"powerline adaptors won't work if the two electrical sockets are on separate breaker boxes"
This is an incorrect statement. The modern models work fine as long as they are on the same main circuit box (so not if one is one the house box and one on the guest house box). I have tested a dozen of the newer models on my 30 year old wiring and they all work quite well across multiple breakers and at distances that wireless cannot handle, particularly between floors. They are quite useful in extending wireless by using them to attach distant access points. Read up on the newer models at SmallNetBuilder.
Moreover, wireless is often a problem with multiple stories -- it really depends on the building materials. If you really want to make...