Walkthrough: replacement dwelling in the Green Belt
An illustrative walkthrough of a Green Belt replacement dwelling, from establishing the lawful baseline and the volume comparison to openness, bats and the conditions to expect.
A Green Belt replacement dwelling application turns on one sentence of national policy and one measurement. The National Planning Policy Framework, in the edition published on 17 August 2026, treats the construction of new buildings in the Green Belt as inappropriate development, with a list of exceptions that includes the replacement of a building in the same use, provided the new building is not materially larger than the one it replaces. If you are inside that exception, the scheme is not inappropriate development and you do not need very special circumstances. If you are outside it, the application will very probably fail.
What follows is an illustrative walkthrough, based on a composite of the way these applications are handled rather than on any real site or decision. The figures are illustrative.
The existing dwelling and its planning history
Take a 1950s brick and tile bungalow on a quarter-hectare plot in the Green Belt, reached by a private drive, with a detached single garage and a timber shed. The owner wants a two storey replacement.
The first job is not design. It is establishing exactly what is lawfully there, because that is the baseline the comparison runs against.
Order the planning history from the authority's online register and, where it does not go back far enough, request a search. Identify the original building and every subsequent extension, and confirm each one was either permitted or permitted development. An unlawful extension that is now immune from enforcement is a more complicated question, and it needs advice rather than an assumption. Then commission a measured survey: floor plans, elevations and sections, with wall thicknesses, eaves and ridge heights and ground levels.
Outbuildings are a separate issue. A detached garage is not part of the dwelling, and most authorities will not let you add its volume to the baseline. Some allow it where the outbuilding is to be demolished and its volume is absorbed into the replacement. That is a local policy question and it is worth asking at pre-application stage, because it can be the difference between a workable scheme and an unworkable one.
The not materially larger test
There is no national definition of materially larger, which is why this is the part of the application that gets argued. Many adopted local plans set a figure, commonly expressed as a percentage increase in volume or in floorspace over the original or existing dwelling, and the figure differs between authorities. Some plans use footprint instead. Some use no figure at all and assess each case on its planning merits.
So read the development plan policy word for word. Three questions decide the arithmetic:
- Is the baseline the original dwelling as first built, or the dwelling as lawfully existing today?
- Is the comparison by volume, by floorspace, by footprint, or by a combination?
- Are outbuildings and previous extensions included?
Get an answer in writing at pre-application stage. A scheme designed against one reading of the policy and assessed against another is the commonest reason these applications fail, and the redesign cost is far larger than the pre-application fee.
Measuring volume and floorspace consistently
Where volume is the measure, calculate external volume from ground level to the outer surface of the roof, measured to the outer face of the external walls. State the ground level datum, because on a sloping site the answer moves.
| Item | Include | Note |
|---|---|---|
| Main dwelling as lawfully existing | Yes | Baseline for the comparison |
| Lawful extensions | Usually yes | Confirm against the policy wording |
| Detached outbuildings | Usually no | Ask the authority |
| Basements | Contested | Often excluded from volume, included in floorspace |
| Integral garage | Yes | Part of the building envelope |
| Open porches and canopies | Usually no | State the assumption |
Present the comparison as a table with existing and proposed figures, the percentage change and the basis of measurement, and put the same table in the Planning Statement and on the drawings. Where a basement is proposed, deal with it explicitly rather than leaving it out and hoping.
Siting, footprint and openness
Passing the size test is necessary and not sufficient. The courts have confirmed that openness in the Green Belt has a visual as well as a spatial dimension, so a replacement of identical volume can still harm openness if it is taller, more spread out, or more visible from a public vantage point.
Three siting decisions carry the most weight. Keeping the footprint on or within the existing one, so the developed area does not spread. Keeping the ridge height close to the existing where it is visible in longer views, which usually means a two storey house with a lower pitch or rooms in the roof rather than a full second storey. And resisting curtilage creep, meaning new hardstanding, walls, gates, lighting and formal garden pushing further into open land.
Domestication of the setting is what conservation and landscape officers object to most, and it is frequently what a refusal notice mentions rather than the building itself.
Landscape and visual evidence
For a site visible from a public footpath, a road or a neighbouring settlement, produce a proportionate visual appraisal: a set of annotated photographs from agreed viewpoints, with the existing and proposed ridge lines shown, and a plan locating each viewpoint.
Full formal assessment is rarely proportionate for a single dwelling, but where the site sits in a National Landscape or on a prominent ridge a Landscape and Visual Impact Assessment following recognised guidance may be expected, and councils usually look for a suitably qualified author with professional review.
Include a landscape proposal that works with the openness argument rather than against it. Native hedgerow and tree planting on existing field boundaries reads as restoration. A formal garden with clipped hedging and lighting reads as suburbanisation.
Ecology, trees and drainage
An old rural dwelling being demolished is a high likelihood bat roost. That is the single most common programme risk on these applications.
Commission a preliminary roost assessment early, because emergence surveys are restricted to roughly May to September and a late start loses a year. A bat survey is work a licensed ecologist must carry out under the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017, and where a roost is affected a mitigation licence from Natural England is needed before demolition. An AI-drafted document can structure the reporting and is a starting point for that ecologist rather than a substitute for the fieldwork.
Alongside that: a tree survey where any tree is within influencing distance of the works, a drainage strategy dealing with surface water on a site that probably has no connection, and a foul drainage solution, which on an unsewered site means a package treatment plant with its own consenting position. Biodiversity net gain applies unless an exemption fits, and on a plot with mature garden habitat the baseline can be substantial.
The submitted pack in full
For an illustrative scheme of this kind, the submission would contain:
- Application forms, ownership certificate and the correct fee
- Location plan and block plan showing existing and proposed footprints together
- Existing and proposed floor plans, elevations and sections at consistent scales
- A volume and floorspace comparison table with the basis of measurement stated
- A Planning Statement addressing the replacement exception, openness and other development plan policies
- A design and access statement or design statement, depending on the local validation list
- Photographic visual appraisal with viewpoint plan
- Preliminary ecological appraisal, bat survey results and biodiversity metric output
- Arboricultural information where trees are affected
- Surface water and foul drainage strategies
- Demolition method and a proposed sequencing condition
Check the authority's local validation list before submitting, because the additional local requirements vary and a missing document costs weeks. The full range of report types is set out in the report index. Our note on the stables and manege walkthrough covers a different Green Belt exception and the same evidence discipline.
Conditions removing permitted development rights
Expect a condition removing some or all householder permitted development rights on the replacement dwelling, and expect it to be justified on the basis that further extension would undermine the not materially larger judgement the permission rests on.
That is a reasonable condition in Green Belt cases and it is usually worth accepting rather than fighting, because the alternative is an authority that refuses on the ground that the size test would be defeated later. Guidance on the use of conditions requires clear justification for removing permitted development rights, so read the wording: a condition removing Class A and Class E rights is common, while one removing every right in Part 1 including painting and window replacement is worth querying.
Also expect a sequencing condition requiring the existing dwelling to be demolished within a set period of the replacement being occupied, or before it is occupied. That prevents two dwellings ending up on the site, and it is the condition that a lender's solicitor will read closely.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What does not materially larger mean in Green Belt policy?
National policy sets no figure. Many adopted local plans express it as a percentage increase in volume, floorspace or footprint over the original or existing dwelling, and the figure and the basis differ between authorities. Some plans set no figure and assess each case on its merits, so read the policy word for word.
Can I add the volume of a detached garage to the baseline?
Usually not, because a detached outbuilding is not part of the dwelling. Some authorities allow it where the outbuilding is demolished and its volume is absorbed into the replacement. It is a local policy question worth settling in writing at pre-application stage, since it can decide whether a scheme works.
Does passing the size test mean the application will be approved?
No. Being inside the exception means the proposal is not inappropriate development, but openness and other development plan policies still apply. The courts have confirmed openness has a visual as well as a spatial dimension, so a taller or more spread out replacement of the same volume can still cause harm.
When should bat surveys be commissioned?
Before the design is fixed, and ideally in the winter before the application year. A preliminary roost assessment can be done at any time, but emergence surveys are restricted to roughly May to September, so a late start loses a season. An old rural dwelling being demolished is a high likelihood roost.
Why remove permitted development rights on a replacement dwelling?
Because further extension would undermine the size judgement the permission rests on. A condition removing Class A and Class E rights is common in Green Belt cases and usually worth accepting. Guidance requires clear justification, so a condition removing every right in Part 1 is worth querying.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.