1. Chickens don't need a big coop. They don't like to be inside, they like to be outside (as noted above, I'm assuming a decently warm climate). The coop is just for sleeping and laying. Almost all the coop designs you'll see on the internet, and all the fancy ones you can buy, are much too big. Not only is it a waste of time and materials to build a big coop, it's a huge disadvantage because it takes up more space and is more work to clean and is harder to move.
2. Don't build a coop you can walk inside. As per #1, the coop should be small, and it should be high (chickens like to be up high to sleep). All you need is a small raised box. You do not need a door for humans or a floor at human height. Do, however, put an entire wall or roof on hinges so that you can open up the whole thing and easily reach every corner.
3. Don't over-engineer. Because the coop only needs to bear chicken-weight not human-weight, there's no need to use 2x4's or half inch plywood, you can use much lighter and smaller construction materials. Again most of the internet designs and coops you can buy are just way off here, way over-engineered. (it does need to be strong enough to be wind-proof and dog-proof; dogs are by far the biggest hazard to urban chickens).
Even if you want a movable coop, you don't really need wheels if you use suitably light building materials and are moderately athletic. It's very easy to just pick up a small coop and move it around the yard as needed.
4. Paint. I painted the inside of the coop, and some sites & people consider this silly and froo-froo, but I think it was a good call and would do it again. A thick coat of high-gloss provides great water proofing and provides a smooth surface, which makes for much easier cleanup and longer life.
5. Rain/Snow. In contrast to #3, you should *not* cut corners in following good practices for weather-proofing. In particular, don't leave exposed edges of plywood or sheathing (they delaminate very easily), do use good shingle-principle for roofing (overlap and cover holes), use a proper drip-edge to prevent water wrapping around, etc.
6. Doors. I put a bunch of doors in the coop and one thing I didn't really consider was that all the poop and shavings and such will constantly be getting in the door jamb, which will prevent closure if it's a tight fit. One option is just to intentionally make a sloppy door that's a loose fit; another is to put some kind of trough near the door so that closing it pushes out the crud into the gap. Many designs, including mine, feature a door hinged at the bottom, so that when it opens it becomes a ramp. This seems clever but is not a very functional door because of the poop-in-the-hinges problem, it just becomes a static ramp. Probably the best type of door is top-hinged, with a raised bottom sill to prevent crud building up there. There's just not a lot of need for doors though; if you make the whole coop open for cleanability (such as via a hinged or removable roof), you can just use that to get the eggs as well; there's no need for the cute little nesting boxes with individual doors that people do.
7. The roost is the backbone of the coop. The chickens will spend 90% of their indoor time on the roosts, so locating the roost is the most important aspect of the design. The coop is really just the roost and the nesting boxes, the chickens want to spend their time outside in the run or free ranging, not on the floor of the coop.
8. The Poop Trough. Because of #7, I've found that almost all the chicken poop that's inside the coop is in a perfect straight line under the roost. I think you could take advantage of that and put an angled trough under the roost so that the poop was super easy to clean out. Another option would be a line of wire mesh instead of solid floor under the roost, perhaps with a removable trough under the wire mesh.
9. Rats. You have to decide from the beginning if you want to try to make a rat-proof coop. Doing so is a major undertaking and requires careful design. For example, chicken wire is not rat-proof. To make a rat-proof coop, first you need a solid stone foundation (for a small coop the easiest way to rat-proof the floor is just to cover the whole floor with pavers or bricks; for a larger coop you wouldn't want to do that, so you have to dig down at least 1 foot underground and surround the perimeter with rat-proof wire mesh or concrete blocks; rats are excellent diggers). Then the entire coop must be surrounded with hardware cloth (wire mesh) or similar. Rats are also superb climbers and jumpers, so vertical barriers will not stop them (you need a closed roof).
Some people try to rat-proof by putting wire on the floor (rather than a solid paver floor or burying a barrier around the perimeter). This is not a great idea. What will happen is the rats will still dig under the coop and create a network of tunnels under the wire floor. The chickens knock their feed all around, so lots (most) of it will fall through the wire mesh into the gap below it, and the rats will have a party living in the dirt under the wire floor. This might be okay with you (at least the rats are not actually in the chicken's space) but I think that overall wire on the floor is actually worse than nothing.
10. Feeders. Lots of people advocate these big automatic hanging feeders that you can fill with feed and it will drop down to let out more. Unless you have made a seriously rat-proof coop, these things are a terrible idea. Rats with an unlimited supply of food like that will multiply incredibly rapidly. You're going to want to visit the chickens every day anyway, so I see no advantage to these gravity feeders, just give them their ration each day so that there aren't a lot of left-overs for the vermin.
11. The Run. You have to decide up front whether you are going to free-range the chickens or not. If you are going to free-range them, then you don't need any run at all, just let them out in the yard. If not, then you need a big run. A tiny run (like under the popular commercial A-frame "chicken tractor") is pointless and cruel. If I had a decent amount of land I would build a simple run by just putting in some posts and wrapping it in chicken wire. (obviously this run is not rat proof). There's no need to cover the top of a large run (assuming as above you do not use a big feeder which would attract other birds).
12. Free ranging in your yard kind of sucks. Chickens love to dig in soft soil, so will go after your new plantings and vegetable beds and dig up your seedlings. They like to sit on railings and handles and poop. You will have poop all over everything. It's not awesome. On the other hand, it is very easy. They will eat a better diet without you having to carefully manage the supplements in their feed. They also naturally return to their coop at night so you don't really have to do any work to get them in and out, they do it themselves.
13. The poop pile. If you are going to try to reuse the poop and shavings you get when you clean out the coop as manure, you need to locate a spot for the poop to rest. You will get a *lot* of waste out of the coop, so you need a big spot, and you need at least two piles so you can cycle the new into the old (like compost; poop needs 2-3 months rest before use). The poop pile should not be near the coop (or run) and should also not be near your planting beds to avoid pest and pathogen transfer. It can be hard to find a good location for the poop pile in an urban yard, so you may want to abandon this idea and just throw out the poop. The poop pile will also attract rats and flies (but of course so will composting); it may also attract justifiably irate neighbors.