I want to know, how can a house be built above water, starting from foundations up to the roof?

This should help: HowStuffWorks "How House Construction Works" (http://home.howstuffworks.com/home-improvement/repair/house.htm)

Same way they build Venice. You need to put in really deep piling so that it sits in the ground where it is no longer wet. That will give you your base and from then up it's just like any other house.