Hi Mo, I'm replying because I have a coop exactly the same. 8' x 4'
I had the time to treat it with timber preservative and let it 'air off'
before adding my pol (4 Marans) I was going to get 6 but glad I didn't
coop not big enough, other than for them to sleep in, or for bad weather days.
I keep them on the back lawn and added small wheels at the heavy end and
a rope across the other end so I could move it every 1 to 2 days without
bothering my oh for a lift. Other modifications needed were adding thicker
sawn off branches as rungs to their ladder as they were struggling to get
up and down the original. Also where the nest box is joined to to the main house,
I had to glue and screw a pvc strip to keep the rain out!!! Have tried to contact
the supplier without success. I suspended their feeder under the ladder and
suspended their drinker at the foot of the ladder, for easier access for myself,
and to give them maximum floor space.
I've installed an electric fence round the back garden to deter cats and foxes,
which the neighbours feed on a regular basis, expensive but what the hell.
It also saves the complete annihalation of the border plants.
I will be moving them soon onto a block paved area, (similar hard standing as
your concrete base) to give the lawn a rest and thought of keeping them 'deep litter'
within the coop, with correx boards round the base to contain the litter.
Getting back to your original question, I would be wary of adding any form of
electric heating, with the fire risk involved with deep litter. (They keep chickens in
northern Canada) A tarp over the roof on a night will do for me, whilst keeping
an eye on their drinker, would hate the glass container to break due to frost.
Enjoy your 'Cocoon' coop, don't they look great. Oh by the way, one half of the
roof blew off on mine, fixed it back with bigger screws ok.