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Foundation and extensions.

Updated: Nov 15, 2022

There is a lot of confusion about foundations and extensions, especially where the requirement is to build over an existing building. Often the question is whether the existing foundations will support the load of the additional building. There are two very simple things to remember about foundations. 1) The width of a foundation and the nature of the soil below dictates how much load it can take. 2) The depth of a foundation is only important to stop the foundation from moving due to fluctuations in the ground below.

In Milton Keynes, where I practice, the clay is usually supportive enough so that even a narrow foundation can support two storeys and therefore the foundation put in to support your single storey building will normally be able to take the load from two storeys. Thus you can often build over without worrying about the foundations.

In respect of the depth of a foundation, if you are building on clay, as in Milton Keynes, you have to take the foundation down to a depth where it will not be affected by seasonal shrinkage, ie dry summers ( remember those) causing the clay to crack and dry, even down to 900 millimetres. This effect is made much worse by the presence of trees where the ground can desiccate/shrink even at depths of 3 meters plus due to the trees sucking up so much moisture. This type of problem will cause cracking in a three storey house or a garden wall alike and thus logically no matter how much the load if you want to avoid any type of movement you have to go deep on clay. Of course, nobody invests a lot of money in a garden wall foundation so no wonder they crack and fall over so often.

It is not the same as building on sand. Sand does not suffer from shrinkage in the same way and so a much shallower foundation may be okay. However the foundation may have to be wider to take even a modest load – it depends on the type of sand.

Therefore most buildings can be successfully built over, using the existing foundations but, if there is evidence of movement such as cracking then it would be unwise to build over since that will probably crack too. All of the above is a generalization and you need an expert to advise, but you should not be put off exploring extending upwards because of fear regarding the foundations. Talk to us and we will try to advise on the best strategy.

Written by Tony Keller – Building Tectonics Ltd.

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