5 things you need to know before converting your basement

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  • Read this before you dig down

    Once you’ve decided not to move but improve, the next decision you have to make is where to add space. Cost wise, it’s cheapest to push out (side or rear extension), than up (into the loft) with going down into the basement the most expensive way to extend your house. However, depending on your area, the type and value of your property, digging down can still be a viable option. A basement is a very specialist job, so always employ an experienced architect and builder, or a dedicated conversion company.

    1. Let there be light

    The problem with any subterranean space is always going to be light as you simply don’t have the same acreage of sky available to you as you would at roof level, say. ‘When converting, our main aim is to find clever ways to bring light through to the lower levels’ say architect Alan Crawford of The Crawford Partnership’. ‘This can be done by in incorporating a light well to create a visual connection to the world above. Even a small skylight that allows a glimpse of blue sky can make all the difference.’ Consider also of things like sun pipes and tunnels, which can harness light and redirect it into dark corners.

    2. Work with what you’ve got.

    Don’t think basement conversions all involve digging under the entire front garden to create a multi-storey gym/carpark/cinema/servants quarters. Many
    Victorian and Edwardian houses already have cellars with sturdy
    foundations making then ideal for conversion. More modern houses can
    sometimes be converted by digging out beneath the property, but this is expensive so check with a specialist to find out the feasibility and

    3. Think about your levels

    ‘Whether you are converting a cellar below the
    house or digging out part of the front or back garden to create the new
    basement, consider excavating an extra 200 – 300mm to increase the floor
    to ceiling height of the new space’ advises Alan. ‘Maintaining the flow like this won’t push the cost up dramatically, but it will instantly make the room feel less
    claustrophobic, especially in combination with a light well or skylight.’

    4. Keep things waterproofYour new basement may be below the level of the
    water table, and if this is the case, there are a number of methods of
    waterproofing available including external tanking to inner drained cavity
    lining. Check with your architect or builder to see which is the most
    appropriate for you.

    5. Be prepared for the long haulHow
    long a conversion will take will depend on the size of the project, but an
    average job of 70-80 sq m will take around 12 weeks to excavate and 12
    weeks to fit out. If you are utilising an
    existing cellar space, allow £1,200 per square m. If the floor needs
    lowering, budget from £1,650 per square m, and if a new basement needs to
    be dug out allow £3,500 per square m.

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