Can't really add any more to what's already been said. It's all good advice.
I cabled my house throughout, running at least a double outlet to every room, and sometimes a pair of doubles to allow for room rearrangement. In reality lots of the wires go unused, but better safe than sorry.
Bring it all back to a central point so it's easy to manage, and can help with isolating noise.
This is what I ended up with:
Very happy with D-Link, TP-Link and QNAP components, all faultless.
Mine is a somewhat simpler system, occupying less space, though of course possibly having less connected (typ 10-15 devices):
8 way unmanaged gigabyte switch sitting next to Fritzbox wireless router/VDSL modem (which has 4 ethernet ports), together with a small NAS (ZYxel NSA325) on a little high shelf on the landing, with mains outlets alongside and phone line routed there. From there 8 Cat 6 cables run to various rooms through eaves space and unshared wall conduit. Further unmanaged gigabyte switches are then used locally where more than one thing is to be connected. (Two 8-way, one 4-way, and another 4-way that is part of an additional wireless access point making use of a redundant Linksys ADSL wireless router/modem.) This keeps flexibility without running multiple cables to every room, other than the lounge that has two outlets in different places because of its size. The switches are all cheap unmanaged ones, all very compact.
Works perfectly, never any issues, other than ensuring no IP address clashes whan adding anything new. Used by two people seemingly constantly for either online gaming or video streaming, plus various other uses. When I played my music from a NAS I sited it in the music room connected to the same switch as the then ND5XS, and nothing else on that switch, and never any glitches. (I no longer need to play music across the network as it is all in the Mac Mini that doubles as renderer). Lack of problems includes video streaming from the NAS to the lounge via three switches and about 25m of cable. The house phone is VOIP also connected to one of the network outlets.
in general with domestic networks I agree with Huge's KISS recommendation, which is what I believe mine is.