Glad you could join the flock! It's sounds like you are really enjoying your chicken adventures!! It's only natural to want to add more to your flock, I'm afraid it happens to us all. The coop section as drumstick said is a great place for lots ideas on building ~https://www.backyardchickens.com/atype/2/Coop_Designs
Welcome to BYC!
As you say you are already planning on many more birds I would definitely suggest that you build for what you will have, not what you do have - take the maximum number you *think* you might want, add a few more, and then use that for your building plan. The coops section mentioned above is a great place to start getting some ideas - and from there you can tweak it to what you want. The "needs" of the birds are pretty basic - enough space and a well-ventilated, draft free area in which to be sheltered from the elements of the climate you live in - - from there all the bells and whistles really boil down to personal preference and what is most convenient for the flock keeper. ......
If you have an idea of the breed(s) you want to add, remember to factor that into your plans as some breeds will have different needs when it comes to coop design.
We are glad you joined us!
Do you like gardening? If so I have the perfect and I mean perfect coop for you! If my flock was smaller I'd have this coop myself! You can make this coop too. I saved a picture of it here it is.
It has an herb garden on top, and egg box on the side, and it is big enough for 4-7 hens. You can make it yourself so you can change stuff about it you don't like or to add something on to it you want.
Welcome to BYC. Glad you decided to join our flock. If you haven't done so already, definitely check out our Learning Center at https://www.backyardchickens.com/atype/1/Learning_Center. There is lots of useful information there. Please feel free to ask any questions you may have. We are here to help in any way we can. Cheers.