I think the NPC should give the simple quest (gather XX resources), then when player gives him what he need the NPC will give House Plans. With Plans player go to desert and use it (or use science or outdoorsmen - whatever is required). Then house appears.
Ye, that would be a good solution.
Just thought, could there be a small privilege to buying a big house, like if you get "Junk Houses" or bigger you get a flint nearby.
If you get "New Reno Style Houses" you get the flint and a gatherable tree.
If you get "Vault City Style Houses" you get the flint, the tree and 2 plants maybe, 1 for fiber and one fruit, or only one of them.
Those privileges aint too much to ask for and i think you deserve them for getting the houses. (and ofcourse it still follows the tent rule, building a new house/tent removes the old one)