Calculating your final basement conversion costs will largely depend on whether you're converting a basement, adding a new one, or upgrading an existing cellar.
If you have an existing underground space, then this will usually cost less than creating one from scratch, although you'll still need to factor in the necessary waterproofing and structural measures to make sure it's fit for purpose.
Factor in a new basement or the extension of an existing one and costs will soon rise. But where will your budget go and are there ways of reducing your potential spend? Quantity surveryor, Tim Phillips helps shed a light on basement conversion costs.
What does a typical basement conversion cost?
A typical basement conversion can cost between £1,275-£1,600 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.
If you intend on using your basement conversion for ideas such as a gym, then additional head height (over and above standard ceiling height) will be a key requirement for lifting weights. This 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. In this instance, the pouring of insulated concrete slabs and reinforcement / underpinning of existing walls will push your conversion cost to £2,250-£4,250 per m2.
It's therefore quickly clear that the scope and scale of your basement conversion is going to have an impact on the overall costs involved, which is why any average cost should perhaps be considered a baseline, rather than a fixed sum.
Breakdown of key costs
- Existing basement conversion: £1,275-£1,600/m2
- Lowering the floor level and reinforcing the existing cellar: £1,650-£2,250/m2
- Underpinning and digging a new basement space: £2,250-£4,250/m2
- Building a new basement beneath the garden: £1,950-£2,500m/2
- Providing a light well / external access costs: £6,100-£10,500
- Structural engineer fees: £1,100-£1,600
- Planning application (if required): from £250
- Building regulations application fee: £795+
- 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."
Conversion factors impacting costs
There are numerous factors that need to be considered when converting a basement into a habitable space, all of which will play a part in the final sum you pay.
And although in the same way that your extension costs will vary depending on the design, professional fees and your location in the UK, there are key differences in the way your new space is constructed, that can cause basement conversion costs to rise.
1. 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.
2. 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 design, 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.
3. Digging out
To create more head height, you may need to consider excavating the existing ground. This is one of the costliest operations to carry out, as well as being 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, or choosing an architect who specialises in basement conversions. They will have encountered your specific needs on many occasions and you can also look at their previous project portfolio.
4. 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 £795. 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 basement 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.
5. 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 your building control officer 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 the regs and will also have installed stairs in previous basement conversions.
6. 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 architectural glazing options such as glazed floors, light wells or even an internal courtyard.
7. Additional surveys
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 the regs, as well as practical examples and solutions on how to achieve compliance for some of the more prevalent building circumstances.
8. 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.
But, 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.
FAQs
Are cellar conversion costs the same as basement costs?
"A cellar conversion is one of the most technically demanding forms of home improvement," says Paula Higgins, chief executive at HomeOwners Alliance. "Before committing, homeowners should first check whether the space is genuinely suitable: adequate ceiling height, safe access, and the potential for natural light all make a huge difference to how usable the finished room will feel.
"Costs vary depending on complexity, but as a guide homeowners should expect to pay around £900–£1,400 per square metre for a standard cellar conversion, with higher costs if underpinning, new stairs or lightwells are required," continues Paula.
"Done well, a cellar conversion can add flexible, comfortable space and boost a home’s appeal; done badly, it can create long-term maintenance issues that outweigh the benefits."
Will a basement conversion add value to your home?
Although above-ground space is typically more valuable than below-ground space, converting your basement is thought to add up to 15% to the value of your home. A local estate agent will be able to confirm the added value post conversion, so you can work out the cost against value prior to commencing works.
Of course, once you've got the structure in place, you'll also need to factor in the fixtures and fittings too, so if you're planning on using your new basement as a kitchen, find out how much of your budget will be needed for new kitchen costs.
Or, if you think your available funds might be best spent on another type of extension, see how much loft conversion costs could set you back instead.
