Start with the property, not just the dimensions
Confirm whether the building is a house or a flat, whether it is listed, and whether an Article 4 direction or earlier planning condition removes relevant rights. The householder allowances do not provide the same route for a flat or maisonette. Listed building consent may be needed separately for work affecting a listed building's special character. [2] [3] [4]
The council's appraisal and local design guidance are then the starting point for a scheme needing permission. They describe the area's significance; they do not grant permission or replace the legal permitted development tests. [5]
Rear and side extensions
For an eligible house in a conservation area, the normal Class A rear-extension limits can still be relevant: up to 3 metres beyond the original rear wall for a house other than a detached house, or 4 metres for a detached house. These depths are only part of the test; height, boundary relationships, previous additions and the other limitations also matter. [1]
The larger-home-extension procedure allowing 6-metre or 8-metre rear extensions does not apply on this designated land. Nor does the ordinary Class A allowance authorise an extension beyond a side wall of the original house or a rear extension of more than one storey in a conservation area. [1]
Treat a side-return or wraparound proposal as its complete shape. Calling it a “rear extension” does not remove a side element from the assessment. Ask for the original walls and the proposed enlargement to be clearly distinguished on the drawings. [1]
Dormers and rooflight conversions
A dormer or other roof enlargement cannot use the standard Class B roof-extension allowance in a conservation area. This includes a proposal tucked away on the rear roof: being less visible may help the design case, but it does not restore that permitted development right. [2]
Rooflights are different from roof enlargements. Class C can allow qualifying roof alterations, including rooflights, subject to its projection, ridge-height and side-window conditions and any restriction on the property. Do not treat a rooflight conversion and a large dormer as the same permission question. [1] [6]
Make the design case specific
Photograph the roofline, neighbouring extensions, gaps between buildings and the parts of the proposal visible from surrounding streets or spaces. Identify which features the appraisal says are important, then explain how the proposed height, position, materials and detailing respond to them.
Under policy HE9 of the August 2026 NPPF, the assessment concerns the area's special interest and the contribution of the features affected. A past approval elsewhere is useful context, but the application still needs to explain the impact of this proposal on this building and area. [7]
Resolve other constraints before fixing the layout
A loft project may require bat assessment where the roof has roost potential. An extension may affect protected trees or their roots. These are separate issues from the architectural acceptability of the design and should be checked while changes to the layout remain possible. [8] [9]