I live in a building with thick stone/brick interior walls and find that they seriously impede my wireless signal. With the wireless router on the same floor as my laptop I need something close to a clear line-of-sight between them (e.g. through a doorway) to get a connection.
However, the signal passes through the ceiling to the flat upstairs much more readily.
So if you have a house with thick walls and more than one floor try putting the wireless router upstairs so the signal can come down through the ceiling into each room.