How much do basement conversion drawings cost in London?

Why this is our highest drawing fee

A loft sits on a structure that exists. A basement creates one. The drawings cover the existing house, the new floor level, the retaining walls, the stair, the lightwell and the junction with the neighbour’s foundations, and every one of those sheets is also a structural question.

Our fee is £1,705 drawings + £150 survey (£1,855 total). That buys the measured survey, the planning set and the coordination with your structural engineer, whose calculations are priced separately at £389 for the standard case. A basement with unusual ground or a deep dig will need more engineering than that.

Why digging down needs planning permission

Converting a cellar that exists is one thing; excavating new space is another.

Section 55 of the Planning Act says that works affecting only the inside of a building are not development. It then carves out one exception: works for providing additional space underground do count. So converting a cellar that exists is usually internal work, while excavating new space is development.

That is why a basement project is a planning application in practice. The council fee for one house is £548, and many councils ask for an engineer’s statement showing how the digging will be done without harming the neighbours.

One borough’s limits, as an example

Boroughs write their own tests. Kensington and Chelsea’s Policy CL7, adopted in January 2015 and explained in its April 2016 Basements SPD, limits a basement to 50% of each garden and to one storey. It also asks for a minimum of one metre of soil above any part of the basement beneath the garden.

That one borough is not a rule for London. Policy numbers change when a local plan is replaced, so check the current plan for your address before you design. We do that check at the start, and draw the garden area and the soil depth where your borough asks for them.

Fire, light and the neighbour’s foundations

Under Approved Document B, a basement with habitable rooms needs either an emergency escape window or external door, or a protected stairway to a final exit. The escape window must have an openable area of at least 0.33 square metres, at least 450 millimetres high and wide, with its bottom edge no more than 1,100 millimetres above the floor.

Digging near next door brings the Party Wall Act in. Section 6 applies where the excavation is within 3 metres of a neighbouring building and goes below its foundations, or within 6 metres on a 45-degree line, and it asks for one month’s notice that says whether you will underpin.

Community Infrastructure Levy is rarely the worry. GOV.UK guidance says development under 100 square metres is generally exempt, unless it creates a new dwelling.