Detached houses have more extension flexibility than any other residential property type — with access on all four sides and typically larger plots. However, detached houses in London are also often in areas with strong planning policies, mature garden trees, and sensitive neighbouring relationships. Understanding what is possible, what requires planning permission, and what it costs helps you make the right decision early.
A rear extension is the most common and lowest planning risk extension type for a detached house. Under Permitted Development, a single-storey rear extension up to eight metres depth from the original rear wall can be built without planning permission on a detached house — subject to standard PD conditions relating to height, materials, and curtilage coverage. This is the largest PD allowance for any residential property type and means that many detached house rear extensions can proceed without a planning application.
A side extension on a detached house adds accommodation adjacent to the existing building. Side extensions on detached houses are typically within PD if they are single-storey and the combined width of the extension does not exceed half the width of the original house. A two-storey side extension or any side extension that creates a terrace or balcony requires planning permission.
A wrap-around extension — combining a rear extension with a side extension to form an L-shaped additional footprint — is one of the most effective uses of a detached house plot. The combined project adds significant ground-floor accommodation and allows a redesigned ground-floor layout that integrates kitchen, dining, and living areas in a connected open-plan space. Wrap-around extensions almost always require planning permission because the combination of side and rear elements exceeds standard PD limits in most configurations.
Double-storey rear extensions on detached houses require planning permission. They provide the largest increase in accommodation of any extension type — adding full-height floor area above an already generous ground-floor extension — but require careful design to address the potential impact on neighbouring properties through overshadowing and overlooking.
Costs for detached house extensions in London are typically slightly higher than equivalent projects on terraced houses, reflecting the additional exposed elevations that require insulation and finishing. A single-storey rear extension (four metres by five metres) costs sixty-five thousand to one hundred thousand pounds. A side extension of similar scale costs fifty-five thousand to eighty-five thousand pounds. A wrap-around extension costs ninety-five thousand to one hundred and eighty thousand pounds. A double-storey rear extension costs ninety-five thousand to one hundred and ninety thousand pounds.
RCB coordinates the design, planning, and principal contractor delivery for detached house extension projects across London and surrounding counties. 07359 872594 | contact@rcbgroup.co.uk | www.rcbgroup.co.uk