How do you decide on a coop design/plan?

I'd recommend a couple things.  First, I would start off with 8 chickens if that is your goal.  It's a lot easier to do one flock than to hassle with integrating more birds in later.  Also, most everyone gets their first batch of chickens and shortly after want to start adding more.  Secondly, I would look into re-used windows.  Habitat for Humanity is great for recycled or donated materials.. however, others have said their Habitat supply shop was overpriced junk.   Craigslist is good too.   If you could find a sliding window you could pop the screen out and run hardware cloth in it's place.  Keep it open during the summer and closed during the winter.   You could probably find a good recycled window for 20-40 bucks. 

I have heard chickens do not fall into the danger zone where they may die until the temperature drops below 0*F.  And you can always run a heat lamp or another heater in the coop for the particularly bad cold snaps.  I have seen many more struggle with overheating rather than getting too cold.  If you wanted to be double sure you could even install two sliding windows across from each other for good air flow. 

Just remember the basics when building, standard rule of thumb is 4 square feet per bird in the coop and 10 square feet per bird in the run(there's a lot of opinions on this, I personally feel 3ft in the coop is enough but also feel you need more than 10ft in the run).   However, I would look for a design that could house more than 8 chickens because there's a really good chance you will soon want more.  As for finding designs, there's good tools here on the main page, outside the forum.   There's a big range from fancy doll house style chicken coops to ragtag recycled homes for those on a budget.  Chickens aren't picky, just like their room, privacy and some security.

You have checked out the coop pages right? First consideration in your case is what will the HOA allow you to have? Number of chickens, size of you property, will they 'free range' or always be in the run? And of course, how much you want to spend. These are all considerations. I would say build it all (run and coop) as large as you can...it is much better and healthier for the birds to have extra space, than to have too little space. I have a 12x20 run for 9 chickens. On my days off I let them free range, but can't do that when I work because I work evenings, I am not here to shut the pen door. So the extra room is good for the days they cant go out.

Well, when dad said we were getting chickens I sort of put all my research into the coop and knew what I wanted.  My coop is 10x12 and can house up to 30 chickens (though planning on up to 20).  For ventilation you can always use the space under the overhang from the roof.
Since I can't explain it well, this is a picture of my coop.

You can see at the top there are the rafters, it's this way on the front and back of the coop.  We put boards up to block the rain and snow from blowing in but it still provides a lot of wonderful ventilation.

If you want more than that you can always make some.

this is the south side of my coop.  There is no shading the coop, so it gets full sun all day so to prevent the coop from being too hot we only put in a 2x2foot pexiglass window for light.  We made a separate ventilation hole for simplicity and so it can be open even during a rainstorm.  At first we put hinges on it so we can raise it up and close it shut, but we later decided we didn't need to do that and made it open permanently at a 45 degree angle instead. 

Another thing you want to consider is if you want to store the chicken food and bedding in the coop you will want more space.

How exciting!  My first thought is that you really can't go too wrong as long as you keep the well-being of your chickens in mind-space, ventilation and safety.  There seems to be quite a bit that chickens will tolerate, so perhaps you should think about other aspects of the HOA that relate to the appearance of outbuildings.  For example, you could convert a garden shed into a coop.

:DSo exciting for you!  Chickie-poos at last.

We're moving, so I get whatever shed, coop, barn, or stable happens to be there.  At this point, the house we want has two sheds, a nice big one and a smaller doorless one.  They'll do great for birds and storage.

One day I would like to have a barn though, because I think it might be easier to rig up for heat in the winter, especially if the power goes out.

If I bought a new shed, it would be the one that looks like a mini-barn but with a tall loft area for feed storage.  I really don't see any point in buying a ready made coop.  You can find sheds on Craigslist and Wal-mart that are much cheaper and can be easily converted.  We're using two converted doghouses right now!

Like most of the rest I spent an extended stay on the coop pages. I finally found 3 I liked but still didn't have just what I personally wanted. So I took the three and sorta combined um and modified the results for things I wanted. Then I drew the plans and tried best I could to measure out everything. I have 8 now and am planning on at least 4 more. My coop is 5X8 feet with plenty of room inside. Mine don't go in there but to roost and lay. Everything else is outside.

  So I suggest and has already been stated, go to the coop page and study. If nothing is found that suits you fancy, modify till you come up with one to you liking. I too would start out with ever how many hens I wanted to end  up with overall. To intergrate new hens you'll need another coop to isolate the new hens. More trouble.

   There you have a good begining.

I agree you should get all the birds at once.
How to plan? First off, you won't need anything too big. Then, think of ways to eliminate unnecessary work, and how to use thewood you buy. For that many birds, working with a 4x8 floor-the same size as a sheet of plywood or particle board, is a good choice. It would be or you, I have A 4X8 foot coop (see my page).
2X4 studs come in 8, 12, and 16 foot lengths. Make your wall height(s) use them efficiently. Set them along the wall at 2 foot intervals instead of 16 inches, it is a chicken coop and you don't need a stud every 16 inches. You want 2 studs nailed together in each corner.
Use nails for the frame, not screws. They are much stronger than screws. 10 penny and 16 penny nails is what you'll want.
A 4 by 8 foot coop should have the floor a couple of feet off the ground. If you use 12 foot 2X4s and want a single pitch roof, I would suggest cutting the 2X4s at 2 feet, then at 4 1/2 feet. For each of the 2X4s you cut this way, this gives you a 2 foot piece, a 4 1/2 foot piece, and a 5 1/2 foot piece. Use the 5 1/2 footers along one side, and the 4 1/2 footers on the other, which gives you a roof that will be a foot higher on 1 side, giving you the run off you want. The left over 2 footers, some of these you can use to frame in a window, or build the frame of your coop doors, etc.
12 foot 2X4s cut in half give you 2 6 foot lengths. Using these as rafters for your coop roof, you get a 1 foot overhang on both the high and low sides of the roof, nice! No wasted wood, it is too expensive to waste.

Well, depending on your purpose for getting chickens (eggs, pets, meat, or a combination), I will disagree about getting all 8 at once.  For example, if you're getting them for eggs, their egg producing capabilities will begin to decline by year 2 or 3 (at which time you'll have to decide what you're going to do with them - keep as pets, eat, rehome to someone else who will most likely eat them).  You could add another 4 birds the following year (or even the year after that) to keep your egg production up without lag time. 
If that's the case, choose a design that can be divided easily with wire for incorporating new chicks/birds down the road.
Another advantage of not ordering all at once is that it gives you time to discover/learn about other breeds out there you might like to try...

Congratulations and good luck!