My (now) wife and I had a cohabitation agreement draw up as we bought a house before we were married and my wife was putting a significant amount of (inherited) money in for the deposit. To keep this going, when we did get married it was turned into a pre-nup, which definitely felt a bit weird…
One thing I will say, is we each obviously had to get separate representation. My wife’s solicitor went about it as if we were splitting up the house at that time and devised some ludicrous formula for working out all parties contributions and scaling them for inflation (or deflation), proposing a deal heavily weighted in my wife’s favour….either that or her solicitor was really rubbish at maths. I think it was my wife’s put in x% of the deposit so would be entitled to x% of the house…conveniently ignoring potentially years and years (I would very much hope) of 50/50 split mortgage payments!
OP, the point I’m making is your other half will have their own solicitor advising them. Their solicitor will not act on behalf of you two as a couple, they will act and advise only in your other half’s best interests. It has the potential to get tricky as you’re already living together and she’s contributing.
NB We’re in Scotland and cohabitation/prenups seem to stand.