Unlike single-storey extensions, which can often be built under Permitted Development rights without a formal planning application, two-storey extensions almost always require full planning permission. Understanding what planning authorities look for — and what commonly leads to refusal — helps you design a scheme that has a realistic chance of success.
Permitted Development does not cover two-storey extensions in the way it covers single-storey ones. Single-storey rear extensions can be built under PD up to certain depth limits. Double-storey rear extensions are not covered by the standard residential PD allowances and require a householder planning application in virtually all cases. Side extensions, including two-storey side returns, also require planning permission.
A householder planning application for a two-storey extension is submitted to the local planning authority. The typical decision period is eight weeks. The planning officer assesses the proposal against the authority's Local Plan policies on residential extensions, the National Planning Policy Framework, and any relevant Supplementary Planning Documents. In London, the Mayor's London Plan also provides an overarching policy context.
The key design considerations for a successful two-storey extension in London are as follows. Impact on neighbouring amenity is the most common reason for refusal. Planning authorities apply a 45-degree daylight and sunlight test to the nearest windows of neighbouring habitable rooms — if the proposed extension projects beyond a 45-degree line drawn from the nearest adjacent window, it is likely to cause unacceptable overshadowing. The height of the extension relative to the host building and neighbouring properties matters: extensions that step down in height at the boundary are more likely to be accepted. The design of the extension should relate sympathetically to the host building in terms of materials, window proportions, and architectural language.
First-floor windows in the proposed extension require particular care. Windows facing a shared boundary that overlook a neighbouring garden or habitable room windows will almost always be a point of objection. High-level windows, obscure glazing, or careful positioning of windows away from the boundary are design responses that address this concern.
Costs: Two-storey rear extensions in London typically cost between eighty-five thousand and one hundred and eighty thousand pounds for build-only works, depending on the size of the extension, specification level, and structural complexity. Planning professional fees add one thousand five hundred to three thousand pounds. Structural engineering fees add one thousand to two thousand five hundred pounds. 07359 872594 | contact@rcbgroup.co.uk | www.rcbgroup.co.uk