With biodiversity net gain (BNG) applying to a large number of different developments all over England, our helpful page explains how the mandate affects change of use planning projects.
Last updated: August 2026
Editor at Arbtech, Rob is a content specialist who manages our ecology and arboriculture services copy to ensure it is accurate, up to date, and insightful for current and future clients.
Under the Town and Country Planning Act 1990 (TCPA), mandatory biodiversity net gain (BNG) demands that most developments leave the natural environment in a better state than before.
It was initially created to advance long-term environmental sustainability alongside national economic growth. Working the BNG regulations into a development site, however, involves compliance with specific guidelines.
A material change of use doesn’t automatically trigger or bypass ecological mandates. In planning terms, whether you need planning permission and how the statutory framework categorises your proposed development will determine the outcome.
If the scheme involves physical works or building operations that disturb existing habitats, BNG applies. Under official planning practice guidance, every use application is evaluated during the planning process to work out whether the works create a mandatory requirement to deliver gains for new developments.
For applicable schemes in England, BNG implementation will only be possible via the right strategic approach. Regardless of the types of development involved, the development must follow certain steps to correctly meet the mandate.
The statutory definition of net gain focuses on the mathematical difference between baseline pre-development value and the post-development value of the application site.
When it comes to assessing site biodiversity, the metric doesn’t judge the primary use or intended use of an asset. Instead, it calculates the baseline value of the site based purely on present land and buildings.
As different uses generate variable development impacts, the ecologist measures baseline biodiversity units as the primary input for all calculations.
Depending on the individual circumstances of the land, a proposed use will typically interact with the biodiversity metric in one of three ways:
If a scheme involves changing the use of a building indoors, the baseline onsite habitat will go unaffected, such as with knocking down internal non-load-bearing walls to convert an office into flats.
As sealed infrastructure or an existing car park would score zero, it means there’s no biodiversity resulting from natural ground to lose.
Whenever a planning project consists exclusively of interior alterations on an applicant’s own property, the scheme remains in its original state ecologically, so the statutory target condition does not force you to deliver BNG.
Wherever outdoor land is negatively impacted, BNG becomes a major factor.
A classic example is converting an agricultural unit into a domestic garden for residential use. Natural grassland has a high baseline score, whereas ornamental turf and patios score significantly lower.
To satisfy the statutory biodiversity gain objective of achieving at least 10% of a net gain, any habitat destruction needs to be adequately mitigated. The applicant will need to introduce targeted site enhancements or dedicated habitat creation to prevent significant harm and secure less harmful impacts.
Bringing a neglected brownfield plot into a new use can create opportunities for enhancing biodiversity.
If an old concrete haulage yard is redeveloped with soft landscaping, rain gardens and tree lines, the planning project will achieve significant habitat enhancement.
In such illustrative scenarios, improving biodiversity through on-site habitat creation easily delivers measurable biodiversity enhancements because the starting baseline was zero.
For planning applications made across England, local planning authorities enforce the biodiversity gain condition as a type of mandatory pre-commencement condition.
As soon as full planning permission is sought, developers need to submit a formal location plan and a detailed biodiversity gain plan to show how the scheme meets the statutory mitigation hierarchy and integrates with local nature recovery strategies (LNRSs).
Under the statutory biodiversity gain hierarchy, developers need to prioritise onsite biodiversity gain before looking elsewhere. If space constraints make on-site delivery impossible, the applicant can secure registered offsite biodiversity gains via established habitat banks as part of their off-site BNG strategy.
Buying statutory biodiversity credits directly from Natural England serves as a strict last resort. Any off-site land or complex on-site habitat must be legally secured for at least 30 years using a section 106 agreement or conservation covenants with a recognised responsible body.
The binding planning obligations form a legal agreement to maintain long-term BNG compliance and satisfy every statutory consultee, as well as simplifying the decision-making process for the local authorities.
Under the Use Classes Order 1987 and the updated country planning use classes, shifting from one use class to another or moving to a sui generis classification often means that you must obtain planning permission.
Whenever physical building work alters the external appearance of a property or breaks ground across open areas, you will require planning permission and need to satisfy all BNG requirements.
From smaller to larger developments subject to the rules, moving one class to another across current use classes involves different use classes and categories. The statutory use process dictates that certain changes across different types will require permission.
Before the point of breaking ground, applicants usually need to apply to their local council rather than assuming that they have an automatic right to convert.
Continuing without obtaining all of the necessary permissions leaves a scheme open to enforcement and penalties. Securing the necessary planning permission at the outset allows your ecologist to establish accurate baseline data before habitats are disturbed.
For commercial and industrial sites, introducing an intensive commercial or industrial process or launching an operational process other than one originally authorised often triggers a material change of use.
Minor modifications may still fall within existing consents, but external plant installations or significant layout shifts mean that planning permission is required.
If your proposals step outside of permitted rights, identifying the exact scope of planning permission required is the first step before calculating your baseline units.
The following structured matrix outlines how changing between classes affects planning and habitat calculations:
| Use Class Transition | Qualifying Developments and Property Types | Specific Activity and Land Constraints | Impact and Metric Triggers |
|---|---|---|---|
| Commercial, Business & Service (Class E to other classes) | Converting a commercial property or commercial business and service unit, including professional services, medical services, research and development, retail sale, a local post office, or local shops. | Sites formerly used for selling essential goods, a day nursery, a community day centre, or units operating under flexible commercial use or expanded food provision. | Triggers BNG if the site area or land required involves breaking ground for car parks or drainage that impacts on-site linear habitats. |
| Industrial & Storage (Class B2 / B8 / Sui Generis) | Alterations to a B2 general industrial plant, B8 storage depot, storage or distribution centre, or sites used for open air storage. | Facilities handling scrap yards, hazardous waste, or chemical treatment (excluding incineration purposes) or schemes with new visibility splays for motor vehicles. | Ground clearance involving motorised vehicles, heavy haulage, or taxi businesses connecting to the public highway triggers BNG if green verges are lost. |
| Residential & HMOs (Class C3 / Class C4 / Sui Generis) | Establishing C3 dwellinghouses (such as a single dwelling house, single-family houses, separate dwelling houses, or a standard single household in a residential area) to serve as an only or main residence. | Creating class C4 houses in multiple occupation or small shared houses (accommodating up to six people where six unrelated individuals share basic amenities), or large HMOs for more than six residents. | BNG is triggered if gardens or soil are replaced by extensions or hardstanding for shared houses occupied by multiple tenants living together. |
| Specialist & Supported Living (Class C2 / Class C3 / Class C4) | Converting property for people living as a single person or household where a foster parent, foster child, person related, or certain domestic employees (such as an au pair or personal assistant) reside. | Providing specialised housing where care is provided, delivering a significant element of care for a person receiving ongoing support, or hosting a small religious community. | BNG applies if external landscaping is converted into hard vehicle access or paved amenity space during the conversion. |
| Institutions & Residential Care (Class C1 / Class C2) | Redeveloping C1 hotels, hotels boarding, boarding and guest houses, boarding houses, C2 residential institutions, residential care homes, nursing homes, residential colleges or boarding schools. | Facilities providing accommodation for individuals receiving care, managing learning disabilities, coping with mental health problems or residing in supported housing schemes (excludes hostels and health centres). | BNG applies if new extensions, footpaths, or utility trenches disturb surrounding turf, trees or surface soil. |
| Secure Accommodation (Class C2A) | Constructing or redeveloping C2A secure residential premises under the provision of secure residential facilities or secure residential accommodation. | Projects involving a secure hospital, military barracks, custody centre, detention centre, short-term holding centre, secure training centre, young offenders institution or secure local authority accommodation. | Mandatory BNG is triggered whenever boundary security works, perimeter fences or access roads clear natural vegetation. |
| Non-Residential & Community (Class F) | Alterations to class F premises for learning and non-residential use or general non-residential institutions. | Works at a state-funded school, public library, art galleries, exhibition hall, public halls, community halls, a local law court or places of public worship and religious instruction. | External expansions that impact landscaped grounds regularly used by visiting members of the local community trigger metric calculations. |
| Assembly, Leisure & Food (Sui Generis / Commercial) | Redeveloping drinking establishments, a public house, a hot food takeaway, bingo halls, dance halls, concert halls or skating rinks. | Developing indoor or outdoor facilities, outdoor sport grounds, outdoor swimming pools, a pay-day loan shop, betting offices or a complex mixed-use scheme often converting rural structures of historic interest. | BNG applies if converting agricultural buildings, other land or unsealed use of land involves other operations that degrade onsite priority habitat or watercourse units. |
Whatever the circumstances, when an applicant undertakes work registerable under planning law, such changes may require a full assessment if the use permitted or use within a parcel ceases to one falling within class standards and creates external impacts across such uses or other use classes.
The following list outlines the statutory grounds where developments are exempt from the national BNG planning condition under the current development legislation:
In terms of assessing if a development falls under one or more than one exemption, applicants need to review statutory BNG exemptions to confirm whether an exemption applies, making the planning project legally exempt from BNG.
Under certain conditions and specific cases, the rules will vary depending on site history. While most cases and conditions maintain the same level of oversight across individual applications, developers need to provide evidence within the formal planning application to be submitted to avoid validation delays.
For further information and help with whether or not certain types of development are exempt, applicants should consult official guidance, review related content from the planning portal, or speak to our team.
Conversions carried out under general permitted development orders do not need full planning consent and are exempt from mandatory BNG.
Even where prior approval is required, statutory 10% net gain doesn’t apply, such as in formal prior approval applications for Class MA or Class Q conversions.
Local councils, however, can issue Article 4 directions to remove permitted development rights within sensitive areas like a national park, forcing the submission of a new application where standard BNG rules apply.
Securing a lawful development certificate or prior approval does not override mandatory building regulations.
If an approved prior approval lapses, or if an original permission is modified via a new permission instead of relying on the underlying use planning permission, the consent may not remain valid without addressing statutory conditions attached, relevant planning conditions or a specific condition requiring ecological review.
Under the de minimis threshold, small developments needing full planning consent are assessed using either the statutory biodiversity metric or the small sites metric.
A planning project is exempt from calculating baseline pre-development biodiversity value if it satisfies all of the following criteria:
When an applicant inadvertently draws the planning boundary within the red line around an entire landholding rather than the immediate development curtilage, it can be known as the red line boundary trap.
If you’re converting a small agricultural outbuilding located in the corner of a 10-acre meadow, drawing the planning boundary around the whole 10 acres forces the metric to calculate baseline units for all 10 acres.
You would then be legally liable to deliver a 10% net gain across the entire meadow, costing tens of thousands of pounds in unnecessary habitat creation or off-site unit purchases.
If the red line is kept tightly to the building footprint, parking bays and direct access tracks, the baseline costs will be near zero.
The distinction is equally vital when:
Complex legislation around change of use needs a level of practical support backed by a proven track record.
While national legislation sets the statutory baseline, many local planning authorities introduce other policies alongside mandatory net gain rules.
The local requirements usually scrutinise landscaping, site boundaries and other things that could influence whether minor works officially constitute development.
If your planning proposal qualifies as a major development or falls into one of the various categories of smaller conversions, you will need to understand more detail behind the baseline calculations.
The expectations are rooted in the national planning policy framework (NPPF), statutory BNG guidance published by the Department for Environment, Food and Rural Affairs (DEFRA), the wider infrastructure bill, and the ecological benchmarks set by the British Standards Institute (BSI).
Whenever you apply for new planning permission, our team works with you to accurately calculate your pre-development biodiversity value and post-development biodiversity value and guide you in meeting your biodiversity net gain goals.
We ensure that the formal BNG plan to be submitted to your local authority contains verified metric figures, helping to ensure compliance right from validation.
All of the technical support we provide is vital, and it could be the difference if you’re dealing with a listed building or a historic site of special architectural significance.
Ecological oversights remain one of the most common causes of delays in local planning decisions.
If an application is contested, referred to an independent body like the planning inspectorate or subject to a judicial review, having clear survey data and metric calculations will provide protection against potentially costly refusals.
In situations where you’re weighing up independent legal advice on permitted rights or curtilage boundaries, having an airtight ecological report keeps your planning route secure.
For clear and dependable planning advice on your development project, reach out to Arbtech by calling, emailing or completing a quote form. As soon as you seek advice from our team, we can work out what you need and book in the necessary service.
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