St Aubyn was undoubtedly assisted in developing his career by his family's prominence in Devon and Cornwall, and particularly in Devonport, where they were the major landowners. He practised chiefly in London and developed a practice which extended all over southern England, but he also kept an office in Devonport for part of his career, and he was employed particularly extensively in Devon and Cornwall. Apart from this local connection, there are clusters of his work in Gloucestershire (no doubt deriving from his years in Fulljames' office), Kent, Reading, Cambridgeshire and Leicestershire.