We just scared off the OP. But my question about securing the separation case was serious. It could also be the goal that she actually acquires shares of the property through her rent payments. How to arrange something like that, I don’t know, except showing up at the notary every year and transferring x% again?
Then you’d have ongoing notary costs... guaranteed expensive. Aside from that, I don’t know if the bank doesn’t have to approve every time because of the mortgage. Even better, out of convenience, people don’t even register the new %s.
The solution is actually quite simple: both in the land register, both in the financing. The percentages could be divided according to contributed equity, income, or whatever, but since each partner brings their full working power (employment income, childcare, or whatever), 50% is the easiest, most common, and in my opinion the fairest solution. Both are liable for 100% in the financing anyway, not just a share.
Or find a partner who has the same financial power as yourself — teachers come to mind spontaneously.