The plaza is defined by an invisible 16x16 'acre' of land, not just the cobblestone it bears. From there, a villager home needs to be 1 space away from that acre - and possibly 2 spaces away if plotting north of it.
To figure out the boundaries of the plaza, try plotting a works project, and check how close Isabelle will have it built on all open sides of it. Like homes, works projects also need to be 1 space away from the 16x16 plaza acre, so you can mark boundaries by how close they can be.