How much does a basement conversion cost? Our quantity surveyor answers your questions

In many cities in the UK, space is at a premium, which means you can’t add an extension to the rear or side of your property. So another option worth considering to create additional space is a cellar or a basement conversion. 

If you have a basement you already have a room to work with, so upgrading this space will be less expensive than creating an above-ground extension. Plus, it's the perfect way to add value to your home with a new bedroom or social space. 

Once you have a good idea of the key basement conversion costs and the various factors that can have an impact on the overall cost of the space, you can make an informed decision on whether to proceed with your own basement transformation. 

What does a typical basement conversion cost?

A typical basement conversion can cost between £1,200-1,500 m2 depending on the  overall basement design, level of finish and the end use of the basement. 

Some homeowners are content to add an additional bedroom and/or bathroom to their basement, while others go all out with their basement conversion ideas opting for state-of-the-art gyms, home cinemas or wine cellars. For instance, The Wine Cellar Company are able to install a modular below-ground hexagonal wine cellar system, which has the capacity for laying down up to 1,700 bottles (not exactly a straight forward project).  

If you intend on using your basement conversion as a gym, then head height is a key requirement for lifting weights. Additional head height would need to be created by lowering the existing floor, which involves lots of labour and plant, and intensive excavation to reduce the ground level. The pouring of insulated concrete slabs and reinforcement / underpinning of existing walls will push your conversion cost to £2100 - 4,000 per m2.

So, you can already see that the scope and scale of your basement conversion is going to have an impact on the overall costs involved.

Breakdown of key costs

• Existing basement conversion: £1,200-£1,500/m2
• Lowering the floor level and reinforcing the existing cellar: £1,550-£2,100/m2.
• Underpinning and digging a new basement space: £2,100-£4,000/m2.
• Building a new basement beneath the garden: £1,820-£2,350m/2
• Providing a light well / external access costs: between £5,750 and £10,000.
• Structural Engineers' fees: between £1,000 and £1,500.
• Planning application (if required): from £200.
• Building Regulations application fee: £750+
• Party Wall surveys: £1,250 and up (per neighbour).
• VAT: 20% added to any work performed by a VAT-registered contractor and all fees charged by a VAT-registered consultant.

"It's also important to note that the VAT rules for conversion projects can be complex," advises Andrew Jones, Managing Director of The VATMan Ltd. "The VAT rate may depend on the specific details of the project, the build-route and if the house has possibly been vacant for a period of time."

Andrew Jones

Andrew Jones is a Vat Reclaim Adviser and Managing Director of The VatMan Ltd. He operates across the UK, providing expert VAT reclaim services to private individuals. 

Conversion factors

There are a few factors that need to be considered when converting a basement into a habitable space. These could include:

• Tanking / waterproofing walls and floors. 
• Digging down to create more head height.
• Introducing adequate basement ventilation.
• Basement insulation. 

It's important to remember that basement construction and conversions must comply with Building Regulations. These include aspects like damp proofing, insulation, ventilation, and fire escape routes. 

Ground gas protection may also be required. "There may already be some in place, but if alterations to the floor or remedial works take place this may need reviewing, it would also form part of planning/building regulations", says Liam Forde from Basement Information Centre.

The Approved Documents section of the Building Regs contain general guidelines on the performance anticipated of materials and construction work in order to comply with Building Regulations, as well as practical examples and solutions on how to achieve compliance for some of the more prevalent building circumstances. 

What will impact the cost of a basement conversion?

• Planning Permission & Design Fees: In general, transforming an existing residential cellar or basement into a living area doesn't typically require planning permission. This is unless it's turned into a separate unit or if you're adding features like light wells that alter the property's external appearance. Consult your local authority to determine if your conversion is considered permitted development and remember, Building Regulations will always apply.
• Waterproofing and water table: Waterproofing below ground level, often called "tanking," involves applying a waterproof material directly to the structure. The process usually entails a waterproof cementitious render on the walls linked to a waterproof screed on the floor. Alternative methods may include using a sheet membrane, asphalt, or other liquid-applied materials. To withstand the hydrostatic pressure of surrounding water, tanking and making a cellar dry is essential. A basement can be under a lot of pressure from the water table, and if the tanking isn't firmly anchored to the ground, it could fail. The smallest flaw can be quickly exploited by hydrostatic pressure, and if a leak develops, it can be exceedingly challenging to find and fix. There are guides available that give general advice on waterproofing solutions in accordance with BS 8102:2022, the code of practice for protecting below-ground structures from ground water. 

• Structural work: Cast-in-situ concrete is a reliable method for reinforcing basement foundations or walls. Its popularity for residential basements stems from its application ease, adaptability, and cost-effectiveness. For retrofit basements beneath existing structures, in-situ concrete is often the only feasible construction method. A structural engineer is the best professional to advice on the exact structural requirements for your basement, they will also liaise and cross examine any specifications and detailed drawings provided by your basement specialist. The Association of Underpinning Contractors (ASUC) manages an accreditation and insurance scheme for existing structure underpinning, which is most typically required for basement refurbishment or conversions.
• Party Wall Agreements: If your home is terraced or semi-detached, you may want to construct near party walls. If this is the case, you must follow The Party Wall Act of 1996. If you share an adjoining wall with a neighbour then you will most certainly require a Party Wall Agreement. Costs will vary depending on the professional advisers engaged. By way of example, there are Party Wall specialists who can offer the complete service for fees of around £1,250, including managing the Party Wall Award, Party Wall Notices and Schedule of Condition and full management of the process.
• Digging out: To create more head height, consider the need to excavate the existing ground to create more headroom. This is one of the costliest operations to carry out as well as time consuming and difficult due to access issues for labour, plant and materials. Depending what the end use of the basement is, this may not be necessary, which will assist your budget. For most basement conversions I’d advise engaging a Basement Specialist to carry out the works required. They will have encountered your specific needs on many occasions and you can also look at their previous project portfolio.
• Introducing services: If you're converting your basement into a bathroom or WC, it'll need essential services like water, electricity, and possibly gas. Extending your home's existing services to the basement typically costs around £750. However, ensure your current systems don't need an upgrade due to the added living space. Basements can feel damp and musty – even when fully damp-proofed – so make provision for adequate ventilation via an electric inline fan. There can be a lot of existing services in a basement - such as drainage, electric, gas and water pipes - which are required for the home above. Remember to make an allowance in your budget for extending, removing or replacing these services.

• Widening an existing staircase: The existing stairs to your basement may not be wide enough under current Building Regulations, they could even be constructed from brick or stone as opposed to a traditional timber stairs. Discuss with Building Control what would be suitable prior to installing or widening an existing staircase. It is possible to add an additional string to the side of an existing timber stairs and fit new risers and treads, however it may be just as cost effective to remove and replace with a new staircase. Discuss your modern staircase ideas with a stair contractor, who will be up to speed with Building Regs and will also have installed stairs in previous basement conversions.
• Introducing new windows or a lightwell: There are many methods of lighting a basement, and each will have different cost implications. The cheapest upfront option is to simply extend your existing electricity supply to include for ceiling and/or wall lights. Although remember natural light is free, whereas in the long-term artificial light will cost money to run. Another budget-friendly option is to install sun pipes that will illuminate a dim area of the basement and the interior space. More expensive options include options such as glazed floors, light wells or even an internal courtyard. 

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