Basement Conversion Cost London 2026: Complete Price Guide by Type and Size

Basement conversion costs in London vary hugely — from £54,000 inc. VAT (£45,000 + VAT) for a simple existing cellar refit to over £300,000 for a full new basement excavation in Central London. Understanding the cost drivers before starting your project is essential because basement conversions have the highest cost variability of any home extension type. This guide gives you accurate 2026 basement conversion costs by type, size and specification, plus the additional cost items you need to budget for beyond construction. LCCL Construction has delivered basement conversions across London and Hertfordshire.

LCCL Construction — specialist basement conversion contractors across London

How Much Does a Basement Conversion Cost in London in 2026?

Basement conversion costs in London range from £54,000 inc. VAT (£45,000 + VAT) to £300,000+ depending on scope. The average project cost is £180,000 – £264,000 inc. VAT (£150,000 – £220,000 + VAT). This range covers everything from simple cellar refits through to complex new basement excavations with underpinning. The specific cost depends primarily on whether excavation and underpinning are required, the size of the finished basement space and the specification of the finished interior.

Basement Conversion Cost by Type

Existing cellar conversion (no excavation): £54,000 – £114,000 inc. VAT (£45,000 – £95,000 + VAT). This is the lowest-cost option because the structure already exists. Works include waterproofing, insulation, plastering, electrics, plumbing, staircase and habitable room finishes.

Basement lowering (with underpinning): £114,000 – £216,000 inc. VAT (£95,000 – £180,000 + VAT). Digs down within the existing cellar to increase ceiling height. Requires underpinning of existing foundations, waterproofing and complete internal finishing. This is the middle option — significant complexity but no need to dig outside the original footprint.

New basement excavation (full dig): £216,000 – £360,000 inc. VAT (£180,000 – £300,000 + VAT)+. Digs out an entirely new basement below the existing house. Requires full underpinning of external walls, extensive party wall agreements, waterproofing, structural engineering and complete internal fit-out. The most expensive but delivers the most space. Central London locations can exceed £350,000 for premium fit-outs.

Basement Conversion Cost Per Square Metre in London 2026

Existing cellar conversion: £3,400 – £5,000 per square metre inc. VAT (£2,800 – £4,200 + VAT).

Basement lowering: £4,800 – £6,600 per square metre inc. VAT (£4,000 – £5,500 + VAT).

New basement excavation: £5,800 – £7,800 per square metre inc. VAT (£4,800 – £6,500 + VAT).

Basements cost significantly more per square metre than rear or side extensions because of excavation, underpinning, waterproofing and structural complexity. Basements are subject to 20 per cent VAT.

What Affects Basement Conversion Cost in London?

The main factors are: type of basement work (cellar refit vs lowering vs new excavation), total floor area, depth of excavation, ground conditions and water table depth, number of adjoining neighbours triggering party wall agreements, planning permission complexity (specific borough policies vary significantly), specification of internal finishes, external light wells or garden basement access, and location within London. The final figure depends on postcode, access and site conditions — the instant estimate applies your area.

Additional Cost Items to Budget For

Beyond construction: planning permission and pre-application costs £2,400 – £9,600 inc. VAT (£2,000 – £8,000 + VAT). Structural engineer design £4,800 – £18,000 inc. VAT (£4,000 – £15,000 + VAT). Basement Impact Assessment (required by many London boroughs) £3,600 – £9,600 inc. VAT (£3,000 – £8,000 + VAT). Party wall surveyors £2,000 to £15,000+ per adjoining owner. Structural warranty £4,200 – £9,600 inc. VAT (£3,500 – £8,000 + VAT). Contingency 10-15 per cent of construction cost is essential due to unforeseen ground conditions. Total additional costs typically add £25,000 to £75,000 on top of construction.

What Is Included in Basement Conversion Cost?

LCCL Construction basement conversion costs typically include: measured building survey, architectural design, planning permission application, building regulations, structural engineer calculations and Basement Impact Assessment, party wall notices and initial correspondence, excavation and spoil removal, underpinning of existing foundations, structural waterproofing to BS 8102:2022 standards including Type A barrier, Type B structural and Type C drained cavity systems as required, superstructure and first floor slab, staircase, first fix electrics and plumbing, sump pumps and drainage, insulation, plastering, second fix and decoration. Kitchen units, bathroom sanitaryware and lighting fittings are typically client-supplied.

How Much Value Does a Basement Conversion Add?

Basement conversions typically add 130 to 220 per cent of construction cost in property value. In premium Central London locations (Kensington, Belgravia, Notting Hill, Hampstead, Chelsea) basement conversions can add £2,500 to £5,000 per square metre in property value — often exceeding the construction cost significantly. See our full basement conversion guide and ROI of a home extension guide.

Frequently Asked Questions About Basement Conversion Cost

How much does it cost to convert an existing cellar in London?

From £54,000 inc. VAT (£45,000 + VAT) for a small cellar refit to £95,000 for a larger cellar with premium finish. This is significantly lower-cost than a new basement excavation because the structure already exists.

Why do basement conversions cost so much more than rear extensions?

Excavation, underpinning of foundations, structural waterproofing, extensive party wall agreements and more complex planning all add significant cost. A rear extension builds up onto existing solid ground — a basement digs down against water pressure and existing structures.

How long does a basement conversion take?

Existing cellar: 12-20 weeks on site. Basement lowering: 20-32 weeks. New excavation: 32-52 weeks. Design and planning add 6-12 months upfront.

To get a free detailed estimate call LCCL Construction on 020 8037 7332 or get an instant quote online.