How Much a Basement Conversion Costs in London

A basement that has been converted into a living room
January 12, 2024

If you can afford the expense and have the space to experiment, then a basement conversion is a great way to add value to your existing property.

It gives you and your family a whole other room in which to live and to use it in whatever fashion you fancy.

Basement conversions have actually been very popular in space-constricted cities, such as London, for a number of years now. They tend to be less expensive than adding an extension, meaning you’ll get more living space for your money.

At the same time, converting an existing basement saves families in need of additional space, the hassle and cost of having to move home. Instead, they get to stay where they love and besides the neighbours they’ve already spent years building a good relationship with.

So, whether you’re looking to create a cinema area, games room, new basement office or want to use the extra space that a basement extension offers for another reason, then read on.

In this guide we provide information on what you will need for your basement conversion (which should take anything from eight to 20 weeks).

We will also share with you the basement conversion costs you can expect to pay in London, broken down into different areas, such as labour and materials.

Average Costs for a Basement Conversion in London

A basement converted into a bedroom

The average London basement conversion cost depends on a number of factors. The biggest price difference is whether you’re having to excavate for a new cellar, or whether you already have the space and simply want to convert it into a liveable room.

On average though, your typical new basement conversion in the UK capital costs around £140,000. That works out at around £1,600 per square metre.

If you’re having to excavate and underpin a new basement area then a typical basement conversion will come in at around £2,800 per square metre.

Other costs to think about when having a basement extension built are the materials you will need to pay for in order to convert the room ie if it’s a games room then you will no doubt look at buying a pool table, install a bar and fridge etc. In some instances, basement conversions are built to hold a swimming pool.

There will always be decorating costs, including plastering and putting down flooring, regardless of what you decide to use the extra space for. Then there are the lighting fixtures to think about, as well as the installation of an electricity and water supply.

Costs Breakdown

Here are some of the main costs for creating an additional living space in your home by excavating a new cellar area or converting an existing basement:

  • Converting an existing basement – £1,400 per square metre
  • Digging and underpinning a new basement conversion and damp proofing it – £3,000 per square metre
  • Digging space below a garden – £2000 per square metre.
  • Creating a lightwell and external access area – £7,500 per square metre
  • Architect and engineering fees – £1,500- £1,800 (basic plan) or £15,000 to £25,000 for a complete package (ie with interior design added)
  • Planning application – £200
  • Building Regulations application – £1000
  • Surveyors fees (party walls, trial pits etc) – up to £800 per neighbour
  • VAT on the total bill at a cost of 20 per cent

Labour Costs

Basement conversion costs comprise a lot of billing for labour. And, although tradesmen costs are often quoted hourly, it’s unusual to be billed this way for such a large job as a cellar conversion.

Most companies will give you a price per job since the number of days involved in this will prove too onerous to work out per hour. Having said that, here will be hourly rates for some tradespeople whose skills will be necessary for parts of the job, such as an electrician, painter and decorator, plumber etc.

The cost of these are typically:

  • Structural engineer – up to £200 per hour
  • Electrician – around £45 per hour
  • Plumber – up to £60 per hour
  • Painter and decorator – up to £200 per day

A building site in a basement

Material Costs

The type of subfloor ie concrete or timber, that you need for your basement extension will be a big part of the material costs. In essence though, the higher the quality of the materials you use, the more money you’ll pay for the total bill.

Having said that, quality materials will last longer and prove more durable in the long run so it’s not a bad idea to buy as good quality as you can afford.


You’ll need electrical sockets and lighting in your new basement extension. It’s not just the fixtures that you’ll need to pay for her, but also the wiring and the installation of plug sockets and switches.


The nicer the décor, the more time you’ll want to spend in your basement conversion so don’t stint when it comes to this. You will have spent so much having the extra space created in the first place, you’ll want to get the most out of it.

Skip Hire

A skip is an essential part of building a new basement extension from scratch and, for that matter, converting a cellar. As such, it should always be included as part of your basement conversion costs.

You can expect to pay anything from £200 a week to rent a skip.

Hire for a longer period and you’ll benefit from a lower daily cost. The cost to hire in London will be higher and you’ll no doubt require a skip permit in the capital and other large cities since the skip may have to sit on a public road.

Factors That Affect Costs

As we mentioned earlier, excavating and building a new cellar will cost more than converting a cellar which is already there. But even with an existing cellar, you may find that additional structural support and underpinning is necessary.

Another factor that will affect the new basement conversion cost is, of course, the size of your intended basement area, together with your future intentions for it. If, for instance, you plan on making the cellar another bedroom then you’ll have building regulations to adhere to.

A converted basement

It may be too that it’s necessary to divert the drains on your property so that they’re not sitting where you intend the basement construction work to be carried out.

The ground conditions on which you’re going to build your basement are important too. For instance, if the ground in your garden contains clay soil or it’s marsh-like, then you could find yourself looking at having to take out a flood risk survey.

If you live in a busy area – much of London being built-up – and you’re close to a main road or there’s a lot of transport links nearby, then you might find you’ll require traffic management plans to be drawn up while the basement is being built.

Depending on your location, you may need to purchase permits for skip licences and the parking of large vehicles during the basement’s construction.

Reasons to Convert Your Basement

There are many benefits from converting your existing unused cellar into another room for your home. You could, for instance, turn the extra space into a gym and improve your fitness.

Or, perhaps it would be better as an office or an additional bedroom. Having the basement as a games room for growing teenagers allows them to have their own living space and disperse all that extra energy out of sight of the rest of the house.

Alternatively, you could have that cinema room you’ve always wanted, the walk-in wardrobe or wine cellar. In other words, you can do whatever you dream of in your extra space.

Another reason why homeowners in London convert their basements is that they need extra living space, but there’s no space to build an extension ie terraced homes are joined and often the gardens aren’t large enough.

When completed, a quality basement conversion can add anything from 10 to 20 per cent to the value of your home, meaning it will pay for itself when you come to sell and move on.

Planning Permission

Planning permission will almost certainly prove necessary if you are thinking about excavating and creating a new basement area. Your architectural designer or the building company responsible for completing your basement conversion can apply to the local authority for this on your behalf.

If, on the other hand, you are simply converting an already existing cellar area you may not have to bother with planning officials. That is, unless, of course, your property is Listed or in an Area of Outstanding Beauty (AONB), in which case you will have to contact Historic England.

DIY Vs Hiring a Professional

A smiling tradesman

There are some DIY jobs that it’s practical to carry out, but a new basement construction isn’t one of them. And for that reason, we would strongly advise against converting a cellar or building a basement from scratch.

Not only will the job require expert skills, physical strength and a lengthy time commitment but, if not carried out properly, the work can cause structural damage to the rest of your home. And the latter can prove extremely costly to rectify.

For all of these reasons, it’s always best to hire the services of a professional company or tradesmen to complete your new basement. They’ll also have the tools and equipment necessary to complete the job – something you would have to hire at great expense were you to go ahead and attempt the job yourself.

Finding Someone to Convert Your Basement with The Page

Converting an existing cellar or creating a new basement from scratch is a big job, which means prices between different companies or tradespeople may vary considerably. That’s why we recommend you receive at least three quotes for the work. That way you’ll be able to get a realistic average. Just make sure you give the same work specification to all those tendering for the job.

Contact us here at The Page with the specifics of the basement construction work you need to be carried out, and we guarantee to get back to you quickly with a curated list of skilled and expert tradesmen for you to choose from.

All of these professionals will have been personally vetted by ourselves, and their work and qualifications checked. Where applicable they will also be members of a professional trade body, providing you with further protection for your project.

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