Extension Size Calculator

Under permitted development, a single-storey rear extension can project up to 3 metres from the original rear wall for terraced and semi-detached houses, or 4 metres for detached houses. The extension must not exceed 4 metres in total height or 3 metres at the eaves, and the total extension footprint cannot cover more than half of the original garden area. These limits are reduced in conservation areas and do not apply to flats. If you go beyond the standard limits, you can apply under the Larger Home Extension scheme, which is a prior approval process rather than full planning.

Permitted development rules limit the total built area of extensions, outbuildings, and sheds to no more than half of the original garden. The word original matters — it means the garden as it existed when the property was first built, not what is there today. If the previous owner added a rear extension in 1998, that extension still counts toward the 50 percent even though you did not build it. Large garden rooms, studios, and existing conservatories also count. Check your title plan and any old planning records before you assume the full allowance is available.

Permitted development rights are reduced in conservation areas but not removed entirely. Rear extensions are still allowed but with tighter depth limits (2 metres for terraced and semi-detached, 3 metres for detached). Side extensions are usually not permitted at all under PD. Cladding, external wall insulation, satellite dishes, and some roof alterations that would be permitted elsewhere also need planning permission in a conservation area. We check the exact constraints for your property during the free site visit and pull the conservation area appraisal document from your council before committing to a design.