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". 
My parents built the house they currently live in back in 1998 (in fact, I think most, if not all, of the houses in my neighborhood were customly built for the owners). I don't remember the process too well since I was around 8 at the time, but they fired the original architect due to visionary disagreements or whatever so they definitely had some sort of creative control over the process. They actually had a Hindu priest bless the then-empty lot prior to construction, as I recall.
