here on the rez no permits are required. but id never build a full garage here because you never actually own it.
a friend just built one here with local help and roughly the size you like for about $24,000.
the only thing id ever build on rez land is a cabin
at least in treaty 3, you can own the structure(house/garage) but you never own the land. thats too sketchy for me, even though it dramatically reduces the cost