Full basement conversion on an average UK home costs around £43,000 and the prices can range between £20,000 to £95,000. For example, the average cost for a basic cellar conversion is between £20,000 and £35,000 and for a larger property with full excavation, complex waterproofing and premium finishes the total can well past £70,000 and sometimes up to £95,000 or more.
A basement conversion quote is not just a simple number as there are a bunch of different costs thrown together to get the final price. Here is what makes up that total.
Size of basement
Most quotes are per square metre so the bigger the basement the bigger the bill. A small cellar might be between 15 and 25m², a typical basement 25 and 40m² and a larger excavated space 40 and 80m² or more. So obviously, more floor area will translate to more costs.
Existing vs New Excavation
This is usually the biggest factor that decides the final cost. Converting an existing cellar is significantly cheaper than excavating a new basement from scratch:
- Existing cellar conversion: £800-£1,500/m². The most budget-friendly option, typically involves waterproofing, insulation and finishing.
- Basement excavation (under house): £1,500-£3,000/m². Requires underpinning, structural support and extensive waterproofing.
- Basement excavation (under garden): £2,000-£4,000/m². Most expensive due to lightwell construction and complex drainage.
Waterproofing System
The type of waterproofing system you choose has a major impact on cost:
- Type A (Tanking): £50-£120/m². Cementitious coating applied to walls and floor. Budget option but can fail under hydrostatic pressure.
- Type C (Cavity Drain): £80-£180/m². Membrane system with drainage channels and pumps. Most reliable for UK conditions.
- Combined System: £120-£250/m². Uses both tanking and cavity drain for maximum protection. Recommended for high water table areas.
Labour
A basement conversion team usually consists of 3 to 6 specialists including builders, waterproofing contractors, electricians and plumbers. Labour is usually 30-40% of the total cost and daily rates vary significantly by area. If you are in London or the South East then you will have to pay premium rates.
Structural Work
Underpinning, steel beam installation and floor lowering are often required for basement conversions. Underpinning alone can cost £1,500-£3,000 per linear metre. If the existing ceiling height is under 2.2m, lowering the floor may be necessary which adds significant cost.
Finish Quality
The level of finish dramatically affects the final price:
- Standard finish: Basic plaster, carpet, standard lighting. £300-£500/m².
- Premium finish: Underfloor heating, engineered wood, quality fixtures. £500-£800/m².
- Luxury finish: Bespoke joinery, smart home systems, high-end materials. £800-£1,500/m².
Region
As mentioned earlier, labour costs in London and the South East are always above the national average — typically 30-50% more. The Midlands, the North of England, Scotland and Wales are likely to be at or below the national average.
On paper, many basement conversion quotes seem simple but in practice there are many things that are routinely missed, not properly mentioned or added later, and it is better to understand them before you compare quotes or agree to anything.
Party Wall Agreements
If you share walls with neighbours (terraced or semi-detached), you may need Party Wall Agreements before work can begin. This process can take 2-3 months and cost £700-£1,500 per neighbour. Some builders include this in their quote, others don't — always check.
Building Regulations and Inspections
Building control approval is mandatory for all basement conversions. Fees vary by council but typically cost £500-£1,500. Some contractors include this, others add it as an extra. You also need structural calculations from a chartered engineer (£800-£2,000).
VAT
20% VAT is included in most quotes but not all make this clear at the beginning. One quote might be £5,000 cheaper than another but that could be simply because it doesn't include VAT and isn't a better deal. Always confirm about VAT explicitly.
Insurance and Warranties
Specialist basement insurance-backed guarantees (like PCA or NHBC) cost £500-£1,500 but are essential. Standard home insurance often won't cover basement work. Make sure your contractor provides a meaningful warranty — 10 years is the industry standard for waterproofing.
Planning Permission
Most cellar conversions fall under permitted development and don't need planning permission. However, if you are excavating a new basement, lowering the floor level significantly, or creating a separate dwelling unit, you will need full planning permission. Listed buildings and conservation areas have additional restrictions. Always check with your local council before starting.
All price figures on our website are real and honest estimates based on current 2026 UK market rates but these are just estimates. Two basements of the same size can still have a huge difference in cost when you consider ground conditions, access, structural requirements and finish quality. The only way to know what your particular basement is going to cost you is to have it properly assessed.
That's where our platform comes in. Read our cost guide thoroughly, get an estimate using our real-time calculator and then get quotes from local basement specialists to get an exact cost.