What do you mean when you say "An apartment doesn't have a single right angle"? Does it just mean this apartment doesn't have any 90-degree angle? Or it has another meaning? If this is the former, then what kind of apartment doesn't have a right angle?
My understanding is that it "doesn't have a single right-angle" is a general comment about workmanship rather than design. It suggests that the quality of workmanship is poor in an apartment where the walls are expected to be (but are not) at right-angles to the floor, or where the the ceiling is expected to be (but is not) the same height off the ground at all points, or the room corners are expected to be right-angles (but are not), or, finally, where the floor is expected to be level (but is not).