I am panning to build a 400sqft shed on an old gravel pad for a workshop/storage space. I do not plan to obtain building permits.

I am looking for DIY foundation options. A full perimeter is not out of the question but it would involve a lot of concrete and the cost of the truck, which is expensive around here.

I was thinking post and peer with a gravel floor to start, with option of adding a slab later. Is there a way to build this rodent proof?

Goals are simplicity, low cost, and critter free. Suggestions welcome.