So if you want 8 chickens, plan for 16.
Our looks like a (very ugly, shanty-town) house. It's 8x8, with a peak roof. Plenty of room. Right now we've got 11 adult birds (Dominiques, mid-size bird) and 5 younger birds all in there and they have plenty of room.
We actually have a window in ours, because there was a window with a little crack in it, and we thought it would be nice to help let as much light in without letting in too much cold air in the winter. It has worked out great so far. See if you can post an ad on CL or freecycle - ours was free, it's a nice double pane energy efficient window that was not installed because of the small crack in the corner of one of the panes.
We are still (a year later!) waiting to finish the roof, right now it's just OSB sheathing plywood but we'll be putting some tin on the roof this spring once we get a couple of warm days to work outside so it will reflect heat as it's out in the sunshine instead of under trees like it used to be.
Anyway, it works great for us. We have a poop board under the two roosts (well, there are three roosts but the birds only use the top two) and we put wire and then a door on the front so underneath the poop board actually serves as a way to introduce new birds - it's a 2.5 x 4ft area, so we can put birds in there and they interract with the flock through the wire, hearing each other's sounds, and we know if anyone has any serious problems before they can get to each other and do damage. After a few days of being in the "coop in a coop" new birds get to go out with the general population. This area will also be a broody area if we have a hen determined to set