Dig deep to add space and value when budgeting and planning your basement extension.

Add space underground2

Image: RDA Architects (020 8299 2222; rdauk.com)

Introducing a basement extension is a tempting way to boost the square meterage of your home. It may be a matter of renovating an unused area to provide a modern extra living space or, more radically, a basement can be added where none existed before – either below the ground floor of your home or by going underneath the garden. Of course, for self-builders, including a basement in the plans for a new home is a viable way to get more value from a building plot.

Considering costs

A basement extension can give you more living space than going into the loft could – or be a way to avoid an expensive move if you’ve already converted the loft. Be wary when you’re assessing the costs and benefits of the work, though. ‘In most cases, basement conversions are only likely to add significant value to properties in more upmarket postcode areas,’ says chartered surveyor Ian Rock, director of rightsurvey.co.uk. ‘Ultimately, the amount of value added will also depend on the use of the new accommodation and whether it overcomes an obvious drawback, such as a cramped kitchen.’

At a basic level, a basement can add storage to your home or give you a utility room for noisy appliances, but they can have huge possibilities. Expansive living rooms can be created, while playrooms for young children, music and hobby rooms, and even home gyms are well located there. A basement can be an area for teenagers to use, and can create privacy for overnight guests, or separate living accommodation for elderly family members or adult returners. It’s the ideal location for luxury additions such as a home cinema, office or wine cellar, too.

Planning ahead

A basement can be future-proofed if it’s constructed in the right way. ‘I advise putting beams throughout to make sure it is open and flexible,’ says architect Ian Hogarth of Hogarth Architects (020 7381 3409; hogartharchitects.co.uk). A basement needn’t be dingy as it can be day lit; it can also have natural ventilation and access to outdoor areas. Sizing up the basement can be worthwhile, too. ‘There is an economy of scale,’ says Hogarth. ‘You might as well make the biggest space you can as it is very disruptive – do it all at once.’

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Image: John Feely Architects (+353 1857 0589; johnfeelyarchitects.ie)

Waterproofing and refurbishing an existing cellar that is 5.5x8m, in a standard London terrace, can be achieved for £48,000 to £60,000 for both dig and fit out (but not sanitaryware and floor finish), according to Robert Wood, sales director at Simply Basement (0800 917 7571; simplybasement.co.uk). If the basement has to be dug out from scratch, though, the same size area could result in a bill ranging from £180,000 to £240,000.

Underpinning existing walls will be a major part of the final bill. ‘Costs also tend to rise significantly where drains need to be diverted or where ground floors are solid concrete rather than suspended timber,’ says Rock.

Getting permission

If you’re converting an existing cellar or basement and you’re not adding a light well that alters the house’s appearance, it may not be necessary for you to apply for planning permission. Digging out a brand-new basement or opting for one that’s going to be used as separate accommodation – or that changes how your home looks from the outside – will probably require an official go-ahead. Work on listed buildings needs consent, so speak to the planners at your local authority.

Building regulations apply to a basement, whether it’s a refurbishment or a new construction. Fire escape routes are crucial, and the regulations also cover elements such as ceiling height, damp-proofing, electrical wiring and water supplies.

Add space underground2

Image: Riach Architects (01865 553 772; riacharchitects.com); Federation of Master Builders (0330 333 7777; fmb.org.uk)

Finer details

Basement work is likely to affect a shared property boundary – or party wall – and neighbours have to be given prior notice under the Party Wall Act. If you’re using a specialist basement company or working with an architect, they can deal with planning, building regulations approval and party wall essentials.

Both the construction and waterproofing of the basement need to be appropriate to the ground conditions. ‘Traditionally, basements were converted by tanking with cement render applied directly to the walls in layers,’ says Rock. ‘But failure and leakage were not uncommon. Modern drained cavity systems are far more reliable.’

Once you have the necessary permissions in place, how long will the disruption last? ‘A tanking-only project could be done in two months. A full dig out and fit out will take at least seven months,’ says Wood.

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