Basement Conversions – All you need to know

Converting your basement can provide you with valuable and flexible extra living space. Plus, depending on your location and the style of the basement conversion, it could increase your home’s value by as much as 30 per cent. But whether you’re after a home office, a kids playroom or a utility room, we’ve put together a handy guide to help get you started.


Basement Conversion

Will I Need Planning Permission?

So long as you’re converting an existing cellar or basement from a storage area to a habitable space, that’s a bedroom or living room, but not a kitchen, you won’t require planning permission. But if you’re creating a new basement, which will enlarge the volume of your home, then it’s treated as an extension and may need planning permission. It’s always best to check any extensions or alterations you’re planning with your Local Planning Authority Local Planning Authority Search before starting work.

What about Building Regulations?

Building regulations are the set of construction standards that ensure all buildings are safe, hygienic and energy efficient. They also cover fire escape routes, ventilation, ceiling height, damp proofing, electrical wiring and water supplies. Regardless of whether you need planning permission or not, all basement conversions that involve a change of use (storage space to habitable space) will need building regulations approval and incorporate more than one fire escape.

You can find more information on this from the The Basement Information Centre & More information from the planning portal

Underpinning and foundation work might also be required and, if other properties adjoin yours, if you share walls, or if you’re going to dig down deeper than your neighbours’ own foundations, you’ll need to consider the Party Wall Act 1996 . This is about getting your neighbour’s written permission for your project to go ahead before work starts. The Party Wall Act provides a framework for preventing and resolving any disputes relating to party walls and is completely separate from building regulations and planning permission approval.

Further reading from the Planning Portal on Building Regulations, Policy & Party Walls

Can any house have a basement conversion?

Most properties can, but if you live in an area with a high water table, or if main services run beneath your house, you’ll find it very expensive. Modern houses built on raft foundations can’t be underpinned, so can’t have basements. You’ll also need to take into consideration the removal of soil and the available garden or street access for excavation works. Your Local Authority will be able to advise you.

How long will it take?

Depending on the size of the property and the amount of work required, most works last between 12 and 20 weeks. Though a simple basement conversion could potentially take as little as two or three weeks, converting and extending a basement beneath an entire property, which would involve underpinning the existing structure, would normally take several months.

What about Waterproofing?

Waterproofing makes the whole house drier and healthier, and increased insulation makes it more energy efficient. Waterproofing below ground level is known as ‘tanking’. It’s the application of a layer of waterproofing material, like a sheet membrane or asphalt. This must withstand the pressure from the water table, as any leak, however small, can be difficult to find and repair later on.

An alternative to tanking is using a cavity membrane. This is an inner waterproof structure covered in studs. The cavity is fully drained by a pump, so any tiny leaks or water that build up are directed down the studs and pumped away by a drain. With even the smallest leaks drained away, there’s never any water pressure against the inner structure, the pump only comes on when necessary, and isn’t noisy. You will need an annual service, but a lot of people consider cavity membranes the most reliable way to waterproof a basement.

How do I find the right Builder?

You’ll want to choose a specialist basement contractor with experience of tanking or cavity membrane systems. You’ll find a list of registered companies at the British Structural Waterproofing Association. Remember to look for an insurance-backed guarantee that’ll pay for repair works even if the contractor goes out of business.

How much will it cost?

So long as there’s adequate headroom, turning your existing basement area into an extra living space will cost around the same as a simple loft conversion. But if you have to start lowering the floor and underpinning the foundations, the work starts to get more expensive. We found these online quotes as follows:

  • Cellarwise of London quote £75,000-£140,000 for a fully finished cellar space for a typical Victorian terraced house.
  • The London Basement Company quotes an average of £300ft² plus VAT for a fully fitted out structural conversion, or £150-200ft² for the structural shell and waterproofing alone.
  • For new builds, the cost is around £300-£450m².

Costs will be higher if the property has solid concrete rather than timber floors, if the drains beneath your house have to be diverted, if your home sits on difficult ground conditions like solid rock, clay, peat or marsh, if the local water table is high, (which would necessitate constant pumping during conversion), or if access to the building site for excavation and removing the soil is an issue.

What am I paying for?

Here’s a breakdown of general basement conversion costs. Please note we haven’t included a contingency fund in the list below, but we recommend you do.

Conversion of existing cellar – £750-1,400/m²
Lowering floor level and underpinning in existing cellar – £1,500-2,000/m²
Digging new basement space and underpinning – 2,000-3,000/m²
Digging new basement space beneath a garden – £1,500-2,000/m²
Creating a lightwell/external access – £5,000-7,500 each
Engineering fees – £1,000-1,500
Planning application (if required) – £165
Building Regulations application – £750 upwards (according to value of works)
Party Wall agreements (if required) – £700 per neighbour
VAT – 20% (added to the cost of all work by a VAT-registered contractor)

Check out some examples of real basement conversions that meet building regulations