Are the houses really secluded just not from cities, but dungeons too? What are the closest landmarks?

I've only built Lakeview Manor, so that's the only one I know about. It is comfortably far from any town, about midway between Falkreath and Riverwood. Whiterun is the closest big city, and it's pretty far haul. However, other locations are nearby. Pinewatch is literally your next-door neighbor. Half-Moon Mill and the Thief/Warrior/Mage standing stones are just down the road (in opposite directions). Helgen is about as far as Riverwood, but east instead of northeast. Those are the only major nearby landmarks i can think of.