Basement work is the most expensive square metre you will ever add to a London house, and it is also the one where quotes vary most wildly. The gap usually comes down to how much earth has to move and what is holding the building up while it does. Here is how the numbers actually break down, from someone who does the digging.
Most enquiries fall into one of three brackets, and they are not remotely comparable on price. Being clear about which one you need is the single fastest way to stop comparing apples with scaffolding.
As a rough guide for Greater London in 2026, expect these ranges before VAT and before fit-out.
Access is the biggest single variable in London. A Victorian mid-terrace in Islington with a 900mm side return means every cubic metre of spoil goes out through the house or by conveyor to a grab lorry parked on a permit bay. That can double the muck-away line compared with a detached property in Bromley with rear access.
Ground conditions matter almost as much. London Clay is workable and predictable, but it shrinks and swells, so heave protection under the slab is not optional. Sites near the Thames, in Fulham, Battersea or parts of Hackney, often hit river terrace gravels with groundwater close to the surface. That pushes you towards a fully tanked reinforced concrete box, dewatering during the dig, and sometimes secant or contiguous piling instead of traditional mass concrete underpinning. Piling alone can add £40,000 upwards.
Depth and party walls do the rest. Every metre deeper is more temporary works, more propping, more concrete. If you share walls with two neighbours you will need Party Wall Awards for both, and each surveyor is a real cost with a real timeline.
Homeowners are often caught out by the professional and statutory fees, which on a full dig-out routinely add 15 to 25 per cent on top of the construction figure.
Budget separately for these before you commit.
Design, planning and party wall matters typically take four to eight months before a spade goes in. The groundworks and reinforced concrete phase on a single storey dig-out under a terraced house usually runs 16 to 26 weeks, with fit-out after that.
When you compare quotes, look at what is included rather than the headline. A shell and core price covering underpinning or piling, the reinforced concrete box, waterproofing, drainage and a pumped sump is a different product from a turnkey figure including staircase, lightwell, glazing, plaster and electrics. Ask specifically who carries the temporary works design, who pays the party wall surveyors, and what the assumption is on spoil volume and disposal.
Be wary of anything materially below the ranges above. Underpinning sequencing, propping and waterproofing are where corners get cut, and those are precisely the things you cannot inspect once the slab is poured.
In higher value boroughs it often is, because the uplift per square metre of added space can exceed the build cost, but in outer London the sums frequently do not work. Get a local agent's view on the likely resale figure before you commit to a full dig-out.
A basement under the existing footprint can sometimes fall under permitted development, but many London boroughs have removed or restricted that, and lightwells, extensions under gardens and any listed or conservation area status will need a full application. Always check with your borough first.
Properly designed and sequenced underpinning, with monitoring in place, is a controlled process and significant damage is uncommon. The Party Wall Award exists to record the neighbouring property's condition beforehand and to set out who puts right any damage that does occur.
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