Welcome to the forum, glad you joined.
I'm not a huge fan of magic numbers for this or much of anything else to do with chickens. Some people would have you think that you need the same thing whether you are on the equator or arctic circle. Different types of ventilation have different efficiencies. Taller coops are generally more efficient at moving air than short ones. There are a lot of variables. General guidelines give you a starting point but are not a law of nature.
Your climate isn't that bad for chickens. From your historical records you can have some pretty hot days so heat may occasionally be an issue but not often. I'd expect your nights when they are in the coop to cool down nicely. Your winters can get pretty darn cold.
Chickens can typically handle cold better than heat, something to do with those down coats they wear, but they do need some help. Mainly keep them out of direct wind and give them decent ventilation so the moisture doesn't get too high. Wind chill is real but with chickens there is another factor. Their down coats keep them warm by trapping tiny pockets of air. Those air pockets gives down its insulating effect. If a breeze strong enough to ruffle their feathers hits them that can release those insulating air pockets.
Another danger to chickens is frostbite. Anytime the temperature is below freezing frostbite is possible, usually on their combs or wattles. But moisture is a big factor. If the air is moist frostbite is more likely. Moisture can come from their breath, their poop, waterers, or other possible sources. You need enough air movement to remove that moisture without creating breezes strong enough to ruffle their feathers.
A much smaller risk is that when their poop breaks down, composts, rots, whatever you want to call it, it produces ammonia. Ammonia can damage a chickens respiratory system, yours too for that matter. But ammonia is lighter than air. As long as you have even small openings up high that ammonia will escape. But if your coop gets a strong ammonia smell you might want another high opening. To complicate this, if your coop is wet you can get a strong ammonia smell even with good ventilation. You want to avoid a wet coop.
I've seen chickens sleep in trees when the temperature never got above 0 F (-17 C) for several days running. People I trust on here say they'e seen chickens spend the winter sleeping in trees in Northern Michigan or in Nova Scotia. Who knows how cold that got. But those chickens were not sleeping on a bare tree limb overhanging a bluff, crowing defiantly into the teeth of a blizzard. They were in sheltered spots out of direct wind but with great ventilation. One problem we can create with out coops, especially those little bitty elevated ones, is that we can build wind tunnels that focus breezes on the birds and they have no place to go to get out of it. Yours should be big enough that you can avoid that.
Warm air rises if there is cooler air to replace it. That is how you get passive ventilation. Warm air also holds more moisture than cooler air. Your moisture sources will be warmer than the other air if it is below freezing outside so that air should rise. So it is really beneficial to have openings up high so that warmer air can escape. In summer when heat is your enemy it is beneficial to have a low opening on the shady side where the air is cooler than on the sunny side to let in cooler air. A breeze on the birds in summer is a good thing. In winter, no. In winter enough cool air to replace the warmer moister air should come in through your higher openings.
What will your 8' x 10' area look like at the roof line? Will it be single sloped, gabled, or something else. How do you plan on providing ventilation? My preferred method (pure personal preference, you'll see a lot of that on this forum) is to have roof overhang and leave the top of the walls open. Have the coop tall enough that the roosts are lower than those openings. That way any breezes pass over the birds heads so they do not get a direct hit but the turbulence caused by any breeze stirs up the air inside the coop enough to remove that bad air. The overhang protects from rain or snow getting inside but a light dry snow can still blow in. Still it is pretty effective.
Other than leaving part of the wall out and covering it with hardware cloth to keep predators out you may be able to use ridge vents, roof vents, gable vents, or maybe a cupola to improve ventilation. I'd be a bit careful of ridge vents where you are, snow may block it. These have different efficiencies, depending on how they are set up, but generally high is better. I'd be reluctant to use a motor-driven fan because of potential dust getting into the motor. In your climate a fan should not be required anyway.
What is going in your husband's part of that extension and how well is that ventilated or sealed off? That may give you an option. If that is open to the atmosphere enough that it gets good air exchange but blocks wind so you can stand inside in a blizzard and be out of a breeze, you could put an opening on your coop into there to give you more good breeze protected ventilation. My coop is set up similarly. But the down side of that is that chicken make a lot of dust. Some of that is chicken dander, bits of skin and feathers flaking off. Hopefully neither of you are allergic to chicken dander. But their scratching also produces a lot of dust. That comes from a dirt floor if you have one, them shredding bedding, or their poop drying out and being scratched into dust. Your husband may not appreciate a layer of that dust in his portion.
Good luck with it. This sounds like a tremendous way to provide a great coop with plenty of room for your chickens. And once again,
