How much does a basement conversion cost in the UK?

A basement conversion costs anywhere from around £30,000 for a simple cellar fit-out to £450,000 or more for a new basement dug beneath a London terrace. The figure depends entirely on which of three projects you're actually undertaking — and most cost guides never make that distinction clearly.

This guide breaks down the three types, what each costs and why, what's included in a quote, and what pushes the final figure up.

If you're in London, our basement conversions page explains how Beams can help.

The three types of basement project — and why cost varies so much

The huge range of figures quoted online comes down to one thing: guides describe three very different projects as if they were one. Before any estimate means anything, you need to know which you have.

Cellar conversion
You already have a cellar. The structure exists — what's needed is waterproofing, ventilation, insulation, lighting, electrics and proper access to make it habitable. This is the simplest and most affordable route, and often proceeds without full planning permission (increasingly subject to borough policy — see planning below).

Typical cost: £1,200–£2,500 per sqm
Typical total for a 30–40 sqm London cellar: £40,000–£80,000

Basement lowering and underpinning
Your cellar exists but lacks head height — you need at least 2.1–2.3 metres for a comfortable habitable room. The floor is lowered and the walls underpinned. Significantly more complex than a fit-out, and almost always requires both a structural engineer and planning permission.

Typical cost: £1,900–£3,500 per sqm

New basement excavation
No basement exists, so one is dug from scratch — underpinning the existing foundations, excavating and removing soil (often by hand on tight London streets), building new retaining walls, waterproofing and laying a new reinforced concrete floor slab. A major structural project, almost always needing planning permission, and among the most expensive residential work there is.

Typical cost: £4,000–£7,500 per sqm, and more for deep or prime-London digs
Typical total for a new 40–60 sqm London basement: £160,000–£450,000+

All figures are indicative ranges excluding VAT, professional fees and interior fit-out. Your builder confirms a fixed price before any commitment.

What's included in a basement conversion cost

A typical quote covers:

• Structural work — underpinning, excavation, new floor slab, steel beams and retaining walls where needed.
• Waterproofing — the most critical element of the whole project, designed and installed to British Standard BS 8102:2022. Our basement conversions page explains the waterproofing systems in full.
• First fix — electrics, plumbing, drainage and ventilation.
• Staircase — access from the ground floor down.
• Basic finishing — plastering and screeding, ready for final fit-out.

What drives basement conversion costs higher

Several factors push costs above the typical ranges.

Ground conditions and water table
London's clay soils and variable water table directly affect the complexity and cost of waterproofing. Properties in flood-risk zones or near rivers need more robust systems and sump-pump specifications. A CSSW (Certificated Surveyor in Structural Waterproofing) assesses conditions before the system is specified.

Depth of excavation
The deeper you go, the more it costs — each additional 100mm of depth can add £5,000–£10,000. If your cellar already has 2.3m of headroom, accepting that height rather than digging for more is one of the most effective ways to control costs.

Access constraints
Narrow streets, no parking for machinery, hand excavation rather than mechanical digging, and restricted working hours all add to labour. Central London projects regularly cost more than outer London ones.

Lightwell
A lightwell — a below-ground recess that brings in natural light and ventilation — needs extra excavation, waterproofing and almost always planning permission. Budget £10,000–£25,000, depending on size and complexity.

Party Wall process
The Party Wall Act 1996 applies to almost every London basement project. Surveyor fees of £1,500–£3,000 per neighbouring property are typical, and complex cases cost more.

Fit-out specification
A basic shell-and-core finish costs far less than a high-specification fit-out with bespoke joinery, a wet room, underfloor heating and premium finishes.

What's not included in basement conversion quotes

Build your full budget around these too:

• VAT — add 20% to all net figures.
• Professional fees — typically 10–15% of build cost across the whole team. A structural engineer runs £2,000–£5,000; building control fees £1,500–£2,500; architect or planning consultant fees on top where planning is required.
• Party Wall surveyor fees — £1,500–£3,000 per neighbour.
• Lightwell — £10,000–£25,000 if you're adding one.
• Interior fit-out — flooring, decoration, bathroom and storage.
• Contingency — 15–20% is sensible on basement projects, more than almost any other renovation. Ground conditions, water table and the state of existing foundations can all spring surprises that are difficult to anticipate.

A basement conversion typically adds 10–20% to your property's value

In prime London, where space is at a premium, the uplift can be higher still. Because it adds usable floor area without changing your home's footprint or eating into your garden, a basement is one of the most space-efficient ways to make a property both more valuable and more liveable.

The conversions that add most are those creating an extra bedroom, home office or self-contained annexe — the spaces buyers pay a premium for.

For how long the work takes, see our guide to how long a basement conversion takes. To get your project off on the right foot, read how to plan a basement conversion.