I think your idea of putting them inside your garage is a good one. I would put the whole wooden coop inside the dog pen and leave the door open so they can go in and out of the coop whenever they want. I would also put shavings, leaves on the floor of the dog pen for them to play in. We have our chickens in a coop 4 x 8 x 4 inside our main barn. The coop is made from lattice. For the winter we have put old horse blankets on the top (our horses go naked during the winter) to keep whatever heat they generate in the coop. we have a small door cut into the outside wall of our barn that leads to a very large outside covered pen. Inside our barn we have one window we leave half open from the top, but is protected from the wind due to a run-in for horses being directly outside of it that is walled on three sides. We have good air going through the barn but no wind. We have our chickens, ducks and guinea fowl all housed inside the main barn. We have elctric waterers for everyone. I think your chicken coop would fair better inside the gararge. No heat lamps, but maybe electric waterer and leave the door open to get better ventilation. This is only my opinon. Your coop is very cute but I fear that a really cold and windy wintery day with lots of snow may bury your coop and therefore limit your access to your hens. I don't know about you but I like to be able to tend to my flocks with ease especially in really bad weather. Again, this is only my opinion, Good luckI'm in Colorado too and in a serious cold spell. It was -9 when I got up this morning and it's only hit a high of 10 today. This is our first year with chickens so I'm learning as I go and have made a couple of mistakes I think. We had a flock of 3 but lost one last month (Mareks?). The other two seem great. My coop is only 1 X 2 with two perches. The girls sleep next to each other and we added lots of extra straw on the floor to help keep them warm.
It was kinda windy the other night, so I hung a "curtain" in front of their window to keep the wind out and closed their coop door. The top corner was open to vent. This morning both chickens had a little frost on their backs!!! I felt horrible. Should I leave that window open to vent better? I'm wondering if their breath added too much moisture in the air.
We don't have electricity out there, so we have 4 small LED lights and 2 small 1.5 watt lights that run off batteries we turn on for 5 hours each night. (They were laying great but one went broody so we had to break her of that, then the other stopped laying and hasn't started back up yet. So no one has laid in a week. There's lots of extra feathers in the coop so I'm wondering if the one is going to molt now. Still trying to learn....) I know that isn't much heat being put off by the lights. I'm thinking of moving them into the dog kennel in the garage tonight as it is supposed to be super cold again. They aren't used to being handled though so I don't know if the stress of catching them is worth the extra warmth in the unheated garage or if it would be better to leave them outside. I did put Vaseline on their combs and waddles this morning to prevent frostbite and will reapply again this afternoon.
They seem okay outside and are scratching and perching. We have clear shower curtains up on two sides of their run to block the wind and hold in some heat plus the run is covered. I gave them warm crumble mash this morning and threw down some scratch too.
Here's a picture of our setup in case anyone has advice on whether I should keep that window uncovered, even in -10 temps.