How Much Does a Basement Conversion Cost in London?

Typical price ranges in London

Builders usually price basements per square metre of finished floor area. The ranges below are for construction only, before VAT and professional fees, and assume a typical terraced or semi-detached house. Treat them as a guide for early budgeting, not a quote.

As a worked example, a 40 square metre cellar under a Victorian terrace that needs lowering and underpinning might come to somewhere between £120,000 and £180,000 for the build, with fees and VAT on top. A new basement of the same size under a house that has none could be £160,000 to £260,000 or more.

• Converting an existing cellar with enough headroom (tanking, insulation, electrics, finishes): roughly £1,500 to £3,000 per square metre
• Lowering the floor and underpinning an existing cellar: roughly £3,000 to £4,500 per square metre
• Excavating a new basement under the house: roughly £4,000 to £6,500 per square metre
• Extending under the garden, or prime central London sites with difficult access: £6,000 per square metre and upwards
• Lightwells: often £8,000 to £15,000 each, depending on size and glazing

What pushes the cost up or down

Headroom is the first question. Most London cellars sit at around 1.8 metres or less, and a comfortable habitable room wants about 2.4 metres finished. Closing that gap means digging down and underpinning the walls in short sections, which is slow, skilled structural work.

Ground conditions come next. Much of London sits on clay, which is stable to dig but drains poorly, while areas nearer the Thames and its buried tributaries can have a high water table that calls for pumping during the works and a more robust waterproofing design.

Access matters more than people expect. On a terraced street with no side passage, every bucket of spoil leaves through the front of the house by conveyor into a skip. Skip licences, parking bay suspensions and restricted working hours all add cost, and they vary by borough.

• Waterproofing type: a cavity drain membrane with sump and pump is the usual choice, and needs annual servicing
• Moving drains, or building near a public sewer, which needs a build over agreement with Thames Water
• Number of party walls affected
• Staircase position and structural alterations to the ground floor
• Specification: a utility room and playroom cost far less than a cinema, gym or wet room

Fees and extras outside the builder's quote

Allow a further 12 to 20 per cent of the build cost for professional fees and consents. VAT at 20 per cent normally applies to the whole project too, so check whether the figures you are comparing include it.

If you plan to move out during the dig, which many households do for at least part of it, add rent for that period to your budget.

• Architect and structural engineer
• Site investigation and trial pits, often £2,000 to £5,000
• Basement impact assessment, required by many inner London boroughs
• Party wall surveyors: commonly £1,000 to £2,000 per neighbour, and you usually pay their surveyor's fees as well
• Planning application and building control fees
• A construction traffic management plan where the council asks for one
• Possible Community Infrastructure Levy where the new floor area is 100 square metres or more

Planning rules and timescales

Converting an existing cellar without changing the outside of the house often needs no planning permission, only building regulations approval. Excavating new space, adding a lightwell or altering the front of the property usually does. Several boroughs, including Kensington and Chelsea, Westminster, Camden, and Hammersmith and Fulham, have specific basement policies that limit excavation to a single storey and restrict how far you can extend under the garden. Listed buildings need listed building consent, and conservation areas bring extra scrutiny of anything visible from the street.

On site, a simple cellar conversion can take 8 to 12 weeks. An underpinned or newly excavated basement more commonly takes 5 to 9 months, with 3 to 6 months of design, party wall agreements and approvals before work starts.

Is it worth the money?

It depends on local property values. In boroughs where homes sell for £10,000 per square metre or more, a well-lit basement can add more than it costs. In outer London and much of the Home Counties, a loft conversion or rear extension usually gives more space for the money, and a basement makes sense mainly when you have no other way to extend.

Whatever the location, ask for a fixed price based on a completed structural design and site investigation. Quotes prepared before anyone has dug a trial pit are the ones most likely to rise.