When you cannot build out or up, you can build down. A basement is the most complex and expensive space you can add to a home, and one of the most rewarding when it is engineered properly.
Everything about a basement, cost, complexity and risk, depends on whether you are converting an existing cellar or digging a new one.
| Type | What it involves, and typical cost |
|---|---|
| Cellar conversion | Making an existing cellar habitable: waterproofing, insulation, services, ventilation and a proper stair. Around £1,200 to £2,500 per square metre. Simpler and lower risk. |
| New basement dig | Excavating new space below the house: underpinning, retaining walls, a reinforced slab and waterproofing. Around £3,000 to £5,000 per square metre, and higher in London. A major structural project. |
A new basement is a serious piece of structural work, and it is led by an engineer from the start.
Excavating below an existing house means underpinning the foundations, digging out and removing the soil, building new retaining walls to hold back the ground, and casting a new reinforced concrete floor slab. A structural engineer designs the sequence, and the ground conditions and the water table dictate the approach. This is specialist work, and it is not the place for a general approach or an inexperienced team.
The single most important detail in any basement. Get it wrong and the space is unusable; putting it right later is ruinous.
Basements are kept dry by one or more of three approaches, usually specified by a waterproofing designer:
Serious basements often combine two systems for belt and braces, with proper guarantees. The water table, ground and intended use decide the design.
What separates a proper room below ground from a glorified store is light and a generous way in.
Natural light transforms a basement. Lightwells (expect to add roughly £15,000 to £25,000 each), sun pipes, and glazed links or a double-height void bring daylight down and make the space somewhere you actually want to be. A proper, generous staircase matters just as much: a basement reached by a mean stair always feels like a cellar.
A new basement is one of the most legally sensitive projects you can undertake, especially near neighbours.
A new basement dig almost always needs planning permission and is highly sensitive under the Party Wall Act, because the excavation and underpinning affect neighbouring foundations. Expect party wall awards, and in some areas specific basement planning policies. A simple cellar conversion, which does not change the external appearance, often needs no planning at all. Early professional advice is essential.
Basements reward patience and a generous budget. The ground rarely does exactly what anyone expects.
A cellar conversion typically takes around 6 to 10 weeks. Excavating a new basement, with underpinning and waterproofing, is a much bigger undertaking, often 4 to 6 months or more. Budget generously and keep a real contingency, because ground conditions, water and access frequently produce surprises. Done properly, though, a basement adds space where none seemed possible, and can be some of the finest accommodation in the house.
Waterproofing is the one area where cutting cost is a false economy. A basement is only as good as the system keeping it dry, and correcting a failed system after the space is finished is enormously expensive and disruptive. Specify it properly, and guarantee it.
We deliver basements and below-ground space with the structural, waterproofing and party wall detail they demand, coordinating the engineer and waterproofing designer and building to their specification. We have carried out reinforced concrete basement work on our own developments, and we will be honest with you about the cost, the programme and the ground risk before you commit.
General guidance for 2026, written for homeowners in Surrey and Hampshire. It is a summary, not professional advice; confirm the details for your specific project.
A basement is specialist work. Tell us your site and your ambition, and we will give you a straight view on what is realistic.