the main house has been underpinned. i want an extension. how deep do the footing need to be to eliminate future underpinning issues with the new extension

Hi there, Normally your foundation will be a minimum of 600mm for a strip found. The underpinning may have been done as the original founds were unadequate depth or thickness or soil issues. You would be best to a structural engineer to have a look at it for you. Andy

Rule off thumb is new footings same depth as underpinninng.I myself had my complete house underpinned to a depth off 3.5m so when i started my extension ( only small off 2 existing house walls) i dug a 3.5m x 1m pad which inspector was happy with.

The building would need to be as deep as the underpinning on the house, close trees within 5 mtrs could also be a problem.

the final depth of any foundations is all dependant on the ground type. The only way to find out is excavating to good ground conditions and calling local council out to inspect, they have the last say on this matter.

There are a great many factors in the depth of foundations, If you contact Building Control they will be able to tell you road by road as to their requirements. With a local tree previso. An Engineer could design you a raft foundation to reduce digging(and costly spoil).