Now if this was in Skyrim, it would add a lot of realism. It could be used for burglary (casing out a house), in quests when you need to find someone, or ding dong ditching. The way I imagine it, since the interior of houses are generally a separate cell, so you can't have the house owner just open the door and talk with you. The game could do it similarly to a conversation, where you can hear them talking from the other side of the door. If they like you they'll invite you in and you accept the invitation and your character will appear in their house. If they don't like you they may just tell you to go away, maybe if you continue to annoy them they will come out of the house and confront you.
Let me know what you think. Ideas, suggestions, comments are all welcome.
p.s. - Obviously most of these suggestions are too late to be added into Skyrim, but possibly implemented into future games.