Harrow Green Belt extension wins despite 76% footprint

A Harrow Green Belt extension has won planning permission on appeal despite a 76% increase in the original house’s footprint. The Inspector rejected reliance on a 30% proportionality benchmark which did not appear in any relevant local policy or guidance.

Inspector P B Jarvis allowed appeal 6013425 on 9th October 2026 at 17 Brightwen Grove, Stanmore. The scheme combined a first-floor side extension with single-storey side and rear works already covered by earlier permissions. The additional first-floor bulk, rather than the full enlarged footprint, was central to the decision.

The outcome offers a useful lesson on national Green Belt policy GB7(b). It does not establish that 76% is a permissible limit everywhere, or that footprint alone determines whether an extension is disproportionate. The judgement turned on the building’s proportions, the approved baseline and its detailed design.

Why the Harrow Green Belt extension passed

The property is a detached two-storey home within an estate of modern houses in the Metropolitan Green Belt. The proposed additions were considered against Harrow Local Plan policy GI1, London Plan policy G2, residential design guidance and the August 2026 National Planning Policy Framework.

The council’s formal refusal notice cited character and appearance, while its delegated officer report had also objected on Green Belt grounds. The Inspector addressed both subjects so that the appeal decision dealt with the substantive disputed planning issues.

The Inspector found that the existing house was lawful and of permanent and substantial construction. It qualified in principle for consideration under NPPF policy GB7(b), which allows building extensions that are not disproportionate compared with the original building.

The council’s 30% threshold was not policy

An important point was the officer report’s reliance on a 30% margin when assessing the development’s size. The Inspector stated that the margin was not set out in the relevant local plan or guidance. Nor did the national policy specify a fixed percentage constituting a disproportionate increase.

The authority was entitled to examine the magnitude of the additions, but it needed to explain the actual assessment. A numerical guide with no identified policy foundation could not take the place of examining the facts.

The lesson is not that extension percentages are irrelevant. The original dwelling’s floorspace, volume and footprint provide useful evidence. But a council or applicant should identify precisely what is being measured and why the resulting change is disproportionate in planning terms.

For a wider explanation of evidential weight and policy, see Planning Geek’s guide to material considerations.

A 76% footprint increase was not a 76% floorspace increase

The council calculated a 76% increase in footprint compared with the original building. It also identified approximately 42 square metres of single-storey additions and 17 square metres of first-floor accommodation. It did not provide equivalent percentage increases for total floorspace or volume.

Because the original house has two storeys, the footprint figure was not representative of the percentage increase in its total floorspace. The Inspector noted that the floorspace increase would be smaller.

This distinction is important when reviewing house extensions. A ground-floor addition can take up considerable land without creating the same proportion of overall building volume or internal area. Equally, a modest footprint could conceal excessive height.

The Inspector did not invent a replacement percentage test. He evaluated the full physical relationship between the proposed work and the original building.

Previously approved extensions changed the balance

The house already benefited from permissions granted in 2024 and 2025, under references PL/2012/24 and PL/0883/25. Those approvals included single-storey side and rear extensions and rear dormers. Building work was underway when the Inspector visited on 7th September 2026.

The appeal proposal retained the same single-storey elements. What was new was the first-floor side addition above the garage. Consequently, the additional accommodation sat within a footprint that already had planning permission.

The proposed first-floor extension would be set back from the main front wall and would have a lower ridge height than the existing house. These proportions allowed it to remain visually subordinate rather than appear as a competing building mass.

Earlier permissions are not a universal guarantee of consent for later works. Their legal status and implementation still matter. However, they can provide an important factual baseline for assessing the additional change actually proposed.

Green Belt policy GB7(b) resolved the first issue

The Inspector concluded that the proposed development would not result in a disproportionate increase over the original dwelling. It therefore fell within policy GB7(b) and was not inappropriate development in the Green Belt.

Having reached that conclusion, the Inspector said that a further assessment of the effect on Green Belt openness was unnecessary for the purposes of the appeal. This should not be misread as a finding that extensions cannot affect openness; it is a consequence of the applicable policy exception.

Where an extension does not meet the exception, a materially different planning exercise may arise, including weighing Green Belt harm and any very special circumstances. Applicants should not proceed to that balancing exercise without first checking the exception correctly.

For the applicable national-policy changes, Planning Geek has a comparison of the 2024 and 2026 NPPF.

Design and visibility were tested separately

Even an extension that avoids being inappropriate development in the Green Belt must satisfy other planning policies. The Inspector reviewed its design, its relationship with the adjoining plots and views from the neighbouring woodland public footpath.

The single-storey work would reach the plot boundary, but the new first-floor element was stepped in and would be smaller in height and width than the original dwelling. From the woodland it would be seen against the house’s existing bulk.

Materials and architectural detail were considered compatible with the dwelling. The Inspector found compliance with London Plan policy D3, Harrow policy GR1 and the council’s residential design guidance.

This was a full planning appeal, not a certificate for permitted development. The constraints differ from those in Planning Geek’s two-storey extension permitted-development guide.

Permission has conditions and a clear scope

The appeal was allowed subject to a three-year commencement period, an approved-drawings condition and a requirement for matching external materials. Applicants should not treat the appeal headline as permission to build any arrangement of similar size.

An ecology officer had recommended a precautionary method statement, but the Inspector considered that condition unnecessary because construction work was already underway. The decision does not remove any separate protected-species obligations.

The appeal involved the particular extension drawings associated with application PL/2985/25. Building control, boundary rights and any operative conditions on earlier permissions remain separate practical considerations.

Householders considering comparable proposals should document the original building, all later additions, existing consents, total floorspace and volume, and the actual design impact before drawing conclusions from a footprint percentage.

Read the Harrow Inspector’s decision

The Planning Inspectorate record for appeal 6013425 provides the formal decision for 17 Brightwen Grove. The Appealbase decision page provides a further route to the full appeal text.

The Harrow Green Belt extension succeeded because the design and overall increase were judged proportionate on these facts, notwithstanding the headline footprint percentage. It is a warning against rigid, unsupported numerical benchmarks, not an automatic approval formula.

Planning Geek takeaway: a defensible Green Belt extension appraisal needs the original-house baseline, the correct policy test and clear architectural evidence rather than a standalone percentage.