Yeah, this is all going to be relationship-based. You can do the market based rent and have of utilities thing for a regular roommate, but with the relationship and the adjustments and commitments one makes to each other based on said relationship add another dimension to the question.

> But what if for example B is currently earning less because a new job was just taken and A encouraged them to take it?

that's a relationship thing, not a finance thing.

Then you've already worked out what's fair and probably no point in asking here.

"I'm ok with you making less" is directly followed by "And here's how we'll agree to cover costs".

If the owner of the house decides to offer a discount to market-based rent, that's going to depend on their overall assessment of the situation, and income change doesn't necessarily change anything about the situation (although it might, and probably would in this case.) Suppose one of the couple didn't have a change in base income, but did receive a huge inheritance. Then what?

Ordinarily if you rent, you don't take the landlord's income into consideration in terms of what you are willing to pay.