Go for powerline wifi extenders, not plain wifi extenders / repeaters.
A basic wifi extender takes the wifi signal and repeats it, but it halves the data rate, plus the extender needs to be in range of the original.
A powerline extender routes the signal through your power wiring and maintains speed, then the other end is effectively another wireless access point. You could do it instead with Ethernet cabling and a wireless access point, but then you have to route cabling round the house. Though a simple access point is cheap, and cable is cheap.