Hello Berg,
I run a small chartered architectural practice in the north west so hopefully I can give you an accurate answer!
The council building inspectors will most likely insist a trial hole is dug to ascertain the exact depth of the founds on the extension and to assess the quality of the ground, i.e. if the footings are within suitable clay. In more recent years most authorities have insisted that foundations for single storey extensions are dug to the same depth as two storey extensions (typically 900mm below ground level minimum) or to match the depth of the existing house footings to cover their backsides incase of subsidence.
If the extension is reasonably recent i.e. built within the last ten years, it may be worth a call to the relevant local authority to see if they still have the original plans on record, If so they will let you look at them in the office.
You can double check any additions on Google Earth and utilising the historic timeline feature.
Hope this helps but if you do need more info drop us a p.m. and I'll give you my email
Mike