My chickens are right up against my house beside my deck. I do not find any problems with predators or smells, I actually find that having the animals so close to my house deters predators. Good ventilation and proper management of the bedding is going to prevent any bad odors. I think it's a great place to have your birds since there's already a roof. I find having them so close to my house means I collect the eggs more often clean more often and feed them more food scraps. I've never had a rodent problem because I keep a lot of cats so that might be something you want to consider, feed storage.
To quote Joe Salatin, if you can smell manure you're managing it wrong...
I think you're still going to need four walls and sub roof under your deck, lots of ventilation the rule of thumb is apparently one square foot of venting per bird. The sub roof could just be hardware cloth. I also find the close proximity of the birds means it's a lot easier to run power to your coop, and depending on your climate you may want heated waters, a light to keep them laying in the winter and some sort of heat source possibly. Water in the coop... you probably gonna want a closed nipple waterer because you don't want extra water vapor causing humidity problems. I think digging down and putting in hardware cloth all the way around the outside of your coop is a great idea, keeping things that dig from digging their way in to kill your chickens.
Another benefit of their close proximity is being able to hear when something is going on, if your birds set off an alarm call it's easy to hear and easy to get to them before something bad happens. Personally I love poop trays under my roosts even if you're doing deep litter they're handy. Poop trays that sit in a frame can be popped out and dumped every day are great. I also really love rollaway nesting boxes so consider adding that to your build, rollaway boxes keep the eggs cleaner and prevent egg eating. Also if you build it right and have enough space in the rollaway compartment you can leave egg collection for a few days if you have to go away... All that said from time to time you coop will get dirtier than you want it to and it may smell it happens to everybody. But as soon as you can smell it you know you need to clean it. Make sure you know what bedding method your gonna want to use before you build and accommodate for being able to shovel it out, make sure a wheel barrel can fit inside it or if you do want to build one of those little backyard style coops make sure you can open it up and scrape all the litter out easily...
Sorry this is so long I just really like the idea!
