How to convert a basement – to add space underground and increase property value

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  • Extending underground is a fantastic way to add heaps of space to your property and increase its value. We take a closer look at what’s involved

    A basement can provide an excellent solution to create more space in your home, particularly if you live in an urban area where land values are high and you don’t want to sacrifice part of your garden. However, integrating ventilation and natural light is a lot trickier when you’re going underground – not to mention more expensive. That’s why clever design and careful planning are both essential.

    Want more extension ideas? READ: How to convert a garage – to make your room work for you

    1. First formulate your brief

    convert a basement

    The first stage of the basement extension process is to create your design brief. This should address what you intend to use the space for and the overall aesthetic you’d
    like to create. ‘Without careful planning, it would be easy to end up with a large, poorly-lit zone with limited uses,’ says Richard Gill, an associate at Paul Archer Design ( Given the great expense associated with subterranean projects, where the lion’s share of the budget will go on excavation and underpinning, it pays dividends to establish an inviting space that adds real value to your home.

    2. Seek expertise

    convert a basement

    Image credit: Jonathan Gooch

    The key to establishing a light-filled, airy zone often involves hiring a professional to formulate the design concept. While many basement construction firms offer their own in-house design services, employing a recommended designer could make the difference between an efficient basement and a spectacular one. ‘Engaging an architect shifts the focus to the creation of beautiful spaces,’ says Richard. ‘They will be able to address issues such as how to connect the basement with the main house and how to maximise natural light, while making it aesthetically pleasing, too.’

    3. Consider the practicalities

    convert a basement

    There’s a raft of design and construction essentials to think about before submitting your planning application. For example, bear in mind that a fire escape strategy will need to be factored into the plans. ‘Soil conditions and the water table also need to be properly investigated prior to any detailed design taking place. A clear waterproofing strategy has to be implemented,’ says Siobhan Maguire, associate director at Paul Archer Design.

    If your priority is to create a bright, airy space, think carefully about how this will be achieved. ‘Don’t fall into the trap of skimping on floor-to-ceiling height,’ says William McGuinness, director at UV Architects ( ‘You need to ensure a minimum height of 2.7m.’

    4. Inform your neighbours

    convert a basement

    Image credit: James Balston

    It’s wise to get your neighbours on side from the outset. ‘Unlike regular extensions, basement projects can cause massive issues with neighbours – largely based
    on stories they might have heard about developments going wrong,’ says William.

    5. Get planning permission

    convert a basement

    Image credit: Colin Poole

    At one time, it was possible to create a basement extension under Permitted Development. These days, however, the chances of that happening are highly unlikely. ‘A lot of authorities have clamped down,’ says William. ‘In some London boroughs, any subterranean development automatically requires formal consent. Or if you introduce a light well, for instance, that will necessitate planning permission because of the alteration to the exterior.’ Consequently, it’s essential to factor in the additional time and cost of planning into your project schedule. Find out more information at

    6. Hire a contractor

    ‘Some are better than others,’ says William. ‘It’s very specialised work, so do your homework and make sure your contractor has relevant experience.’

    7. Manage your budget

    convert a basement

    Image credit: Mark Luscombe-Whyte

    It’s vital to keep a close eye on the pot of money you’ve allocated to your basement extension throughout the build. Plus, as you would with any extension scheme, it’s a good idea to set aside a 10 per cent contingency fund in case any problems arise. Typically, you’ll need £3,500-£4,000sq m for a fully-fitted basement.

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