Basement Extensions: The Pros and Cons

From storage space to a new kitchen, your basement could add extra living space – and value – to your home. Laura Pank investigates
by Laura Pank
26th November 2012

The premium on space has seen interest in basements soar as they become elevated from mere storage rooms to day-to-day living  spaces. If you have a cellar, it may make economic sense to exploit it, perhaps enlarge it and turn it into a habitable basement.

“Depending on location, current value and style of the basement conversion, it could increase your home’s value by as much as 30 per cent,” says Maggie Smith of The London Basement Company. “For example in certain areas of London, such as Fulham, converting a basement is now the most popular form of extension and business is growing by 50 per cent a year.”

Your first priority if you want to join the trend is to check with a local estate agent that developing your cellar would add to the value of the house. No such concerns apply if you are planning a new build, provided your plot is suitable – that is well drained and not subject to flooding, and allowing good access for excavation work.

Including a basement can provide a whole floor of useful extra space. At this stage you can plan a design with windows into a well or courtyard – the ideal way to let in plenty of natural light.

Retro-fitting a basement into an existing house is the one option that needs the most careful investigation, because it is likely to be the most expensive of the three types of basement. You need to make sure you will redeem any outlay you make, and this will very much depend on property values.

In city centres such as London property values are now so high that almost any type of basement build could be financially viable.

A suitable case for digging

Are there any conditions where a basement cannot be fitted? Most obstacles can be overcome, but high water tables and particularly where there are main services running beneath the property may make costs prohibitive. “Local authorities have lists of areas that are close to rivers – the closer you are, the higher the risk and the more difficulties to overcome,” says Keith Defoe of Cellarwise.

“It’s also more difficult to install basements where there is a solid floor.” There are some cases where it really is not feasible to fit a basement. Maggie Smith says: “We have fitted basements to some modern houses, but modern terraces and townhouses that are built on raft foundations can’t be underpinned, so can’t have basements.” It’s also important to have good access for the excavation work: too close to a boundary or the road and it could be difficult for a JCB to get in.

Cavity drain membrane system

Until 10-15 years ago, basements simply relied on being dug out and finished with cement, with no extra waterproofing. The result was that after several years leaks would inevitably occur, perpetuating the reputation that basements have always had for damp and susceptibility to flooding.

Today, basements are excavated and their foundations underpinned with waterproof concrete, but in addition the whole area is tanked on the inside with a heavy waterproof membrane covered in studs.

This cavity membrane system ensures that any water that may build up is directed down the studs into a gulley that runs all round, and thence into a chamber where a pump – or more than one if it’s a large area – drains it away.

The pump comes on only when necessary and is not audible. “The pump is serviced annually,” explains Maggie Smith. “You can also have an alarm fitted to monitor the pump and a back-up power source.”

Planning requirements

If you are excavating beneath your house to create a basement, you will usually need planning permission. But if you are simply adapting an existing cellar and all the work is internal, you probably won’t need planning permission. To be on the safe side, check with your local planning authority.

As with all new building work, it has to meet Building Regulations. Contact your local authority’s building control department for advice. If you share a wall with a neighbour, you will need their written consent in a party wall agreement before you can begin any building work. This also applies if you are going to dig down deeper than your neighbours’ own foundations.

If you are using your basement as habitable space – bedroom or living room but not kitchen – you also need to incorporate more than one fire escape, so you must have a window or door.

Depending on the size of the property and the amount of excavation required, 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.

How long will it take?

Depending on the size of the property, the amount of excavation and building work required and its complexity, the work will last typically for 12-20 weeks. Although excavating tons of earth is bound to be disruptive, the job is done from outside rather than from inside your house, which is effectively sealed off at ground level.

Generally a wooden hut is constructed in front of your house and tunnelling is done from within this structure, the excavated earth funnelled along a chute out of the hut straight into a skip. Before deciding, consider the following points:


  • When you gain space from a basement you don’t lose any external space. And a basement is much closer to the living area of your house than a loft.
  • Securing planning consent is generally easy, especially if there are already basement conversions in your street, and if you have windows or light wells outside.
  • In urban areas, a basement can add value.
  • Underpinning can actually stabilise an old building.
  • Waterproofing makes the whole house drier and healthier and increased insulation makes it more energy efficient.
  • If you live in a terraced or semi-detached house, soundproofing can make the room quieter for you (and your neighbours) than the rest of the house.
  • A basement will free up space in other parts of the house, and it’s an adaptable space that can change depending on your family’s needs. Or it can be completely self-contained with its own access.


  • The job, though greatly refined and improved over the last 10-15 years, is still complex. It’s bound to be disruptive, noisy and messy especially if you are enlarging or creating more headroom and so need to underpin.
  • You need to have an annual (at least) inspection of the pump and drainage.
  • Even though the ‘burrowing in’ to enlarge or retro fit is usually done from outside and the breakthrough isn’t until it’s time to install a connecting staircase, there is still dust and disruption and the neighbours won’t like it.
  • There may be unforeseen problems, especially if there is a party wall.
  • You may not be able to get enough light in and the room may end up gloomy or always dependent on artificial light.
  • Safety may be a concern and you may end up having to lay out more money on another access route.
  • It’s expensive and there is always a risk that you may not recoup costs when you sell.

Case Study: The 1920s semi

The London Basement Company converted the basement of this 1920s double-fronted semi detached house in Putney, London, in 2004 to provide a family games room, gym and wetroom plus utility room. Work took 24 weeks. The house had a 15m² (161ft²) cellar.

The cellar was excavated, the ceiling height raised from under 6ft, the space was underpinned and waterproofed to create a basement of 84m². There are two windows and lightwells at the front of the basement, which originally had no natural light, and one at the rear in the gymnasium.

The London Basement Company also carried out all of the finishing works, including all carpentry and joinery, stud partitioning and plastering, electrics, plumbing, heating, wetroom installation, utility room units, media fit out, flooring and all decorations. Work is guaranteed for 30 years.

Case Study: The edwardian terrace

Timberwise converted the damp, unused basement of this Edwardian house in Ilkley, West Yorkshire in early 2006. The total area is about 7m by 12m, and it now provides a kitchen, dining area, utility room and cloakroom.

The basement already had its  own external access and natural light from windows in the front and rear, and the conversion took five weeks in total, including all the fitting out. Timberwise installed a cavity drain membrane system and an insulated moisture resistant floating floor, erected partition walls, carried out plumbing and electrics and replastering. The work is guaranteed for 10 years.

Main image: This cellar conversion by London Basement shows how underground storeys can offer high-quality accommodation

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