While traveling through London a few weeks ago, my wife and I were able to cut costs by staying with friends. Since buying their place a year ago, our friends were keen to show off the home’s special features, including a bright garden lined with dark brown, antiquated railway ties. Nice touch.

But it was the parts of the house I couldn’t see that I was most interested in: the second floor. You see, in London, at least, owning a home doesn’t necessarily mean owning the whole house; co-ownership is common, especially as house prices climb, pushing potential buyers outside of the market. Live downtown? Would love to, if only I could afford to buy and not just rent; plus, the houses are just too big. Solution: shared ownership.

In North America, where this type of co-ownership isn’t as common, sharing a house usually means finding a buying partner and then retrofitting the place, either vertically (dividing the house in half from basement to attic) or horizontally, with one owner taking the bottom and one taking the top. In most cases, the two units will not be defined legally as separate dwellings, so the house title and the mortgage will be singular. In other words, unlike the U.K., where a single floor—a flat—can be defined as a dwelling, and carry its own mortgage and title, North Americans must choose their partners wisely or risk mortgage-payment fiascos and other shared-property problems related to renos and repairs.

Still, risk acknowledged, shared ownership can be a smart move so long as you’re prepared to do some extra planning and, let’s face it, live with other people. Because even if you build separate entrances, you’re still sharing space, so make it formal by writing up a set of rules and protocol for things like emergency repairs, large-scale fixes (what happens when the roof needs an update in a few years?), and, most importantly, what to do if one person wants out of the agreement. Like any investment, co-ownership carries risk and reward.