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    Basement conversions

    Summary: Planning a basement conversion, is your house suitable for a basement conversion, financing a basement conversion, digging a new basement and tanking a basement.

    Following the craze for loft conversions and home extensions, basement conversions are the new big thing, particularly in town houses where a dingy basement or cellar can be transformed into an extra bedroom or living space, adding considerably to the value of your home. Basement conversions have been praised in recent years for the socio-economic benefits of increasing inner-city dwelling space, but let us not forget the valuable addition of space to country houses as well. Those without a basement or cellar can create a 'retrofit' basement, which involves digging down under the footprint of the house.

    The versatility of basements is a principle attraction. Whatever home additions you dream of, with some imaginative design and competent contractors, you can turn your basement into virtually any room. Some common uses of underground spaces include: kitchens, lounges, kid’s playrooms, studies and studios, utility rooms, luxury wine cellars, granny annexes or rented bedsits, even a home gym, cinema, or indoor swimming pool. And once you have made the area habitable, its use can always be changed.

    Is your house suitable

    Before you get carried away with planning how you will use your newfound rooms, make sure your house is suitable for a basement conversion or creation. If you live in a modern town house or an older house, the foundations may not be strong enough to support a new basement. Equally, if you have a modern terraced house, the way that the building is underpinned may prohibit a new basement. You will need to speak to a structural engineer and possibly an underpinning specialist to assess the feasability of digging under the house and supporting the structure. If you do decide to go ahead, bear in mind you may need to move out while this is being done, unless your house has timber-suspended flooring, enabling work to be carried out externally.

    Other restricting factors include underground cables and pipes, a permanently high water table and flood risk, or soil contaminants and ground gasses on brownfield sites. Ask your local water board or council for information. These problems might not necessarily rule out a basement but are likely to increase construction and waterproofing costs. Damp problems and ventilation issues can usually be solved except in severe cases but it is vital to carry out a full survey. If you already have a cellar, the ceiling will need to be at least 2.3m (7½ ft) above floor-level so the floor-level may need to be reduced.

    You will also need to obtain planning permission and building control approval – more about these later.

    Financing a conversion

    You should see a basement conversion as an investment, adding to the value of your property by up to 30% and significantly more in a commercial property. You could even make the conversion pay for itself by making into a room for a lodger or a self-contained flat for renting out. It is worth consulting several estate agents to see if the conversion is financially advantageous. Bear in mind that alterations to the building could affect your home insurance so make sure your proposed plans will not push up your premium.

    The cost of the project will vary greatly. One study estimates that a 40–50m2 basement creation costs around £80–£100K but structural work can be very costly. On the other hand, if you already have a basement, you could be looking at a much lower figure. Your major expenses will be plumbing and electrical work, damp proofing and ventilation. When you have carried out a survey, get quotations (not estimates) from a number of contractors to find the best price.

    Financing options for the project include taking out a loan or re-mortgaging. When considering re-mortgaging your home, find out what extra fees you will be liable for, and remember you can re-mortgage with a new lender so shop around for the lowest rates. If you have a fixed-rate mortgage, you may be able to obtain a secured loan from your lender to cover the costs at the same rates as your mortgage, but watch out for arrangement fees. If you choose an unsecured loan, your property will not be held as collateral but interest rates can be much higher.

    Digging a new basement

    If you are digging a new basement, the house will be underpinned and supported on steels and props as the area is excavated. Existing first floor joists may also have to be supported, such as by a steel beam or beams. Once the walls and floor are in place, they must be waterproofed. Interior waterproofing will only do so much, and if at all possible, external measures should also be taken. One of the most effective methods is to build a secondary wall inside a waterproofed lining. A specialist damp proofing company will suggest options to suit your basement, such as fixing corrugated iron piping around the perimeter of the foundation walls to channel water away or installing a sump and pump.

    Tanking a basement

    If you already have a basement or cellar, there are a variety of Tanking methods to help protect the walls. Which ever method you choose should be resistant to ground water pressure and approved by the British Board of Agreement (BBA) or equivalent body. Adequate ventilation will also help prevent damp and should be achieved with the installation of air bricks and perhaps a mechanical ventilation system or dehumidifier. A cellar expert will be able to help with this as well as suggesting ways of bringing more natural light into the room.

    Before you can plaster the walls, insulation and soundproofing will also be required. You may also have to add an external door or window to the room in case the primary access staircase is blocked in a fire. Your contractors should be familiar with regulations governing fire safety but if the room is to be used as a workshop you will need to consult the local fire brigade for any additional safety measures.

    Basement lighting

    Much can be done with artificial lighting schemes to make a room seem bright but it is important to bring as much natural light into the basement as possible. Natural light inspires a more uplifting mood than electric lighting, and will also help with damp problems. If part of the basement extends above ground you can add windows, otherwise light wells can be dug to provide a skylight effect. If you are concerned about security problems that a window may bring, use glass blocks or shatter-proof glazing sheets instead. You might also consider creating a sunken terrace from the back of the basement to the garden with glazing panels or French windows. Keeping the room open-plan and installing an open staircase without risers will also maximise the light in the room. If money is no object, you could even replace the ceiling with a glass floor to allow light to filter through from above.

    Interior work

    With the walls waterproofed and any structural work done, you can turn your attention to the implementation of plumbing, electrics and heating. Depending on how the room will be used, you may decide against plumbing work but do spare a thought for how the room might be used in the future – you may want to have the option of installing a sink or toilet later on. If you opt for overhead plumbing, a suspended ceiling will conceal pipes and cables, and leave them accessible. Routing pipes underground could save you money but will also make access restricted.

    If you have an old house, there may already be electric wiring in the cellar. Be warned, this may be old and unsafe so get it checked over by a qualified electrician. Adding or extending circuits requires some skill so if you are considering doing the work yourself, you will need to get Building Control approval and arrange an inspection. Plan the lighting and power circuits to suit the room’s use. If it is to be a kitchen, bathroom, or office you will need task lighting over cookers, desks etc. and sockets for appliances. It is a good idea to include as many sockets as possible in the room, at least one double socket on each wall for future uses. You will need to make a detailed diagram of the lighting before circuits are routed.

    Basements tend to be chilly places, and whilst this may be a relief on hot summer days, you are likely to need a heating system for winter. Electric heaters can be expensive to run so radiators supplied by your boiler may be more cost-effective. If you find radiators unsightly you could enclose them or choose a reproduction vintage radiator. You will need to make sure your boiler is powerful enough to supply an extension and ensure any work involving gas is done by a Gas Safe registered fitter. Underfloor electric heating is a luxury worth having if you are planning a bathroom or cosy living room.


    Most basement conversions do not require planning permission unless there is a ‘change of use’ such as from a living space to a granny annexe, separate flat, commercial office, or garage. You will, however, need permission for a retrofit basement or any alterations to a listed building both inside and out. You may also find properties in Conservation Areas are subject to slightly different rules than elsewhere.

    Even if planning permission is unnecessary, the conversion must still comply with Building Regulations governing all structural building work; Fire escape routes; Ventilation; Damp; Insulation; Electrical wiring; Water supply and drainage. You will need to submit a full-plans application to your local Building Control department to gain the necessary permission so it is worth consulting an architect, builder, surveyor, or structural engineer first to make sure the regulations are satisfied.

    Don’t forget that, if building work affects a shared wall or boundary, you must also gain permission from your neighbours in accordance with The Party Wall Act 1996.

    When choosing contractors or builders, always check their credentials with regulating authorities. Having ascertained that they have the qualifications necessary, do double check that they have insurance and check to see what is covered. Specialist basement contractors may also be able to provide testimonials or a portfolio to help judge the quality of their work. Always hunt around for the most competitive quote and get it in writing. Some questions you might consider asking are:

    • How do they want to be paid, on completion or in stages?
    • Are they happy to accept a retention clause?
    • Will they give you a final completion date?
    • Will they agree to a penalty clause for failure to complete on time?
    • Will they agree to independent arbitration should a dispute arise?

    Author: C J Mills Google+

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