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Q&As
Spreading the load
I recently submitted plans for Building Regulations approval for a two-storey extension to a house that has a lower basement – in other words, a floor below the ground floor. Building Control told me that the foundations for the return walls to the extension that are at right angles to the basement wall should go down to the basement level. (There was no mention of the walls that are parallel to the basement wall.) Apart from made-up ground, is there any other reason for this?
Answers
It is usual to assume a 45 degree spread of load from the underside of a foundation so the reason Building Control has asked for this is to ensure that no load is applied to the external walls of the existing basement. It would be usual for the perpendicular walls initially to be founded below the basement floor, generally taken down to match the existing basement wall foundations. These can then be stepped up as you come away, provided you are founding in good ground and that any step height is matched with a horizontal distance between steps of of two times the height. (Steps are generally limited to less than 0.5m.) With regard to the parallel wall at the rear of the extension, provided it’s founded in good ground this can be shallower. However, it should still be no more than 0.5m higher than the corresponding depth of the return walls at the corner. The minimum depth in any event will be 1m on clay and 0.75m elsewhere. Expect it to be deeper based on the stepping criteria above, though.
