Floating footings for single extension as on clay?

Hello, There are a great may types of clay. We need to know the type and plasticity in order to make an informed decision. I suggest that you contact the local authority building control and ask what type of foundation is likely, first. If you are building over or near an adopted drain then a build over agreement will be needed as well. Some trees within 30m of the proposed extension can be detrimental to the ground conditions and foundations designed to take this into account. You may need to get a soil survey carried out which typically costs £900. Kind regards Andy

Not exactly although this may be the case. The soil the foundations sit on are important as they will have to carry the load of the foundation and the extension on top of that. Also the soil will react to water content (heavy rain and dry spells). The way some clay soils react to water content can cause some issues (expanding and contracting). Generally a strip foundation 1 metre deep is sufficient as most ground movement only occurs up to a depth of 0.75 meters. However the only way to know for sure is to contact your local building control department. They will be able to advise you the depth of the foundations required for the soil type in your area. If the depth required for the soil type in your area is substantially deeper than 1 meter then it will be cost effective to construct a raft foundation (floating foundation). I hope this helps.