There are lots of things to think about ranging from the quality of the speakers, the thickness of the speaker cabling required, fire ratings (you need fire hoods for speakers installed below living areas), sound penetration in adjacent rooms, what your sources will be, how to get those sources to the various rooms, how to zone and group the rooms, individual room volume control / grouped volume control /whole house volume control, audio sync - will the rooms play the same source without timing lags causing echos, what streaming sources you might want to use and the compatibility of the file formats, whether you want a central music library for your own iTunes/MP3 library, ease of use for others in the house......... and when you've gone through most of that it's then a case of working out how much it will cost and whether you can afford it.
Here's some threads along similar lines
//www.diynot.com/diy/threads/radio-in-open-plan-lounge-kitchen-diner.427264/#post-3344798
//www.diynot.com/diy/threads/bose-soundtouch-535-surround-sound-system.445404/#post-3495427
//www.diynot.com/diy/threads/metal-speaker-enclosures.427507/#post-3324121
I've fielded questions from forum users and from clients who started off with seeing cheap in-ceiling speakers on Ebay for around £20-£60 per pair and who though those would be a good start for multi-room. They're not. They sound lousy. I know this because I've been to homes where customers had the electrician install these and it sounded rubbish so they called me in to fix it. There isn't much that can be done after the fact. Unfortunately the answer is almost invariably "rip it all out and start again with proper gear". I've done that a few times. On other occasions the home owner has said "I'll think about it" which is code for "
not a blooming chance and you'll never hear from me again"
What they're left with then is a white elephant; it's a system that's a waste of money.
Ceiling speakers worth installing cost a minimum of £250/pr + the cost of fire hoods - £70-£100/pr.
For voltage drop over distance you compensate with cable thickness. 100% pure copper cable is the thing to have, not the cheap copper coated aluminium (CCA) rubbish. As a rough guide 20 mtrs of thin bell wire will lose around 30-50% of all the power you put down the cable, and losses will be higher if it's the CCA rubbish. 1.5mm 49 stand or 79 strand pure copper cable will lose some power but not nearly as much; roughly 10% at the same distance. A 2.5mm copper cable will lose about 6%
If you're going for a central hub location in the house which houses the amps and sources then you'll need amps capable of delivering quite a lot of current and some real wattage. If you want a system to deliver 100W then you need amps capable of delivering around 3.5 Amps of current. You won't use 100W per channel but you do need the headroom (power reserve) to minimise audio distortion. This is not the time to use those little T-amps from Ebay. They crap out at around 8-12W for a clean-ish signal. A good 6 zone multi-channel amp is around £1400.
Finally you come to the source and control gear.
If you've got this far and are thinking "
Holy crap!... I thought I could do the whole house for £500-£1000" then you're not alone. This is why I sell a lot of Sonos. It solves most of the above problems in a really neat way. It's cheap too once you start comparing the cost to doing proper multi-room.