A side return extension infills the narrow strip of land beside the rear projection of a Victorian or Edwardian terrace. It typically adds 8 to 15 square metres, transforms a dark galley kitchen, and usually requires a planning application rather than permitted development.

A London extension typically takes six to twelve months from first conversation to completion. Construction is only twelve to sixteen weeks of that for single-storey work. The rest is design, consent, party wall notices and tendering, most of which runs before anyone arrives on site.

Moving typically costs 8 to 10 per cent of a property's value in transaction costs alone, before you gain any additional space. Extending converts that same money into square metres, provided your street has room above its ceiling price.

Planning permission and building regulations are two separate approvals granted by two different teams against two different tests. Almost every extension needs building regulations approval, whether or not it needs planning permission.

Comparing builders' quotations only works when every contractor prices the same drawings and specification. Where they do not, the cheapest number usually reflects the most that has been left out rather than the keenest price.

An Article 4 direction is a local council decision that withdraws permitted development rights across a mapped area, meaning work that needs no application elsewhere requires a full planning application on your address.

A loft conversion is often permitted development within volume limits of 40 cubic metres for a terrace and 50 for a semi or detached house, but conservation area status usually removes that. Head height decides feasibility before any rule does.

A London rear extension costs £3,000 to £5,000 per square metre to build, depending on specification. Once professional fees, statutory costs, contingency and VAT are added, the total project cost runs roughly 45 to 55 per cent above the construction figure.

Many London rear extensions are permitted development and need no planning application, but the answer turns on depth, height, boundary distance and whether your property is listed, in a conservation area or covered by an Article 4 direction.