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Conversions

Basement and cellar conversions

Dry, insulated rooms below ground, from cellar conversions to new basements. Designed, approved and built by SK Residential, with a fixed price in writing for the agreed specification.

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What is a basement conversion?

A basement conversion turns an existing cellar into dry, usable rooms, or excavates a new basement under a house. Converting an existing cellar is far simpler and cheaper than digging a new one, and often does not need planning permission.

Converted basement family room with lightwell glazing and built-in joinery

Everything below ground depends on keeping water out. We design the waterproofing system (usually a cavity drain membrane with a pump, or tanking) to the relevant British Standard guidance, then add insulation, ventilation, light and an escape route. New basements involve underpinning and a party wall process and are designed with a structural engineer.

Typical time on site
6 to 30 weeks
Guide price
£1,900 to £6,600 per m²
Building regulations
Required

What is included?

  • Damp and structural survey
  • Waterproofing design and installation
  • Underpinning or lowering the floor where needed
  • Lightwells and windows for daylight and escape
  • Ventilation, heating and electrics
  • Finished floors, walls and ceilings

How much does a basement conversion cost?

Converting a 25 m² cellar typically costs around £50,000 to £75,000 including VAT.

Basement and cellar conversionsGuide price incl. VAT
Existing cellar conversion£1,900 to £3,000 per m²
New basement excavation£4,200 to £6,600 per m²

Unit: per m² of floor area. Guide ranges only; your price is fixed in writing after the survey.

Use the guide price tool for a figure based on your size and specification, or see all guide prices.

Does a basement conversion need planning permission?

England

Converting an existing cellar with no external changes usually does not need planning permission in England. Digging a new basement or adding a lightwell often does, and many councils have basement policies. Listed buildings need consent.

Wales

In Wales an existing cellar conversion usually does not need planning permission; new excavation and lightwells may.

Scotland

In Scotland new basements and external lightwells normally need planning permission; all basement work needs a building warrant first.

Northern Ireland

In Northern Ireland new basements and external changes normally need planning permission; building control approval is needed.

What about building regulations?

Building regulations approval is needed for ceiling height, waterproofing, ventilation, fire escape and structure.

What should I think about before starting?

  • Groundwater levels drive the waterproofing design and cost.
  • New basements nearly always involve the Party Wall Act in England and Wales.
  • A pump for the drainage system needs a battery back-up.
  • Headroom of about 2.1 m or more is needed, which may mean lowering the floor.

How does the project run?

  1. Tell us about the project. Call, use the guide price tool or send the form with a few photos and a postcode.
  2. Free site visit. We measure up, look at access, drains and the structure, and talk through options and budget.
  3. Design and fixed price. You get drawings and a written price for the agreed specification before anything is ordered.
  4. Build and hand over. Our teams build to the programme, building control inspects, and you get the completion paperwork at the end.

Questions about basement and cellar conversions

How much does a basement conversion cost?

Converting an existing cellar costs about £1,900 to £3,000 per square metre including VAT. A new excavated basement is around £4,200 to £6,600 per square metre.

Does a cellar conversion need planning permission?

Usually not if nothing changes outside. New basements and lightwells often do.

How do you keep a basement dry?

With a designed waterproofing system, usually a cavity drain membrane draining to a pump, sometimes combined with tanking.

How long does it take?

About 6 to 12 weeks for a cellar conversion and 16 to 30 weeks for a new basement.

Will my neighbours be affected?

New basements usually need party wall agreements. We handle the notices and surveyors.

Related projects

Guides

Where do we build basement and cellar conversions?

We take on basement and cellar conversions across England, Wales, mainland Scotland and Northern Ireland. Choose your area:

Planning an extension, loft or new home?

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