Costs · Basements
What Affects the Cost of a Basement in London
Basements are the most cost-intensive home projects. This guide explains what drives the price, from excavation and waterproofing to party walls and borough policy — with no headline figure.
Why basements cost the most
Per square metre, a basement is usually the most expensive way to add space to a London home, because so much of the cost is invisible: excavation, temporary support of the existing building, specialist structural design and a designed waterproofing system, all before the space is fitted out. Unlike an extension, much of the spend goes into making the space possible at all.
Our fixed-fee packages
Design fees, inc. VAT — separate from the council fees above
| Package | Fixed fee |
|---|---|
| Planning Route ReviewA paid route check before drawings or applications. | £395 inc. VAT |
| Standard Planning Drawings PackageFor straightforward householder schemes. | from £1,450–£1,950 inc. VAT |
| Full Householder Planning PackageDrawings plus application preparation and submission. | from £2,250–£3,250 inc. VAT |
| Licence to Alter / Flat Refurbishment Drawing PackLeasehold flat alterations and freeholder consent drawings. | from £1,500–£2,750 inc. VAT |
| Building Regulations / Technical Drawing PackageTechnical drawings once the planning route is clear. | from £2,750–£4,950 inc. VAT |
| Listed / Conservation Planning PackHeritage-led drawings and consent strategy. | Quoted after review |
Fixed fees quoted before you commit, credited from the £395 route review if you proceed. Larger, listed or complex projects are quoted after a paid review rather than sold at a fixed price. See all packages or find your package.
Structure and waterproofing
The structure must support the building above during and after construction, and the waterproofing must keep water out reliably for the life of the building. These are specialist designs, and they are not areas to economise — a failure is extremely costly to put right. They are a major part of a basement’s cost for good reason.
Party walls and neighbours
Basements almost always engage the Party Wall etc. Act, often with neighbours on more than one side, which adds surveyor costs and time. Protecting and, where necessary, underpinning adjoining structures is a real cost, as is managing construction impact on a tight, occupied street.
Borough policy and assessments
Many boroughs require a Basement Impact Assessment and other supporting reports, and limit the depth and extent of what can be built. These add professional cost and shape the design. In boroughs with restrictive basement policies, the achievable basement may be smaller than hoped, changing the cost-to-benefit calculation.
Programme and disruption
Basements take longer than above-ground work and are more disruptive, which has cost implications — including, for some owners, the cost of living elsewhere during the works. A realistic view of time and disruption, alongside the build cost, is essential before committing.
Getting to a real number
Because basements are so dependent on ground conditions and engineering, an early feasibility assessment and specialist input are the route to a realistic budget, followed by a competitive tender once the design and engineering are resolved.
Frequently asked questions
Why are basements more expensive than extensions?
How do I get a realistic basement budget?
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