Minimum footprint:  8ft x 12 ft  
Concrete floor.
one row of concrete block on the concrete slab all around the perimeter. 
leave a gap for a 3 foot wide door.
  made a 4 inch sill under the door.
make the door swing inward. (in snow country)
    make the sill plate with treated lumber.
 stud the walls 16" on center.
 Pitch the roof towards a wall that the door is not at.
Use ribbed metal roofing.
put a window on each of the three walls that the door is not on. 
    window that open are best.
INSIDE:
If you can afford it, insulate the walls.
cover the insulation with paneling or boards.
   hang the egg nests on the wall about 2 feet above the floor. 
  put the roosts across the 8 foot span.
two roosts, hung on the walls so you can remove them for cleaning the coop if you want to. 
    I put the lower roost about 16 inches high and 12 inches away from the wall.
the next roost is about 12 to 16 inches higher and 2 feet from the wall.. 
   you will appreciate this arrangement when you have to shovel the litter from the floor.
    If you have access to electricity.
put at least one ceiling light in. two is better.
   a receptacle  or two.
    make sure  you have adequate breaker.
In winter you might want to heat the water, or add a heater.
    DO NOT use heat bulbs. They sometimes throw sparks when they get broken or burn out.  then say goodbye to your coop. besides they do not last long anyway. use a ceramic heater.
   provide plenty of ventilation.