Answering a Green Belt openness refusal with volume evidence

A refusal reason that reads "the proposal would result in a disproportionate addition over and above the size of the original dwelling and would cause harm to the openness of the Green Belt" is answered with measurements, not with argument about character. The case officer has reached a view on how much bigger the building gets. If you want that view changed on appeal, you have to put the volume and floorspace figures in front of the inspector, with the original building defined and the calculation shown.

Most Green Belt openness appeals are lost because the appellant argues about design quality and landscaping while the inspector is looking for a number.

What an openness refusal reason alleges

Two separate claims usually sit inside one sentence. The first is that the development is inappropriate in the Green Belt, because the addition is disproportionate and so falls outside the exception the Framework allows for extensions and alterations. The second is that it causes harm to openness, which is a further matter the decision maker weighs.

Inappropriate development is by definition harmful, and the Framework requires substantial weight to be given to any harm to the Green Belt. So the appeal has to deal with both limbs. Showing the addition is proportionate removes the first. Showing openness is not materially reduced narrows the second.

Read the refusal notice carefully for which limb is actually relied on, because the evidence differs. A refusal on openness alone, without a disproportionate additions finding, is a weaker reason and is sometimes answered with sections and a visual analysis rather than with volumes.

Spatial and visual aspects of openness

Openness is not the same as visual amenity and it is not a design judgement. The Supreme Court confirmed in the Samuel Smith Old Brewery case that openness has both spatial and visual dimensions, and that visual effects may be a relevant consideration without being mandatory in every case.

In practice this means two strands of evidence:

• the spatial strand, which is about built volume, footprint and the amount of land covered by development
• the visual strand, which is about whether the development reads as encroachment in views from public vantage points

A conservatory tucked into a courtyard behind an existing range may add volume while being invisible from anywhere. A low single-storey addition across an open paddock frontage may add little volume and be plainly visible. Say which strand the refusal engages and answer that one properly.

Measuring volume and floorspace change

The original building is the starting point and it is a legal concept, not an aesthetic one. For planning purposes the original building is the building as it stood on 1 July 1948, or as first built if constructed after that date. Everything added since, whether under permitted development or with permission, counts as an addition.

So the first piece of work is documentary. Historic Ordnance Survey mapping, council tax and rating records, aerial photography and the planning history together establish what the original building was. Where the authority and the appellant disagree on the baseline, that disagreement is usually worth more than any argument about percentages.

Then present the arithmetic in a table the inspector can check:

Those figures are illustrative and not taken from a real appeal. State the measurement convention used, whether external or internal volume, and whether below-ground space is included, because the two sides often measure differently and an unexplained discrepancy undermines the whole submission.

The disproportionate additions test

There is no national percentage at which an addition becomes disproportionate. The Framework uses the word proportionate and leaves the judgement to the decision maker, and many local plans and supplementary design guides then set their own figures, commonly somewhere between 30 and 50 per cent of the original volume.

Those local figures are policy, so check whether the adopted plan or an adopted SPD actually contains one before conceding the point. Where it does, the comparison has to be made on the same basis the policy uses. Where it does not, cumulative effect over time becomes the argument, because a series of modest additions can produce a building several times the original.

Volume alone does not decide it either. A single-storey addition that spreads the footprint across previously open land can be more harmful than a loft conversion of greater volume, and inspectors have treated footprint and sprawl as part of the proportionality judgement.

Very special circumstances and their evidence

If the development is inappropriate, permission should only be granted where very special circumstances clearly outweigh the harm. That is a high bar and it needs evidence rather than sentiment.

Circumstances that carry some weight when evidenced include the removal of existing unsightly buildings as part of the scheme, a demonstrable need connected to agricultural or equestrian use, provision for a disabled occupant's requirements supported by an occupational therapist's assessment, and biodiversity or landscape improvements secured by condition or obligation. Personal preference and family circumstances alone rarely succeed.

And the arithmetic of the balance has to be set out. List each benefit, state how it is secured, and explain why together they clearly outweigh the substantial weight given to Green Belt harm. An appeal statement that asserts very special circumstances without that structure gives the inspector nothing to adopt.

Previously developed land in the Green Belt

Where the site is previously developed, different exceptions apply, and the December 2024 revision of the Framework added the grey belt concept covering previously developed land and other land that does not strongly contribute to Green Belt purposes. The current Framework was published on 17 August 2026 and organises policy by code rather than paragraph number, so verify the wording and the code reference on gov.uk before citing it in a statement of case.

The evidence for a previously developed land argument is factual: what was lawfully there, what remains, the extent of hardstanding and buildings, and whether the land has reverted to a natural state. Photographs with dates, measured surveys and the planning history do that work.

Presenting measured evidence

An appeal on openness should arrive with a measured survey of the existing buildings, a volume and floorspace schedule, drawings showing existing and proposed footprints overlaid, and a short visual analysis from the public vantage points the officer referred to. Put the methodology on the face of the document so the authority's response has to engage with it.

Where the authority's committee report or delegated report contains its own figures, address them directly. If their volume calculation included a building you say is not part of the original dwelling, say so and show the evidence. Unexplained competing numbers are usually resolved against the appellant, and the same discipline applies across refusal reasons generally, as set out in the guidance on reading a refusal notice and turning it into a plan.

A Planning Statement drafted on PlanScope sets out the Green Belt policy tests, the proportionality comparison and the very special circumstances structure, with the volume figures left for you to insert from a measured survey. Amenity reasons frequently sit alongside Green Belt reasons on the same notice, and those are handled differently again, as covered in neighbour amenity refusals for overlooking and overbearing impact. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.