Partner A is going to own the house and pay the mortgage whether or not they are dating and no matter where the date is living. Offering to have their date live with them doesn’t change anything financially except it saves Partner B from paying housing costs. Even if Partner A normally has a renter, I doubt the renter shares the same bed with their landlord.

If Partner B wants their own room in the house, then determine fair market value of that room and sign a lease agreement. That way, if the dating suddenly stops, then Partner B isn’t also suddenly homeless.

The other financial aspects of dating won’t change. Partner B has their own expenses for utilities, cellphone, groceries, and personal care. Most couples do this on a casual basis such as, “I’ll pay the electric bill this month” or “I’m going to the grocery store, do you need anything?” Partner A continues to pay for all house related expenses because they were going to pay for those anyway.

In the meantime, Partner B saves their housing budget. As mentioned above, they are at risk for suddenly finding themselves homeless so they might need that money.