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Basement Pros and Cons

A basement can be the key to unlocking potential and getting the most out of your plot. Here's how.

With our diminishing building space and fast rising land prices, the return of the basement makes sound economic sense. So why are they still so rare in new houses in the UK?

Basements were fashionable with the Victorians and Edwardians when labour to dig the cellars was cheap, but with the coming of World War One the rate of building fell dramatically. After the war the number of household servants radically diminished so the need for accommodation below stairs was greatly reduced. With the rush to the newly-expanding suburbs in the 1920s and 30s, thanks to improved public transport, more land became available for building and so the need for below ground accommodation diminished.

Today it is estimated that less than two per cent of new homes being built in the UK have basements, although as many as seven per cent make use of sloping sites to build a partially submerged basement. Nevertheless they are becoming less rare according to Alan Tovey of the Basement Information Centre. "Many self-builders are seeing the obvious advantages of going down to provide extra space as plots get smaller and more expensive," he says.

"However, the real impact will come when the volume housebuilders move into basements. At present they perceive them as simply an extra cost, but new planning directives from the government plus Westminsters drive to increase housing densities will, I am sure, nudge more and more housebuilders in the direction of basements."

What are the Benefits?

Space and better value for money: It has been estimated that in the UK we need 4 million new homes in the next 20 years, so naturally one important option when it comes to using our available building land better is to go down. By adding a basement to a two storey house you can get as much as 50 per cent more floor area without having to increase the size of the footprint or having to sacrifice any more garden.

In Somerset, self-builders Cyril and Samantha Healy have built a new timber-framed house with a basement built into the foundations. "It made a lot of sense," says Cyril. "We have a large extended family and wanted a big guest suite, including a kitchen and utility room. It is not yet finished but we reckoned that when we were having the house built it would be a lot easier and more economic to do this at the same time."

Cyril has estimated that by building the six-room basement for little more than 20 per cent of their total build cost of £90,000 (two years ago) they have added more than a third to their living space. For their basement the Healys used the Beco permanent polystyrene shuttering system.

Growing Families: Basements have a big appeal for growing families who do not want to move house. Gordon and Louise Craigen decided to add a basement to their Victorian Terrace house in Parsons Green, West London.

"We loved the house and its position. It is convenient for my husband's work and the children like it, so we decided to extend by going down," says Louise Craigen.

"When we worked out the cost of moving, once we had paid two sets of solicitor's and agent's fees plus stamp duty on a house that was then worth around £500,000, it would have amounted to well over half the cost of the basement."

For £80,000 they have a new family room, a utility room and a lavatory, increasing the area of their home by around 25 per cent. The cost fully finished worked out at about 2,000/m². Louise says: "We thought it a good investment. It meant we could stay here and has added greatly to the houses value."

Better Site Utilisation: If you are having to dig deep foundations for any reason, it might make economic sense to add a basement. This is what happened in Highbury, North London, when architect Colin Hodson was building a terrace of three mews houses. "I was going to live in one of the houses and we were required to put in foundations three metres deep and a metre wide because of the the proximity of the trees and the site was on clay," he says. "Having put in such enormous walls beneath the house, it made sense to dig out the area between, add a concrete slab and turn the area into a basement. At first we used it as a playroom for our son, then I turned it into a home office, from which I ran my practice for two years."

Basements can also make good use of a sloping site. Rather than level it why not create a semi-basement, if necessary by doing a cut and fill, digging out from the slope and then levelling to form a firm base upon which to build? This is what Heath and Ruth Rowland did with their self-build on the side of a valley in a wooded site in Cheshire.

"We had little option but to build into the embankment because it was such a steeply wooded site," says Heath, a civil engineer. "Building it this way has enabled us to have five bedrooms and a house of 430m²." Although the entrance is through the basement floor, all of the bedrooms are upstairs. Heath constructed the basement using a tanked masonry system. "It was expensive but we think it has been worthwhile," he says.

Another example of good site utilisation is MP Bob Marshall-Andrews new house in a desolate spot on the Pembrokeshire coast. Some years ago he and his wife Gill bought a plot of land with an old wartime hut on it that had served as a lookout point. They gained permission to replace it with an earth-sheltered structure. The result is a stunning £250,000 abode, designed for them by London-based Future Systems (www.future-systems.com), that looks rather like a flying saucer that has landed and stayed there - and gradually become covered with turf. It is an unusual structure because most of it is built from two metal pods, containing a kitchen and the two bathrooms, that were craned onto the site.

A similar situation faced Erlend and Clare Copeley Williams. When they moved to their eight year-old chalet bungalow on the edge of Shaftesbury, Dorset. They decided to utilise the slope and maximise the size of the house by extending at the rear. This would give them a magnificent view over the Vale of Blackmoor and the ability to build a semi-basement into the slope beneath.

Clare, a ceramicist, uses it for her studio and because it is built into the hillside has a view through the window almost as good as the view from the room above.

Gaining Extra Space: Basements can be a good way of gaining extra space in sensitive sites where there are height restrictions.

A good example was the self-build of Steve and Phillipa Evans in a village on the Kent/ Sussex border. The site is in an area of outstanding natural beauty and their permission was to demolish the existing old bungalow and build a new one. By building a full basement using the Styro Stone expanded polystyrene formwork system they were able to add 90m² to their original permission for 167m². The basement has two light wells and is used, amongst other things, for sleeping accommodation for their two daughters.

Energy Efficiency: A house with a basement has fewer directly exposed external walls.

Wall and floor insulation are improved and the means of construction incorporates additional mass. The effect of this is that the building does not overheat or cool down too fast. It therefore helps to retain heat and to control fluctuations in temperature. This is why the Basement Development Group says that a house with a basement can be up to 10 per cent more thermally efficient than one built entirely above ground and will also bring about savings in the quantity of insulation material needed.

Reduced energy demand is one of the major advantages of his basement, according to Nottinghamshire-based architect Allan Joyce. He and his wife Anna built a complete storey of their house below ground in response to the difficulties of the site - it would have been very hard to get permission for two storeys above ground - and because of Allan's commitment to an ecological approach.

"The house was designed as a thermal store and with our family of seven all sleeping in the basement we find it very warm," says Alan. "There is no wind chill and the ground temperature outside is regularly 14°. We have a below-ground courtyard and a lot of natural light coming in through rooflights and on a sunny day in winter the temperature in the basement has been as high as 25° with no heating on." The basement section of the house is of solid masonry construction, with an external membrane and thick polystyrene insulation between this and the soil.

However not everyone likes the quality of light achieved by basement living. While the Joyces were able to gain light from a long angled skylight this is not always possible. Several architects do not like basements beneath houses for this reason. As one architect says: "With basements you usually have to dig a hole [a lightwell] in order to light them."

Multi-purpose use: One of the joys of a standard basement is that it can be used for a variety of purposes and graduate from being a children's playroom to a television and games room for them when they grow older. Unusual specialist uses include basements specifically designed to provide swimming pools, as in the house of businessman Jeremy Paxton on the Thames near Reading. The basement contains a guest bedroom, study, bathroom and wc, while the front section has been flooded to create a dock for his motor launch. In London, Irene Lukas, a lover of jacuzzis, has just completed a basement containing a whirlpool large enough to take four people, while near Wolverhampton, a self-build currently under construction has a basement specifically designed to house the owners koi carp collection.

What are the Cons?

Causes of possible failure: The British Structural Waterproofing Association and the British Cement Association do not recommend building basements particularly those that rely entirely on tanked protection on sites where the water table is too high. This is defined in the Waterproofing Design Guide (available from the Basement Development Group) as where the water table is above the underside of the lowest basement floor slab. A very useful risk assessment chart is printed in the guide.

Problems of this type are likely to occur when a head of water brings pressure to bear on the walls of a basement. If there is an external membrane for waterproofing it is always possible you might get a leak. A hydrostatic head is totally different from percolation of water. A head of water refers to depth, so if, for example, a burst water main were to saturate the ground on the outside of the basement, the head of water would reach the top of the basement walls.

If you have a leak in the membrane it should be easier to carry out repairs if it is fixed inside rather than outside the structure. It is possible to open up external systems for inspection, but this may not be practical if there is hardstanding above or, as is often the case, where the basement is partial that is, beneath only a part-section of the house.

Peter and Freda Dawson encountered a very similar problem to this when they were completing their five bedroom self-build home in Surrey. They had not originally intended having a basement but found the site contained a great deal of made up ground as a result of previous construction in the area. As the excavations therefore needed to be substantial they decided they might as well build a full basement containing a study, storeroom and games room.

Peter, who trained as a builder, constructed a blockwork system, using concrete blocks, reinforced in the centre with solid concrete. On the inside he used a proprietory bituminous membrane system sandwiched inside an inner skin of Thermalite blockwork.

The plastic membrane extended beneath the floor screed and polystyrene insulation beneath, but while installing this, disaster struck. Heavy rain in winter 2001 penetrated beneath the membrane, which then began to rise up. Despite 30 years in the building business this was Peter's first basement. He decided to remove the membrane beneath the floor and sealed the bottom edges of the structure where the walls join the bottom concrete raft with a well-established glass-fibre based sealing system.

It failed. Water constantly seeped in and Peter had to call in specialists Polycrete Basement Systems to rectify the problem. They did this by installing a new internal membrane system that drains to two new sumps situated beneath the floor of the building.

Peter says: "It was hell. We were eventually able to move into the building before the remedial work was complete but it caused a huge amount of inconvenience and nearly seven weeks work to rectify the leaks. The total cost was around £16,000 and we are seeking redress from the engineers who designed the system, which we have been told would not have worked anyway because adequate drainage was not designed into it."

Independent consultant Phil Hewitt is a past chairman and current vice-chairman of the British Structural Waterproofing Association, and nowadays spends much of his time troubleshooting. He says: "My advice is to seek the opinion of a basement specialist before you start. So many domestic clients have come to me over the years saying 'if only we had known'. You do have to be very careful as there are companies that will take your money and when you find you have a problem a few years later they are no longer there.

"On the other hand there are some very good companies around. I have to say that I would have no hesitation in buying a property with a failed basement, because I know I would be able to get it dry. And if I was in London in particular I would know that the cost of the increase in the value of the property would be greater than the cost of rectification."

Another leading name in the basement business, Ray Foulkes of Polycrete Basement Systems - who supervised the rectification work for the Dawsons - takes a slightly more cautious line. "Personally I would never rely on any tanking system alone to hold back water pressure," he says. "With tanking alone I believe you can only rest at ease when there is very low water pressure. In scenarios where there is a high water table, tanked protection is not recommended and in my view in situations where, due to the prevailing soil conditions there is variable water pressure, I would only use membranes in conjunction with other means of protection."

One other potential drawback is that if you are contemplating a basement in your new self-build you might find your favourite architect may not be willing to take on the brief. The reason is likely to be fear of litigation should the basement fail.

One architect in a small partnership says: "If there is a failure they fear the client might take them and the structural engineer to court." A number of very famous architects, including Frank Lloyd Wright, were against basements beneath houses. In this country it is generally the smaller practices who are against building basements in domestic schemes on the basis of: why take the risk? In towns where basements in new buildings are the norm, the big practices are usually quite able to cope.