I want to build a detached garage at my cottage in Quebec. I would like to build a 20' x 24' with a loft, and am wondering whether a floating slab will provide long term stability, or a full 48" deep foundation with footings is required. I realise that soil conditions and local codes play a huge part in this decision, but I am looking for general guidelines or thoughts.

Definitely need foundations extending below frost line.

In a cold climate with 48" or more frost, it is hard to beat a building with deep footings and frost walls/stems and an interior slab (4") between, but not tied to, the surrounding walls. This gives the stability of soild footings and no frost heaves.

Just make sure you have good compacted granular fill under the interior slab and that you have the top of the stem/block wall abour 6" or more above the slab so you can easily keep it clean when you flush out the winter residue.

A structural slab (6" to 8") on grade must be thicker and is exposed to different conditions. I can also do funny things since the frost gets under it. It can heave. If it moves uniformly, there is not much of a problem. If the soil is not uniform or if the building shades the sun, the freezing and heaving can be different and the building will move and may not come back to the height is was. You also end up with a plate directly on the slab that can rot.

Thanks, that is pretty much what I thought, nice to have it confirmed by experts.