Morning Mark,
I'm pretty new to this too so this might not be the best advice, but I'm currently trying to do something similar so I can tell you what I've found so far.
Amp-wise, are you looking to run a 5.1 or higher set up as well? You can easily find something that runs both movies and music nicely, providing you pair the right speakers with it. The what hifi reviews usually cover off the music credentials of the a/v amps. I'm looking to get a Yamaha RX A2010 (last year's model going for a very good price) to pair with B&W M1 speakers. It really depends on your budget though.
It sounds like you should also invest in an amp that can serve 2 zones - this means you could wire it to your kitchen set-up with out the need for investing in another amp as a slave amp. That way you can feed it to your kitchen. Did you want to be able to feed a tv in the office though, or is that just for music?
Music-wise, I'd strongly recommend getting a NAS if you don't have one already - you can attach this to your router and then stream your itunes collection throughout the house to various devices. I've got a Synology DS 212j and it's been great for streaming music and film around the house. Again, what NAS you'd want would depend on budget, the size of music collection, if you wanted to stream film as well etc.
Then to go with that, you could look at Sonos system to distribute your music around the house. Sonos do speakers that stream the music (Sonos Play 5 or Play 3), an amp that connect directly to speakers (Sonos Connect Amp) or just a box that streams through an existing amp (Sonos Connect). These can all be controlled by a free IOS or android app, and can be configured to stream the same or different combinations of music to the different zones you have set up.
So as an example, you could have a Sonos Connect attached to your A/V receiver, a Sonos Play 3 in the kitchen and a Sonos Connect Amp connected to some dedicated speakers in the office. Many A/V amps these days can stream music from a NAS without the addition of a Sonos system or similar, but the benefit of having a connect would be having one system throughout the house that can be controlled through one app.
If you're building an extension, you may also want to consider ceiling speakers. You generally take a drop in quality with them, but they have their benefits to suit asthetics etc.
And if you're wiring up the house anyway, make sure you put in some cat 5 or cat 6 to support any of the above - while most kit supports wireless these days, wires are always preferred like you say!
Hope that helps a little.