Whether your partner is Japanese or gaijin, you might come across some problems living together prior to marriage in some rural areas of Japan.
When I moved here with my fellow English-jin woman we were told to lie to the Board of Education about our marital status. "Say you're engaged" said our boss. And that's no small deal. In Japan that's basically unofficial marriage. Japanese women are not interested in a long engagement.
We did what she advised because she said we may not be able to live together if we didn't. It was very strange 9 months later when we actually did get engaged and had to keep it a secret from the BoE. They did find out though, and just looked a bit confused. It's possible our (now fired) boss was just a bit batty.
Nevertheless, living together is a big deal here. A lot (a lot) of people don't leave home until they are married, and then move straight into the marital home (or husband's family-home). Not that anyone will consider you a proper person for a long time anyway, but beware of getting funny looks for your living situation.
Also, if you're living in kyoinjutaku (teacher's housing), some backwards countryside BoEs might not let you live together (as such) and give you two houses.
If you live in a big city, virtually none of this applies.
If you have any experience of this sort of thing, please let us know in the comments box below.