Final answer:
Exterior footings should be placed according to local building codes, which can often exceed historic rules of thumb, potentially recommending 15 diameters deep for improved stability.
Step-by-step explanation:
The standard depth at which exterior footings should be placed varies depending on several factors including local building codes, soil conditions, and the building load.
While a historic rule of thumb once suggested footings being placed 5 side-to-side and 7-8 inches deep, advancements in understanding have led to recommendations of footings being placed as much as 8 diameters side-to-side and 15 diameters deep. It's important to always follow current local codes and best practices which may dictate specific measurements.