No one literally builds their own house today - at least, not alone. Some people like to have as much control over plans, construction etc as possible and also muck in and "help" / annoy on the building site, however. I guess if you neither picked it from a catalogue nor simply bought a new house that you have no or next to no influence on how it looked (say on a brand new privately built estate) - in which case of course you're still making the decision to buy before it's actually finished - I'd call it "building a house".