I use Devolo powerline extenders; 50m from a separate office where the router is installed, 2 WiFi transmitters in our large Victorian house, one upstairs, one down.
The office is supplied by its own power circuit back to the main switchboard.
It works perfectly, far better than any signal booster system could have done, and was simple to install.
However, I never thought about the possibility that the signal may escape to a neighbour; would that be protected by the network key?