Semi-detached houses are one of the most common property types in outer London, and they offer good extension potential compared with terraced houses because they have a free side boundary that can be built against. Whether you want to extend to the rear, to the side, or above, a semi-detached house typically provides more options than a terrace. Understanding the planning rules and the practical constraints is the key to making the right choice.
The most popular extension for a semi-detached house is a single or double-storey rear extension. Under permitted development, a single-storey rear extension to a semi-detached house can extend up to three metres from the original rear wall without a planning application, or up to six metres under the Householder Prior Approval scheme. A double-storey rear extension requires a planning application and must be at least seven metres from the rear boundary.
Side extensions on semi-detached houses are also possible but are subject to permitted development restrictions. The new building must not exceed half the width of the original house, must not be higher than the eaves of the original roof, and must use materials that are similar in appearance to the original. In many London boroughs, the council has used Article Four directions to remove permitted development rights for side extensions, requiring a planning application even for modest additions.
A two-storey side extension, sometimes called an infill extension, involves building a full-height addition on the side of the semi that matches or is slightly subordinate to the main house in scale. These require planning permission and are most commonly approved where the design is well-considered and the addition does not dominate or harm the visual balance of the street.
Wrap around extensions, combining rear and side elements, are particularly effective on semi-detached houses because the free side boundary removes the party wall complication that applies to terraces. The side element can use conventional masonry construction against the side boundary wall without the need for party wall procedures.
Loft conversions are another strong option for semi-detached houses, particularly hip-to-gable conversions where the sloping hip at the side of the roof is replaced with a vertical gable wall, creating significantly more internal space and headroom in the loft.
RCB Design and Build carries out extensions and loft conversions on semi-detached houses across London, from planning through to delivery as a single design-and-build package.