowners providing approved biodiversity units.

Natural England’s Biodiversity Gain Site Register is the official source for finding offsite unit options. Private companies (such as habitat banks) are actively offering units both through this scheme and direct to developers.

It’s important to distinguish between biodiversity units and credits - a common source of confusion. Biodiversity units are measurable improvements that can be delivered either onsite or offsite. Credits, by contrast, are a government-operated fallback mechanism used only when biodiversity units cannot be delivered through either of those means. Buying offsite units is not the same as purchasing credits.

Any off-site biodiversity gains must be registered on the Biodiversity Gain Site Register and allocated to the specific development in order to count towards that development’s 10% requirement​. Onsite gains do not need to be registered.

Offsite gains also need a legal agreement (typically a Section 106 agreement or conservation covenant) securing the off-site habitat for 30 years and linking it to the development.

There are often questions around where onsite starts and offsite begins. Onsite is within the development’s red line boundary. But, as mentioned earlier, it cannot be in an area of private demise as this would not fulfil the requirement of 30 year management.

Risk Multipliers

There are also two risk multipliers (adjustment factors) built into the biodiversity metric.

Spatial Risk Multiplier

Applies to: Offsite units only

Does not apply to: Onsite units or statutory biodiversity credits

Purpose: To reflect the ecological value of proximity. Gains delivered far from the development site are considered less beneficial and are penalised with a multiplier (<1.0), while gains in strategic locations may be neutral or even favoured.

Detail: if an off-site unit is delivered within the same local planning authority area (or other agreed local area), it counts fully (no deduction), but if it’s in an adjacent authority it’s typically valued at 0.75 (25% reduction), and if further away (anywhere else in England), only 0.5 (50% of its value) counts​.

Temporal Risk Multiplier

Applies to: Onsite and offsite units

Does not apply to: Statutory biodiversity credits (since these are priced by habitat type and distinctiveness, with delivery risk factored in by Defra)

Purpose: To account for how long it takes newly created or enhanced habitats to reach their target condition. The longer it takes, the greater the penalty.

Advance habitat banking (creating habitat in advance of impact (or having it already established)) is one way of mitigating this.

BNG credits are a government-run system for developers unable to meet BNG onsite or offsite.

Key points to note:

• Credits are expensive - designed to discourage reliance on them.

• Approval is needed before purchasing credits.

• Developers can buy credits via Natural England's credit scheme.

• The money from biodiversity credits goes towards habitat creation and restoration projects managed by Natural England.

Statutory credits are already pre-priced to reflect delivery risk, including spatial and temporal factors, so these multipliers are baked into the cost, effectively doubling the number of credits needed to equal one biodiversity unit​. One biodiversity unit shortfall will require two credits, making credits a costly fallback.

Prices vary significantly depending on the habitat type. For example:

Low distinctiveness habitats (e.g. urban, cropland, individual trees): £42,000 per credit.

Medium distinctiveness woodland or intertidal sediment: £48,000 per credit.

High distinctiveness woodland types (e.g. upland birchwoods): £66,000 per credit.

Specialist high distinctiveness habitats (e.g. floodplain wetland mosaic or priority ponds): £125,000 per credit.

High alkalinity or low alkalinity lakes: £650,000 per credit.

Note that these prices exclude VAT.

We have an actual pricing framework (published by Defra in 2023). It’s also worth mentioning that credits are intended to be rarely used. As of early 2025, very few have been purchased. The first annual report on statutory credits is expected to show minimal uptake, since most developers will find cheaper local unit options.

For the full breakdown, see the Statutory Biodiversity Credit Prices on GOV.UK

BNG is enforced primarily by Local Planning Authorities (LPAs). Their responsibilities include:

• Assessing biodiversity gain plans during planning applications.

• Ensuring compliance through monitoring and enforcement.

• Issuing penalties for failure to meet BNG commitments.

In some cases, Natural England and Defra provide additional oversight.

While the Biodiversity Gain Plan (BGP) is formally a pre-commencement condition, in practice, most Local Planning Authorities will expect a clear biodiversity net gain strategy, including a metric calculation and indicative BGP, at the planning application stage.

What's required

Submitting a full or draft BGP with your application will help avoid delays and increase the likelihood of permission being granted. LPAs use a template BGP form provided by Defra to assess net gain proposals.

The typical BNG documentation required at application stage includes:

• Biodiversity Metric calculations

• BNG Statement or Strategy

• Draft or full Biodiversity Gain Plan

• Ecological assessments and condition surveys

Timescales

The statutory target for BGP approval is 8 weeks. Developers should budget for this time and factor it into project timelines.

However, some LPAs in early 2024 were still building capacity, so delays or differing interpretations may occur. It is recommended to involve a qualified ecologist early and, where possible, consult the LPA ecologist.

Developers should anticipate a 1–2 month process to get their BGP signed off post-planning consent.

Local variations

Always check local policy. Some LPAs have policies exceeding the 10% requirement or require local off-site offsets.

Registration

Off-site “statutory” units must be formally allocated via the register before the LPA can approve the Biodiversity Gain Plan​, meaning a developer can’t just claim future off-site plans – they must have secured and registered them.

Phased and Outline Developments

For phased developments, an Overall BGP sets the framework to ensure net gain across the entire site.

Individual Phase-specific BGPs must then be submitted and approved alongside reserved matters applications.

Some habitats are considered irreplaceable due to their age, uniqueness, species diversity or rarity. These habitats are excluded from the 10% biodiversity net gain (BNG) metric requirement, as it would be inappropriate and often impossible to replace them through traditional offsetting or biodiversity metric credits.

Instead, developments that impact irreplaceable habitats must undergo bespoke compensation arrangements, agreed with the Local Planning Authority (LPA), and are subject to strict planning controls. Planning permission for loss of irreplaceable habitats is only granted in exceptional circumstances.

Irreplaceable Habitats include:

• Ancient woodland

• Ancient and veteran trees

• Blanket bog

• Limestone pavements

• Coastal sand dunes

• Spartina saltmarsh swards

• Mediterranean saltmarsh scrub

• Lowland fens

Key rules for developers:

• Impacts on irreplaceable habitat cannot be measured using the biodiversity metric and are automatically removed from the baseline calculation.

• Enhancement of irreplaceable habitat can be recorded in the metric but cannot be used to offset losses.

• A compensation strategy must be agreed with the LPA on a case-by-case basis.

• The 10% BNG requirement must still be achieved on the non-irreplaceable habitat within the site.

For example, if a site contains an area of ancient woodland that will be harmed, the biodiversity metric will ignore that woodland in the net gain tally, but the developer will have to propose a bespoke plan (which could be extensive) to compensate for that harm, and still deliver 10% net gain for all other habitats affected on site.

Developers must:

• Record all irreplaceable habitat within the BNG calculation tool (even if not affected).

• Provide detailed information about the type, extent, and condition of irreplaceable habitat in the Biodiversity Gain Plan (BGP).

• Show how adverse impacts will be avoided or minimised, and set out a plan for any required compensation.

For full guidance, visit the official Irreplaceable Habitat Guidance on GOV.UK

BNG and UGF are both tools to encourage greener developments, but they serve different purposes and are measured in very different ways.

Biodiversity Net Gain (BNG) is a legal requirement under the Environment Act 2021. It focuses on ecological value, using Biodiversity Metric 4.0 to measure changes in habitat type, condition, distinctiveness, and location. The goal is to leave biodiversity in a measurably better state than before development - with at least a 10% uplift, maintained for 30 years.

Urban Greening Factor (UGF), by contrast, is a planning policy tool. Most notably required in London under the London Plan (Policy G5), it measures how much green infrastructure is included on a site - things like green roofs, rain gardens, trees, and permeable paving. Each surface type is assigned a score, and the aim is to reach a minimum overall target (typically 0.4 for residential and 0.3 for commercial developments).

The key difference is that UGF is about how green a site looks and functions in urban terms, while BNG is about how biodiverse it actually is. For example, a sedum roof may contribute to UGF but offer little value under BNG. Conversely, a native hedgerow or restored grassland could score well for BNG without significantly impacting the UGF.

In many London boroughs, both BNG and UGF apply, and they need to be considered side by side. One doesn't replace the other - and a high score in one doesn't guarantee success in the other. Some other areas, such as Southampton and Guildford are also now trialling the Urban Greening Factor.

BNG is a complex but essential requirement for sustainable development. Developers must carefully plan their approach, whether achieving BNG onsite, purchasing offsite biodiversity units, or, as a last resort, buying government-backed credits.

If you need assistance navigating BNG requirements or finding offsite biodiversity solutions, get in touch with our team for expert guidance.