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Foundation for wood framed extension


I've just bought a house with a concrete yard at the rear. So i was wondering what foundations i would need for a single story extension. It would be using three existing walls as one of the yard walls is made of granite and goes up to first floor level, and the other is made with block and is 5 foot high.
So i imagine it would be the weight of the roof that would be the deciding factor

2 Answers from MyBuilder Extension Builders

A trench foundation 750mm x 600mm, foundation depth would be 1m below ground level as standard but it depends on the site conditions if your going onto solid chalk you should be ok going 600mm deep, I would advise you dig a trial hole and get your local building control officer to confirm the depth they require.


Answered 9th Nov 2015

You have not mentioned if your property is a Terrace, Cottage, semi detached or a detached. If it is adjoining property you will have to address the Party Wall Act as the two side walls will be shared and you cannot assume that you can build off the walls. If the property is a detached than you will be able to upgrade the specification of the walls. The first thing to do is excavate trial holes so you can physically found out the depth and construction of the existing foundations. You then need to get these inspected by a professional. You could ask an experienced builder, building inspector, Structural engineer. Usually you would have to excavate new foundation for the new wall to a depth of 900mm from finish floor level and this would usually be 600mm in width. If you do not have neighbours than the trial holes will reveal the depth and construction of the walls foundations. These may be found adequate or you may have to strengthen the foundations by under pinning the side wall foundations.


Answered 14th May 2019

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