How much do rear extension drawings cost in London?

Start with the length, not the price

Every rear extension in London is first a measurement. Class A of the permitted development order allows a single-storey extension to go no more than 3 metres past the rear wall of a terraced or semi-detached house, or 4 metres past a detached one, and no higher than 4 metres. The extended part may not take the total built area beyond half the garden.

Because those figures are limits, the drawings have to prove them. We measure the house, draw the extension against the original rear wall and mark the length, the eaves and the ridge, so the council can check each number from the sheet.

Three routes, three sets of bills

Inside the limits, the extension is permitted development. You pay for drawings and building control, and nothing to the council. Our fee is £475 drawings + £150 survey (£625 total).

Between those limits and the larger ones, a householder can go up to 6 metres, or 8 metres for a detached house, through a prior approval application. The council fee for that is £249. Our drawings for it are £375 drawings + £150 survey (£525 total).

Above that, or on land such as a conservation area where the right is cut back, you need a full householder application. The council fee is £548 for one house, and our rear extension drawings, £475 drawings + £150 survey (£625 total), are the planning set.

What happens in a prior approval, and why neighbours matter

The neighbours’ view of daylight and overlooking decides the result.

A larger extension is not a free pass. The council must write to every adjoining owner, who get at least 21 days to comment. If any neighbour objects, the council decides whether the extension harms their amenity, and it looks at all adjoining homes, not just the ones that wrote in.

If the council says nothing within 42 days, you may begin. That is shorter than the 8-week limit that applies to a normal application, but it is the neighbours’ view of daylight and overlooking that decides the result, so our drawings show both.

Costs outside the drawing fee

Building control and a structural engineer sit beside the drawings, because a wide opening in the old rear wall needs a steel beam. We price the calculations at £389.

A deep foundation next to the neighbour’s can need a notice too. Section 6 of the Party Wall Act applies when you dig within 3 metres of their building below its foundations, or within 6 metres where the dig cuts a line sloping down at 45 degrees from the bottom of their foundations. It asks for one month’s notice.

Light is the other cost people forget. A neighbour whose window has had light for twenty years uninterrupted holds an absolute right to it, so a deep extension beside a long-established window is worth drawing carefully.