Check the dynmap for the Worldguard boundaries, but yes... aside from any areas (Roma, for example) that will be made into cities, you can still build wherever you want.
Owning a house area inside a city is just a luxury (no mob spawns, your entire plot is worldguard protected from other players/griefing, and you are close to the voxport hub as well as the merchant district etc.
Oh, and players get to see your stuff every time they die or want to buy something hahahah