If the proper name is Johnson, Mark Johnson, then it would The Johnson's, as in, the house that belongs to the Johnson family.
If the proper name is Johnsons, e.g., Barry Johnsons, then it would The Johnsons', as in, the house that belongs to the Johnsons family.
Referring to a married couple, you could use plural, e.g, "The Johnsons (surname: Johnson) throw great parties." But it seems awkward or dangerous to then mark plural with possessive, when you are referring to a singular proper noun. Back to your example, it isn't the case the the husband OWNS the house and the wife OWNS the house -- rather that they own the house together.