Every situation is unique, I've known houses in the same street have totally different foundation designs for near identical extensions, so one should always employ a structural engineer to design the foundations and structural framework to suit the local conditions, rather than rely on what other people have done. A SE will carry out preliminary checks/tests on the soil conditions, for instance a common problem in London is tree roots causing the clay soil to 'dessicate', thus making it unsuitable for standard deep foundations, and instead requiring a series of very deep pile foundations. A SE will also allow for differential movement between the old and new, as should any decent builder e.g. movement joints...