The short answer
A cellar conversion finishes space the house already has, while a full basement dig-out creates new space by excavating beneath the property, and the difference in cost, work and risk is large. A cellar conversion mainly involves waterproofing, insulation and fit-out, often around £1,000–£2,000 per square metre, and a programme of weeks. A dig-out, by contrast, requires underpinning, excavation, structural design, party-wall agreements and full waterproofing, typically £3,000–£5,000+ per square metre over many months. Both can add value, but the dig-out is a major structural project suited to high-value areas or homes with no usable cellar. Choosing between them comes down to what your property already has below ground.
These two projects sound similar but differ enormously in scope, cost and risk. The sections below explain what each involves, how they compare, and how to tell which one your property actually needs.
At a glance
- Cellar conversion~£1,000–£2,000/m²
- Full dig-out~£3,000–£5,000+/m²
- Cellar timescaleWeeks
- Dig-out timescaleMany months
- Dig-out needsUnderpinning + party wall
What each project involves
A cellar conversion takes an existing below-ground space, an old coal cellar, a storage basement, a vaulted cellar, and turns it into usable accommodation. The structure is already there, so the work focuses on making it dry, warm and habitable: applying a waterproofing system (tanking or cavity drainage), insulating, adding services, improving ventilation and light, and fitting out. There may be modest underpinning if the floor is lowered slightly, but the building is not being structurally re-founded.
A full basement dig-out creates new volume where little or none existed, by excavating beneath the house and lowering the floor level, or digging out entirely. This requires underpinning the existing foundations bay by bay, removing large quantities of spoil, casting new structural walls and slab, and installing a robust waterproofing system, all engineered by a structural designer. It is a heavy civil-engineering job carried out beneath an occupied or vacated home.
Cost, time and disruption compared
Because the dig-out builds new structure and the conversion finishes existing space, they sit far apart on every practical measure. The conversion is cheaper, quicker and far less disruptive; the dig-out is a major investment of money and time with significant disruption and risk. The table below summarises the differences.
| Factor | Cellar conversion | Full dig-out |
|---|---|---|
| Cost/m² | £1,000–£2,000 | £3,000–£5,000+ |
| Time | Weeks | Many months |
| Structural work | Minimal | Underpinning, new structure |
| Party-wall agreement | Sometimes | Usually |
| Disruption | Lower | High |
Indicative comparison for guidance. Figures vary with access, ground conditions and finish.
Which does your property need?
The right project depends on what is already beneath your house. If you have a sound cellar with adequate headroom, a conversion is usually all you need, and it is the more economical, lower-risk route to habitable space. If your cellar is too shallow to stand up in comfortably, you may need a partial dig-out to lower the floor, which sits between the two in cost and complexity. If you have no cellar at all but want basement space, a full dig-out is the only option, and it should be approached as a serious structural project.
Value enters the decision because the dig-out's high cost only makes sense where local prices reward the extra floor area, typically in high-value urban locations, or where the household genuinely needs the space and will stay to enjoy it. A conversion, being cheaper, has a far wider range of situations where it pays. Whichever route applies, both require proper waterproofing design, building-regulations compliance and, for anything involving a shared wall, a party-wall agreement. The key is to be clear from the outset which project you are undertaking, because budgeting and planning for a conversion when the property actually needs a dig-out, or vice versa, is where costs and timescales spiral. An honest assessment of the existing basement, ideally with a structural engineer, settles the question before you commit.
Frequently asked questions
What's the difference between a cellar conversion and a dig-out?
A cellar conversion finishes existing below-ground space with waterproofing, insulation and fit-out. A dig-out excavates and underpins to create or deepen basement space, making it a major structural project costing far more.
Why is a dig-out so much more expensive?
It involves underpinning the existing foundations, excavating and removing large amounts of spoil, casting new structural walls and slab, and installing robust waterproofing, all engineered and carried out over many months, often with a party-wall agreement.
How do I know which one I need?
It depends on your existing cellar. A sound cellar with good headroom usually needs only a conversion; a shallow cellar may need partial floor lowering; no cellar at all means a full dig-out if you want basement space. A structural engineer can advise.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.