Good job on the coop build and yep having chickens is an exciting adventure along with educational and enjoyable .. my only regret with having chickens is that I did not get them sooner
I am in Aus so probably not the best person to comment but some things I thought of that you can either rule out or consider depending on your circumstances are:
What type of predators do you have? Anything that could dig it’s way in? Might be worth considering pavers or a buried wire skirt around the bottom to keep them out.
Are you free ranging? If not, is there plenty of room for 12 chickens? If they do not have enough room boredom might kick in along with squabbling and feather picking.
It looks like you have great ventilation and as long at their roost is draft free, they can snuggle for warmth.
I am guessing the roost is in the top section with the nest boxes? Does this close off from the run below? I am not sure of your climate but if it does not and it gets really cold, you might consider fashioning something to wrap around the lower section to aid in keeping them that bit warmer.
Again, living in Aus, I should probably consider air conditioning the coop, not heating it but there is a recent thread on whether to heat the coop or not and the dangers of using a heat lamp in the coop:
Also, there are lots of great articles in the Learning Centre:
Good luck on your new adventure and as you probably have already discovered, BYC is overflowing with knowledgeable, friendly folks who are more than happy to answer any questions so do not hesitate to ask.