How much does a basement conversion cost per m2 in the UK?

Costs vary significantly by type: a simple tanking conversion (where existing headroom is adequate) costs £800-£1,200/m². Lowering the floor to gain headroom (which involves underpinning) costs £1,500-£2,000/m². A brand new basement excavation costs £2,500-£3,500/m². London and the South East tend to be at the higher end. These figures include waterproofing, insulation, electrics, and basic finishes.

Do I need planning permission for a basement conversion?

Converting an existing cellar or basement usually falls under permitted development rights, provided the work does not extend the footprint of the building. However, Building Regulations approval is always required for structural work, waterproofing, fire safety, ventilation, and drainage. In Conservation Areas or with Listed Buildings, planning permission and/or Listed Building Consent will be needed.

What is the minimum headroom for a basement conversion?

Building Regulations Approved Document K recommends a minimum headroom of 2.0m for habitable rooms, though 2.1m is considered the practical minimum. Most conversion specialists aim for 2.3-2.4m to allow for floor buildup (insulation, screed, floor finish) and ceiling finishes. If existing headroom is insufficient, underpinning to lower the floor is needed.

Is a basement conversion worth the investment?

In high-value areas (London, South East, major cities), basement conversions typically add £2,000-£4,000 per square metre to property value. Where the conversion cost is £1,500-£2,500/m², the return on investment can be significant. In lower-value areas, the cost may approach or exceed the value added. A local estate agent can advise on likely value uplift.

How long does a basement conversion take?

A simple tanking conversion takes 4-8 weeks. Lowering the floor with underpinning takes 12-20 weeks. A new-build basement takes 16-30 weeks depending on size and complexity. Underpinning work is sequential (you cannot do all sections at once for structural stability), which extends the programme significantly.

Basement Conversion Calculator — Free Basement & Cellar Tool

How We Calculate This

This calculator estimates basement conversion costs based on floor area and conversion type. Costs are derived from UK industry data and include allowances for waterproofing, structural work, services, and basic finishes.

Conversion types

Simple tanking (£800-£1,200/m²): Existing cellar with adequate headroom. Work involves waterproofing (tanking), insulation, new floor, electrics, lighting, and finishes. No structural underpinning needed.

Lowered floor (£1,500-£2,000/m²): Existing cellar with insufficient headroom. Requires sequential underpinning of walls and a new deeper floor slab, plus all tanking and finishing work.

New-build basement (£2,500-£3,500/m²): Full excavation beneath an existing or new building. Requires structural concrete walls and slab, comprehensive waterproofing, and all associated works. The most expensive but adds significant space.

Additional costs

The calculator adds contingency (typically 10-20% for basement work due to unknown ground conditions) and professional fees (architect, structural engineer, party wall surveyor — typically 10-15% of construction cost).

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.