I've seen plenty of people add tarps and awnings over small coops so rain doesn't get inside the vents. Chickens can't live in dark windowless boxes with heat lamps and poor ventilation. Yes they can live without lights but they might not like it or go in it willingly
Easy to raise the coop up off the ground with cinder blocks (or attach legs) You can also add ventilation simply by drilling holes in the side the triangle over the door(both sides)
Last but not least add a light bulb on a timer so the coop isn't dark inside then train your chickens to roost in...
I made the same mistake when I built my first coop to short and too small without enough ventilation. I wish I'd abandoned the idea of trying to 'fix' it because it was so much work.How I fixed my roosting bar and ventilation problems was I took the roof off and made the walls taller before...
My chickens have a predator proof run made out of pallets and hardware cloth. I staple plastic on the exterior walls so wind doesn't blow on the chickens on the roost and a tarp does the rest. Its cool in the summer and comfortable all winter. Chickens can bear the cold better than heat because...