the uchihas had big houses probably because they built the houses themselves.....
notice how in the after-effects of the war, everyone was building stuff? I'm sure back in whatever time period naruto is set in, people are alot more independent and versatile, and clans can work together and build their own communities, without having to mess with contractors and hiring builders.

Itachi's dad: "Itachi, go get us some more lumber to add another wing to the house"
Itachi: ".........." <pulls out katana and slices down several trees>