Reminder: if you create this form of relationship explicitly, be expected to pay your significant other as much as the FULL mortgage, not half. Half-mortgage is what an owner pays, knowing they have incidental costs.

A renter, however, is expected not just to pay in excess of mortgage to cover all of those costs like appliances and repairs (landlords do not make a habit of losing money on their investments) but also expected to pay fair market value, including yearly raises in rent to keep up with the market.

If I was in this position of splitting a mortgage and someone came at me with "renter / you pay for appliance" the rent I charged them would double instantly.