Your unarmed neighbor's house happens to be on the land of an existing nation, so you would have to deal with their military before claiming rights to said house.
The question of "legally" invading someone's house is odd anyway, because legality implies some authority having deemed it appropriate. In the case of a neighbor's house, it is the government of the country containing the house. I can't think of an entity that establishes the "legal" way of creating/invading a country.
Of course this is all messed up. There's no reason a nation shouldn't be considered a nation just because it doesn't have a military.