Yes, here's the full set of numbers for you:
For each adult, standard-sized hen you need:
- 4 square feet in the coop (.37 square meters)
- 10 square feet in the run (.93 square meters),
- 1 linear foot of roost (.3 meters),
- 1/4 of a nest box,
- And 1 square foot (.09 square meters) of permanent, 24/7/365 ventilation, preferably located over the birds' heads when they're sitting on the roost.
12 hens
- 48 square feet in the coop. 6'x8' is more practical than 4'x12' since a long, skinny coop like that would be difficult to work inside.
- 12 feet of roost
- 120 square feet in the run. 10'x12' or 8'x15' -- 8'x16' means fewer odd cuts than either of those. 6'x20' is possible, especially if your run is an open-topped, fenced area instead of fully-enclosed with a solid and/or wire roof but risks social problems because subordinate hens need to be able to pass the dominant hens at a respectful distance.
- 12 square feet of ventilation.
- 3 nest boxes.
Your plan to have extra space for future additions is wise. Especially since these numbers are only suggested minimums and integration of newcomers takes extra space.
I don't think that anyone has ever regretted giving their chickens extra space.
I had my DH make me a feeder/waterer shelter for out in my run:
https://www.backyardchickens.com/articles/an-outdoor-shelter-for-the-feeder-and-waterer.76487/