Make the decision for yourself not the girl.
That said, figure out what your parents are expecting out of this. They are building you a fucking house, are they expecting you to live there for the rest of your life? If you're only living there for a year or two I doubt your parents will be too happy with building you what amounts to 15k/month temporary housing. Are they even expecting you to live there or would they build the house anyways knowing that you may not live there?
I could never accept something like that from my parents. I would feel shame in depending on them. Plus, if you're only living there for a short time you're wasting their money. A house isn't cost effective unless you're living there for a long ass time.
Sounds to me like your parents are trying to hold on to you. You know they are going to expect regular visits and spending every holiday with them. I know you're young, but living that close to your parents is odd and letting them hand you something like that won't make you look all that great to most people. Although, if it won't hurt your ego, don't worry about it.
Your parents will respect you if you just tell them you want to earn it on your own.
There's also the possibility of keeping both places.
If you decide that the house with your parents is something you really want go for it. Explain to your girlfriend your reasons, how it would help both of you, etc. If it's still a no go, maybe take her over there once it's built and show her that it won't be weird with your parents. Some people fear change until the benefits are forced in front of their faces. Otherwise just live there, tell her that you'd love to live with her still but the money would really help you accomplish your goals. If she's not okay with what you want, get rid of her.
Final thought: Would you want to depend on your girlfriend's parents like that?? It's not exactly surprising that she's reluctant.
Hopefully that wasn't too random and incoherent.
