Having a basement or a cellar gives you the perfect solution to creating more space in your home, without having to move.
It’s a big investment, but installing a basement or cellar does add value to your property. You can use your converted basement to create an entertainment space, a playroom for the kids or a teenager’s den.
Many householders move their kitchens downstairs. They convert their basement into a kitchen or diner while freeing up more space above.
The costs of a cellar conversion are similar to a loft conversion, but you’ll likely create more floor space with a cellar conversion.
If you decide to excavate under your garden, you can make yourself an even larger space.
In this guide, we’ll tell you about the average costs of converting your cellar or basement, what affects the costs of a basement conversion, how you can save money, what work is involved in a basement conversion and how you can find and hire a builder to do the job for you.
Finally, we’ll answer some frequently asked questions.
How Much Does a Cellar Conversion Cost?
The table below gives you an idea of the basic costs of a basement conversion and the type of costs to expect. Later in our article, we go into more detail and explain some of the options available.
|Type of Job||Estimated Cost||Time Required|
|Basement conversion||£1,000 to £1,800 per square metre||Three to four weeks|
|Basement conversion with added ceiling height||£1,500 to £2,500 per square metre||Four to six weeks|
|Basement conversion with added ceiling height and underpinned garden extension||£2,000 to £3,000 per square metre||Two to three months|
|Basement conversion with added ceiling height and underpinned garden extension (London only)||£4,000 to £6,000 per square metre||Two to three months|
|Adding a lightwell or outside access||£3,000 to £9,000 each||Two weeks|
|Surveyor||£500 to £1,500||N/A|
|Structural engineer||£1,000 to £1,500||N/A|
|Architect||3 to 7 per cent of build cost||N/A|
|Planning application||£202 to £291||Two months|
|Building regs application||£750 and up||N/A|
|Party wall agreement||£700 and up||N/A|
|Skip hire||£90 to £390 per week||N/A|
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What Affects the Cost of a Basement Conversion?
Several factors will affect the cost of a basement conversion and the final price will vary from household to household, depending on what exactly each individual wants to be done and the size of their property.
Complexity of the Job
Whether you have a straightforward cellar conversion or you choose a more complex basement conversion will impact the final cost of the job. A basement conversion could include excavating to make the basement larger.
Let’s take a look at two examples.
One homeowner may want to convert an existing cellar of 18 square metres. The costs for this are likely to include:
- Hiring a surveyor and structural engineer
- Obtaining planning permission and building regs approval
- Skip hire
- Hiring the builder
For this type of conversion, the basic costs are around £20,500. This price excludes any costs relating to a party agreement. It also doesn’t include flooring, lighting, plumbing and electrics.
Another homeowner would like to convert an existing cellar of 18 square metres to make a basement of 27 square metres. For a job this size, the cost will include:
- Hiring a surveyor and structural engineer
- Obtaining planning permission and building regs approval
- Skip hire
- Hiring the builder
- Party wall agreement
- All fitting costs such as lighting, plumbing and electricity
For this type of conversion, you could expect to pay around £58,100.
In London where costs are higher this type of conversion, without the fitting costs will be in the region of £113,800.
These two examples demonstrate just how much the size and complexity of the job could impact the final price.
Size of the Basement
How big your basement is will have an impact on the final cost. If your basement is larger, you can expect the final cost to be higher than someone whose basement is smaller.
More materials and labour will be involved in converting a larger basement.
Quality of the Fixtures and Fittings
How you decide to furnish your basement conversion will also have an impact on the cost.
You may be hoping to cut costs, and so choose lower quality fixtures and fittings. This will keep your total cost down but do keep in mind that lower quality products may need to be regularly replaced and can be subject to wear and tear.
Number of Windows
Adding windows to your basement conversion will add another layer to the work. This means that it will increase the final cost of the job.
You should take a look at your budget beforehand and decide if you want to have windows included in your basement conversion.
If you’re hoping your basement conversion will grant outside access through a back door, this is also another cost.
The tradespeople will need to take into account any structural changes that may need to be made as well as potentially adding steps to the design.
You will also have to pay a planning application to alter a single dwelling. The costs are:
- £206 in England
- £202 in Scotland
- £230 in Wales
- £291 in Northern Ireland
Party Wall Agreement Costs
If your conversion involves a party wall you have to get your neighbour’s agreement before you can do any building work which includes:
- Excavating within three metres of your neighbour’s home or at a lower level than their foundations
- Making holes in a shared wall so that beams or flashing can be inserted
- Excavating within six metres of your neighbour’s home at an angle traversing 45 degrees from the bottom of their foundations.
The cost of a party wall agreement is about £750.
You must send your neighbour a party wall agreement at least two months before the work is due to begin. If they agree they must sign the agreement and return it to you.
If they don’t respond within 14 days of receipt of the agreement, it is assumed they don’t agree.
If they don’t agree, this means your neighbour can appoint a surveyor to investigate the problem. You will need to pay for your neighbour’s surveyor.
The surveyor will collate a report which will go to arbitration. If the report recommends you can go ahead you can then start the work even if your neighbour is unhappy.
This whole procedure could cost between £3,000 and £5,000 or more for two surveyors working between 10 to 20 hours each. Party wall surveyors charge between £150 and £250 an hour.
Rather than sending your neighbour a letter that comes out of the blue, try and make a date to talk to them. Explain exactly what you want to do and how this will have an impact on your neighbour’s property.
Download the government’s booklet about party walls and copy one for your neighbour.
The government booklet also contains letter templates which you could use to send to your neighbour, rather than using a surveyor. Don’t forget to send it registered post.
If there isn’t a dispute with your neighbour your surveyor’s costs will be less. You’ll probably pay between £500 and £1,500 for their services.
Architect and Structural Engineer’s Fees
Architect’s and structural engineer’s fees will depend on where you live. For example, prices in London are higher than elsewhere in Britain.
If you need a ballpark figure for your budget, then it’s best to allow between 3% and 7% of your build costs.
So, with a build cost of £20,500, you should allow between £615 and £1,435 for architect’s fees unless they offer you a flat fee.
With a flat fee, you’ll know exactly how much you need to put aside.
Structural Engineer’s costs are between £100 and £200 an hour. For example, if you wanted them to provide a report for load-bearing walls or RSJ calculations you can expect to pay between £1,000 and £1,500.
You’ll also have a building application to make. Costs depend on the type of conversion you are having done, but you should allow £750 or more.
When you have created your space, you are going to need heating and cooling, lighting and, if you’ve installed a bathroom, water too.
Other factors to include are drainage services. You may need to divert drains or set up a pumping system if the water table is high in your area.
You also might need to tank your basement walls or use cavity drain membranes to waterproof the basement.
If you are planning to build under your garden, you could well need a tree survey. This is so that the planning authorities can see how your basement conversion will affect the trees.
A tree report or survey will cost in the region of £400 but you’ll pay more if you live in London.
You might also need a tree surgeon if you want to chop down a tree or trees in your garden. You don’t need planning permission.
Costs to fell a 30ft tree in a garden with easy access, including waste removal will be in the region of £400.
If you are creating waste while converting your basement you may need to hire a skip if this isn’t included in your quotation. Skip hire costs from £90 for a mini skip up to £350 for a large skip which holds approximately 80 plus bin bags of rubbish.
The fitting out costs will very much depend on how you want to use your basement. If you fit:
A kitchen – you’ll be paying between £7,000 and £22,000 to fit out a complete kitchen. This will include cupboards and worktops, any carpentry, plumbing and electrics.
Bathroom – a standard priced bathroom will cost around £5,000 but if you want premium fixtures then you could spend £10,000 or more. This includes plumbing and tiling.
New flooring – you can pick an affordable laminate floor or basic carpet for between £15 and £20 square metres, or you can lay a hardwood floor or premium wool carpet for between £40 and £50 a square metre.
Have a look at our article about the costs of installing new radiators.
You can also instal underfloor heating, but this will add to the costs of your conversion. Prices depend on the type you have and the size of your basement.
If you want stand-alone heating, you can use electric heaters.
Cooling – for the summer months, you can have air conditioning installed while the basement is being converted. Or you can buy a portable unit if there is a window for the hose.
The prices for installing an air conditioning unit in an average-sized basement are around £2,000.
Lighting – lighting, plugs and sockets will also be added to your conversion. An electrician will cost between £150 and £250 a day, depending on where you are in the UK.
Spotlights start at around £5, a strip light for the kitchen starts at around £7 each and pendant lights are between £25 and £100 or more.
Ventilation – ventilation is important in a basement conversion to prevent damp and mould. There are various ways of adding ventilation and they include:
- Windows will provide adequate ventilation if your basement opens onto the garden. If you only have an egress window you may need to consider adding a secondary form of ventilation.
- Air bricks can be fitted as the conversion is being built. These bricks have holes so the air can come into the basement and they are usually placed in different locations around the room.
- If the staircase that runs from the room above down into the basement is open, (in that there’s no door), this will provide adequate ventilation
- Ceiling fans can be added as a secondary method of providing ventilation.
- Extractor fans can be installed in the ceiling or on the walls of the basement. They reduce humidity by getting rid of stale air and replacing it with fresh, clean air.
If you have a basement conversion you could have patio doors leading out into the garden. This will certainly add the wow factor to your conversion. You can choose:
- French doors – French doors are similar to double doors and they look perfect in smaller openings.
- Sliding patio doors – You can have 2 sliding glass doors in a small or large opening. They bring in a lot of light and save space.
- Bi-folding doors – If you have a large opening then a bi-folding door gives you more space as they open up completely to let the outside, in.
Prices vary according to the material you choose. You can have timber, which is mid-priced. Aluminium, which is the most expensive or uPVC which is the most affordable.
- French doors costs between £450 and £1,700
- Sliding patio doors costs between £650 and £2,800
- Bi-folding doors cost between £1,150 and £3,500
Windows or Light Tunnels
If you dig under your garden, you can install windows or light tunnels. Both will bring in the daylight into your new basement so that you have natural light.
The costs can vary depending on the number of windows and the type you have, but uPVC windows will normally cost around £300 each.
Egress windows which are sometimes required in cellar conversions cost between £1,200 and £3,500.
As you can see there is plenty to consider when you want to convert a cellar or a basement, in a nutshell, you have to think about:
- The condition of your basement
- Your soil type
- The water table
- Fire safety
- Fitting out
How Can I Save Money on a Basement Conversion?
There are a few tips and tricks that you can do to make sure you save some money on the total cost of a basement conversion.
Don’t make your basement or cellar conversion bigger than you need. Remember that a smaller conversion will cost less.
Create a budget and plan ahead.
Consider hiring an interior designer or an architect so that they can use their expertise to make the most of your available space.
It’s easy to use HouseholdQuotes to find trusted tradespeople. Simply fill in the form online and we’ll find vetted and professional tradespeople in your area to give you quotes for a basement conversion.
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What’s Involved in Converting a Basement?
First of all, there’s the investigation to make sure the project is viable. You’ll need to:
- Check with your local planning department about the regulations for basement conversions
- Talk to building control about their regulations
- Investigate your soil type, drainage, and impact on any nearby trees
- Inform your neighbours and send out Party Wall Agreements
- Find an architect or structural engineer to draw up your plans
Next, it’s the planning stage: You need to decide:
- How big you want the basement to be
- How much headroom do you need?
- Do you want natural light?
- How are you going to use your new basement?
- Where do you want to place the staircase?
If there are no disputes the builder or team can begin the work.
First of all, they will do any necessary excavations and underpinning (if you want to read more about underpinning, have a look at this article).
They will level the floor if necessary. Next, they will construct any new walls and then they will waterproof and insulate all the walls.
After the walls are insulated and waterproofed, they will install the staircase.
They will then plaster the walls, leaving the basement build completed and ready to fit it out.
You’ll also find plumbers and electricians on site dealing with the drainage and electricity for the basement.
If you only converting your cellar into a room, you may only need to tank and insulate the walls. There won’t be any need for excavation or underpinning.
How Do I Find and Hire Professionals to Convert My Cellar Into a Basement?
Start by asking your family and friends, as well as colleagues at work. Someone you know may have had work done recently, or they might have a builder in the family.
If you prefer to use the internet, then HouseholdQuotes can help you! Fill in the online form (it takes less than a minute).
Give us a brief description of what you need. We’ll then find builders and other tradespeople in your area to give you no-obligation quotes.
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Ensuring the Professional Is the Right Fit
Use the below questions to make sure the professional you want to hire meets good standards:
- Have you got a lot of experience with basement or cellar conversions? If the job requires a great deal of skill you might prefer to hire someone with plenty of experience. If the job is straightforward then you could give a newer company a chance to show off their expertise.
- Do you have a website? It isn’t necessary to have a website, but they are useful. Most websites have photographs of previous jobs and testimonials from customers. If they don’t have a website, ask if they have a portfolio with customer feedback and photographs.
- Are you a member of any trade associations? Trade associations will only grant membership to tradespeople with a good work record. They also need to have high standards of workmanship. The construction industry’s longest established association is the National Federation of Builders (NFB)
- Do you have insurance? Is your public liability insurance up to date? Public liability insurance is vital for all tradespeople. It protects them and you from any injury or damage claims should an accident occur during your cellar or basement conversion.
- Do you arrange for all the waste to be taken away and disposed of? If waste disposal isn’t included in your quotation, it will be up to you to arrange for the hire of a skip.
- What guarantees do you offer? Any good builder will offer a guarantee against their work.
As you can see converting a cellar or a basement isn’t cheap. But if you love where you live and you don’t want to move, then a basement conversion is the ideal way to create much-needed space.
Use the checklist below to give you a good overview of the steps for planning a basement conversion project:
- Visit your local authority planning department. Find out about planning permission and building regulations
- Find an architect to draw up your plans
- Hire a structural engineer to advise you about building regulations and how to protect the structure of your home
- Talk to your neighbour about your plans and send the Party Wall Agreement at least 2 months before you want to begin any work
- If there are no disagreements, and you have a signed Party Wall Agreement, arrange for the work to begin
- Let HouseholdQuotes help you find the builder you need for your basement conversion project.
All you do is fill in the online form. We then search our extensive database to find local builders to give you no-obligation quotes!
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Frequently Asked Questions
Do I Need Planning Permission or Building Regulation Approval for a Basement Conversion?
However, if you are reducing the floor level because you want more height you may need to get planning permission. This is because the conversion might then be treated as an extension.
Even if you don’t need planning permission, you will need building regulations approval. It is needed for a basement or cellar conversion you want to use as habitable space.
Building control will want to be sure the new space complies with the regulations for safety, hygiene, and energy efficiency.
You can get more information and a guidance document from The Basement Information Centre website.
Do I Need a Party Wall Agreement?
You need to send your neighbour a Party Wall Agreement for them to sign. If they don’t agree it may cost £1,000 or more to go to arbitration.
Could a Basement Conversion Increase the Value of My Home?
A basement or cellar conversion is usually a good investment if you live in a high-value or popular area.
An estate agent will be able to tell you whether your property will increase in value as a result of a conversion. Speak to one close to where you live.
Can I Extend My Basement Conversion Into My Garden?
You should also consider whether there is a party wall, as you will need the agreement of your neighbour to get the work done.
Are There Any Limits on the Size of a Basement Conversion?
However, this will need planning permission and the cost of the work may be more than the value that’s added to your home.
How Can I Use My New Basement?
- Home office
- Entertainment room
- Utility room
- Den for your teenagers
- Granny flat
- Second living room
- Kids toy room
- Spare bedroom and en-suite bathroom
- Storage and washing room
- Walk-in wardrobe
What Does It Mean When You Tank Walls?
Can I Do the Basement Conversion Myself?
If you are doing your own excavating you need to know about supporting the structure of your building and, if you have a party wall, your neighbours too.
Are There Any Rules About the Height of the Ceiling in a Basement Conversion?
There are building regulations about the height of the ceiling where you have stairs. The rules state there must be at least two metres over the stairs.