Hi DUCKGIRL89...I hate to burst your bubble, but the 8 x 4 house equals 32 sq. ft divided by 12 ducks +6 chickens, or 18 = 1.78 sq. ft per bird. I don't think it's large enough for full size adult ducks and chickens, even with the chickens roosting at night. How much room does one of your WH take up when nesting? Measure that out and multiply it by 12...then you will know how much space you need for them. The house is a great idea and I know you'd like to have them all together, but I agree with a BYCer that maybe two houses would be better. If the floor space is totally occupied by ducks, then all the poop from the hens on roosts will land on them and they won't be happy. If you would build a coop that is raised, the chickens can sleep/roost in it and six chickens + would be fine. Under the coop there should be room for 12 ducks to have nests...so 32 sq. ft under the coop divided by 12 ducks = 2.66 sq. ft per bird. Do you think that is large enough? Then, of course, you will need a run....oh! I see where you are planning on free ranging them. Also, as other BYCers have suggested on various threads, check out Craigslist for used coops.I will be on the lookout for coops that are reasonable and when I see one I'll PM you.
Don't lose faith! It will all work out and all your babies will be in a warm and safe place.