House extensions in London, designed and built by one team

It depends on floor area, ground conditions, the glazing specification and how much of the existing house is disturbed. Foundations and drainage move the figure more than finishes do, and neither is knowable before the trial holes are dug. Our cost guides set out the variables in detail. A firm price follows a site visit and the drawings, so you can see which part of the number is fixed.

Often not. Permitted development allows a single-storey rear extension of three metres on an attached house and four on a detached one, with six and eight metres available through the prior approval route where neighbours are consulted. Height, materials and how much garden is covered all carry limits. Permitted development is removed in conservation areas, by Article 4 directions, and for flats entirely, so the answer is address-specific.

On site, a single-storey rear extension on standard ground usually runs twelve to sixteen weeks, and a double-storey or wraparound twenty to twenty-eight. Approvals run before that and frequently take longer than the build itself, which is why we start planning, building control and party wall notices in parallel rather than one after another.

For a small single-storey rear extension with a simple specification it can be. For a wraparound with structural glazing it will not be. The figure is set by floor area, ground conditions and glazing rather than by ambition. Tell us the budget at the first conversation and we will say which version of the project it buys, rather than finding out together in month three.

Three metres from the original rear wall on a semi-detached or terraced house, and four metres on a detached one, under the standard allowance. The larger home extension procedure doubles both figures to six and eight metres, subject to prior approval and a neighbour consultation. Separate conditions apply to height, eaves, materials and how much of the original garden may be built on.

Almost certainly, if you are building on or near a boundary. The Act is triggered by building on the line of junction, cutting into a party wall, or excavating within three metres of a neighbouring structure to a lower depth than its foundations, which extension foundations normally do. Notice periods are one or two months depending on the section, so notices are served at design stage rather than when the crew is booked.

For a rear extension, usually yes, until the break-through. The extension is built as a separate structure alongside the house, and the disruptive phase is the few days when the existing wall is opened and the two become one space. Where the extension takes the kitchen, plan a temporary kitchen rather than improvising, because months of eating out costs more than the temporary kitchen does.

We do. The survey, the plans, the structural calculations and the building control application are part of the contract, and we deal with the inspector at each stage through to the completion certificate. Where a planning application or a party wall notice is needed we prepare and submit those too. You pay the authority fees directly, so you can see what they are.