Do you have a geotech engineer in the area that you know. They're the ones to ask, they always know. I'm sure there's one that you are on a first name basis with. Hell, if you cold called someone they would probably give you an answer.
Thanks. 4 feet is same as Toronto, which seems strange unless perhaps it was a non-frost susceptible soil. I will try calling the Huntsville building department.
Most of the time 4'-0" is accepted in that area, however, I have had geotech reports for that area specifying 5'-0". However, 4'-0" is the norm, 5'-0" is the exception.
Cottage footings used to be regularly installed insufficient for frost heave or any form of frost protection. Sad, but true.
Bear in mind (bah-dum-dum-dum) that most structures throughout Canada have full height basements for very good reason: It is simply the cheapest and most reliable form of construction.
If you are going with piles, sonotubes, or any other "point" support you need to size them for more than just the frost depth. If you are going to be providing strip footings at below 4 or 5 feet, I frankly see little reason to deny the client significantly more space for little additional cost. Not your choice, of course, but most clients will opt for a little more money for a great deal more space.
While cottages have been largely abusively built from a foundation point of view in the past, they are actually some of the most challenging foundations to get right. They are frequently unheated for long periods of time, and frequently at or near water sources which are unmaintained in the winter and thaw. Be careful with your decisions; Sumps can be a bad idea, and draining walls are hopeless.
It was built last year. I had nothing to do with the design or construction. I was asked by a friend to review some things about it after it was completed. I have never seen a small seasonal cottage, accessible only by water, that has a basement. Also, part of it is on rock. The piers have small spread footings under them. Some are on rock and some are on soil.
