I've built a few different ones over the years in existing locations,
so never had a chance to start from scratch. A lot depends on how big
of birds you have & how many. Don't skimp when it comes to putting
wire around. Making a wood structure encased in 3/4" hardware cloth is
idea. That's tough enough to stand up to a racoon if you've got good
wood over it & small enough to keep mice & snakes out, too.
You should put a piece of plywood that slopes down to you from the wall
under the perches. We mostly had Bantams or Game chickens, so we could
fit 3 perches staggered down in a slope with about 8" between them,
stepped a few inches up as they got to the back. Keep the back one at
least 6" off the wall for those birds. You'll have to change for other
sized birds, of course.
Depending on how deep, you probably want to keep the perches a good
foot above the plywood so you can reach up there with a trowel & scrape
the crap off into a bucket. Doesn't want to be a huge slope, an inch
or two per foot is plenty. If you overhang it by 6", other birds can
use nesting boxes underneath.
Make nexting boxes modular & easily moved. You'll want to pull them
once or twice a year & clean up behind them. A galvanized metal trash
can is one of the best holders for feed made, IMO. You should have
windows that can be closed fully for cold weather & opened fully for
the summer. I prefer a top hinged, short window if I can. that keeps
out the worst of the rain, but still lets plenty of breeze in. I live
in Maryland, so that's the weather I'm writing for. YMMV
You don't need much height. Don't go making 8' ceilings if you're only
6' tall. Make the ceiling 6'6" or a bit less. Chickens like something
more enclosed for the night & if you want to catch one for eating or
whatever, they'll just hurt themselves flying around. You want to be
able to corner them easily & that means grabbing them in the top rear
corner of the perch area - so don't make that terribly deep either, if
you can help it.
I think the best one we ever had was built out of an old corn crib -
the long narrow kind that sit at the bottom end of a barn. We had just
one roost & a 1' strip of plywood (no slope when that narrow) to catch
the crap down one wall with nest boxes underneath. It already was
wired well. All I had to do was put up a lower ceiling. The chickens
didn't like being strung out that much, though. They prefered to
cluster more & I wound up putting a tiered perch at one end. We left
the one on the side & some used it.