Basements and below-ground space, answered plainly.

A new basement dig typically runs £500k to £900k. Converting an existing basement runs £180k to £300k.

Excavation is only part of the cost. We also price engineering, temporary works, access, waterproofing and drainage.

Neighbour interfaces and the finished fit-out add further cost.

Use our build cost calculator for a range built from our own rate card.

Our basement cost page sets out the 2026 figures in full.

Next Ask us for separate allowances for shell, services and finished accommodation.

Suitability depends on planning, ground and water conditions, existing foundations, neighbours, utilities and access. Feasibility work sets out the main constraints before we develop a detailed layout.

Next We start with engineering and planning input for your actual site.

This needs planning and engineering review specific to your property. Trees, retained planting, drainage, access and the effect on neighbours all need consideration.

Next We check current local policy rather than assume a permitted size or depth.

Deeper excavation can significantly change the structural, water, access and planning risks. Existing basement drawings and approval records matter a great deal here.

Next We carry out a feasibility assessment before developing accommodation plans.

It is a coordinated specialist design for the intended use and ground conditions. The construction method is designed in alongside it.

Structure, joints, penetrations, drainage and maintenance access all need to work together.

Next We identify the waterproofing designer, design responsibility and warranty terms.

That depends on the drainage and waterproofing design.

Where pumps are needed, we assess redundancy, alarms and power-loss response. We also plan ongoing access for servicing.

Next We explain how water is managed normally and if a failure happens.

Engineering, temporary works, sequencing and monitoring are central to our approach. We never promise zero movement just because monitoring is installed.

Next We provide the monitoring plan and how trigger levels lead to action.

Access, conveyors, storage and collection are all planned around the street. The property itself shapes these arrangements too.

These logistics affect cost, programme and disruption to neighbours.

Next We show you the site logistics strategy before agreeing a programme.

Room proportions, light, ventilation, acoustics and the stair connection matter as much as finishes. We plan lightwells and rooflights alongside structure, privacy and water management.

Next We set a daylight and comfort brief for each proposed room.

Each use needs its own structural, acoustic, ventilation and environmental requirements. A wine room and a gym should not share assumptions about temperature or humidity.

Our Chelsea townhouse basement, 1,200 sq ft, holds a mirrored gym.

Next We define the equipment and operating conditions you need before design starts.

A pool or spa needs specialist feasibility for loads, structure and water treatment. Humidity control, plant and maintenance access all need checking too.

We treat it as a separately defined specialist package.

Next Send us the space and budget so we can assess the specialist team needed.

We define the boundary explicitly for every package. Structure, waterproofing, drainage, slab, stairs, services and finishes can be allocated differently.

A shell price should never be compared with a fully finished living floor.

Next We agree a package inclusion schedule with clear interfaces.

We follow the actual waterproofing, drainage, pump and ventilation maintenance schedules.

Keep inspection access clear and keep service records. Warranty terms can depend on this maintenance being kept up.

Next We include basement systems in the handover and aftercare plan.