So the people selling permits are trying to take advantage of other players(who are either uninformed or lazy). Theyre taking something from the equivalent of an NPC merchant and hoping to sell it at a markup(potentially a huge markup). Buy your plot, take a few days to save up if you need to(i think youre right about having 45 days, but obviously try not to wait til the last minute), then buy the permit straight from the placard. Your plot will only allow one size of house(cottage for small plot, house for medium, mansion for large). Whichever option you pick for the permit(wood, stone, etc) gives you a "starter house". Then while on your land theres a housing menu you can access, which allows you to remodel your home. At this point, you can mix and match some of the options(wooden walls with a stone roof plus a more ornate fence? sure) to get a house uniquely yours(you will need to provide the parts you want to change - so if you want a new stone roof you either need to craft or buy one from someone). There are also housing "skins" that fill all the customization slots to allow for special "themed" houses(if you walk around your housing district youll probly see some people with otter carbuncle or moogle designs as examples)