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My boyfriend and I have a strong relationship, and we’re planning to move in together. The catch is he already owns a house, whereas I rent my apartment. We make similar incomes and feel comfortable splitting expenses evenly. I would want to contribute to housing costs, but it seems a bit unfair to split a mortgage payment down the middle when it’s going toward the house he ultimately owns.

His house is not in a location where I would choose to rent otherwise, and it doubles my commute time, but I’m still happy to move there as a next step in the relationship. Of course, if we end up life partners, the question of contributing to his mortgage would be a wash, but I also want to be realistic. If the relationship ended, should I be okay with walking away having paid off a potentially large portion of his mortgage? Is there a typical practice you know of in these situations?

Your question neatly distills the problem that every couple eventually faces: How should they split expenses that appear to benefit one person disproportionately? And, perhaps more important, how should they talk about it?

Let’s start with the facts. Under law, your boyfriend’s home belongs to him, and in many states, it’s his and his alone until (or unless) your name goes on the deed papers — even if you get married, because it qualifies as a premarital asset.

Unfortunately, there’s no numerical formula for the equity situation, despite the fact that cohabitation before marriage is more normal than ever. However, there is a relatively simple way to break it down, in concept: Treat housing expenses as though your boyfriend is your landlord. Detached, yes, but also fair.

“Approaching this arrangement from a landlord-tenant perspective may seem cold, but it is objective, straightforward, and meets the interests of both parties,” says Sarah Asebedo, a certified financial planner and professor at Texas Tech University. “The bottom line is that if your boyfriend owns the house, he bears all of the associated risks. These include a decline in the home’s market value, any expenses associated with its future sale, and necessary maintenance and upkeep.” If a dog-walker happens to slip and fall in the icy driveway, guess who’s liable? Not you.

In return for shouldering these risks and expenses, your boyfriend will build equity and perhaps one day sell the house for more than what he paid for it. But for now, that’s not for you to worry about. “It’s not fair to expect to receive these returns if you are not bearing any of the risk,” explains Asebedo. “To justify asking for part of those returns, then you would need to become a joint owner and borrower of the property. But given your relationship status, it sounds like you aren’t ready for this.” Then there’s the fact that you wouldn’t even want this house anyway. “If you wouldn’t buy it on your own, it makes no sense to ‘buy’ it now,” she adds. (Another option would be for you to become a “tenant in common,” which would give you a percentage stake in the property — but, again, that seems like more trouble than you want.)

Now for the trickier part: How do you decide how much to pay him for “rent”? It’s reasonable to think that a 50-50 split of the mortgage payment would be fair, but Asebedo says that might not be the case. Instead, she suggests that you check out current rental rates for similar properties nearby. “Either pay half the mortgage or a fair rental rate for a similar property — whichever is less,” she says. Then, of course, you can divide the rest of your living expenses — utilities, cable, groceries, etc. — the same way you would if you were both renting together. (Asebedo also recommends getting your own rental-insurance policy to cover your personal property.)

In other words, think about this and discuss it — but don’t obsess. “As a renter, you’re already paying part of the mortgage for whoever owns your home, so in that sense, nothing will change,” says Clinton Gudmunson, a social scientist who studies family economics at Iowa State University. “You’re still paying for a place to live, and that’s worth any person’s money. This is just a slight shift in mind-set.”

That said, this needs to be a conversation that takes both of your feelings into consideration as well as your finances. You don’t think of your landlord as someone you live with; you think of them as someone who finally fixes your drains after you email and call them repeatedly. And that’s why you’ll need to have an open mind, a sense of humor, and a willingness to acknowledge when things aren’t going well and should maybe change. (Perhaps your rent might get raised at some point — hear him out!)

“Having honest conversations with him about the financial arrangement is essential,” says Asebedo. “Be careful to listen and understand his perspective.” Obviously, there will be complications — maybe you want to repaint the living room or hate his ugly entertainment console — and that’s when you’ll have to communicate and, ideally, have some faith that things will come out in the wash. You wouldn’t split the dinner bill based on who ate more bites of sushi, at least I hope. Research has shown that spouses who cohabit before marriage do not always demonstrate more “positive problem-solving” than couples who don’t — which may come as a surprise but goes to show that regardless of your relationship status, sharing space is always a learning process. But in being intentional about what you’re willing to pay for, who owns what, and the differences between your financial positions, you’ll set yourselves up for a much smoother ride if and when your finances blend further.

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