How to convert a basement

If you don’t have the room to extend your home above ground, a basement conversion might be the best way to add space and value. Here's how to convert a basement

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Basement conversions are a great way to create extra living space in built-up urban areas where outdoor space for extensions is limited. A basement, naturally closer to the main living areas and garden than a loft extension, is a brilliant way to add more practical living and entertaining space. 

There are two main types of basement conversion – the first, and most cost effective – is turning an existing cellar into usable living space. The second is excavating a new basement, which will require more significant structural work and is, therefore, more expensive. Bear in mind that it can be much more cost-effective to convert a basement if you are also adding a new extension above at the same time. However, if you have a small garden and lack living space, both can be worthwhile options.

Read on to find out how to create a basement conversion – and read our ultimate guide to extending a house for more advice and tips.

How much does a basement conversion cost?

Turning an existing cellar into habitable space by tanking and insulating the walls will cost £720 to £1,800 per square metre – around the same as a loft conversion. Factors that will increase costs include the need to move services, underpinning, and lowering the floor level to increase ceiling height.

Constructing a brand new basement under an existing house and underpinning the walls will cost £1,920 to £2,640 per square metre for a shell space. These costs will increase if there is the need for any party wall agreements with neighbours whose boundary walls will be affected, of if there are difficult ground conditions or poor access to the site for the work to be completed.

Regardless of which option you choose, installing a light well or new external access door will cost £5,500 to £9,500 each.

Bear in mind that the figures quoted above do not include professional fees, which come in at around 10 to 15 per cent of the build cost plus VAT. Assuming you are employing an architect, expect design fees to be three to seven per cent of the build cost, with a minimum of around £2,700 to £4,000 for the planning drawings. Construction drawings will cost a similar amount. A structural engineer will cost between £500 to £1,000. Expect to pay from £500 to £1,500 for a measured survey of the existing house.

Use our extension cost calculator to work out your basement conversion costs.

basement conversion in terraced house image by jody stewart


(Image: © Jody Stewart)

The planning fee (if needed) for a basement conversion in England is £206. If planning permission is not needed, it may be worth applying for a certificate of lawful development, which costs £103; discharging planning conditions costs £34 per request. 

A building regulations application will cost £750 upwards, according to value of the works.

Party wall agreements typically cost from £700 to £1,000 per neighbour. 

See the next page for more details on planning permission, building regulations and party wall agreements.

Factors that will raise your basement conversion costs

  • If you need to divert drains beneath your house;
  • If your home has solid concrete rather than timber subfloors
  • If your house is sited on difficult ground conditions (clay, made-up ground, sand or marsh); 
  • If the local water table is high, necessitating constant pumping;
  • If access to the site is poor;

What does it cost to fit out a basement?

The cost of fitting out a basement conversion will very much depend on the function of the rooms within it, but expect to pay:

  • For a kitchen: between £5,000 to £20,000.
  • For a bathroom or shower room: between £4,500 to £11,000.
  • For bi-fold or sliding doors on to a basement terrace: £1,500 to £2,000 per linear metre. 
  • To extend an existing central heating system: £150 per day excluding materials for the plumber; a new boiler costs around £2,300.
  • For flooring: from £25 to £100 per square metre.
  • For wall and ceiling finishes: around £85 per square metre for plaster, plus paint.

Is it worth converting a basement?

Whether it is financially worth converting your basement depends on a number of factors: 

  • The cost of the work versus the value added to your home (city-central, high value areas with little available land will give you the best profit potential. Ask a trusted local estate agents for an idea of how much space is worth per square metre in your road. You can compare this to the cost of the basement conversion.
  • If, of course, you are planning an existing cellar conversion rather than a basement dig-out, your spend will be lower, and your profit potential higher.
  • If this is the case and it's a toss up between a planning a loft extension at a similar cost, you'll need to weigh up whether you need more living space or more bedroom space.
  • Check your property's ceiling value with the local estate agent – it's unwise to spend significantly more than you will add to your property's value on any project.

Planning a basement conversion >>

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