I'm going to disagree with the previous posters, here, and say that's actually *not* going to be the easiest coop to use in a very cold climate such as Alberta. Chiefly because it is so small. I would absolutely not put more than 3 chickens in that for a Northern winter, myself, and honestly if I were going to build something for 3 chickens I'd build something different and with much bigger 'indoors'.
But, that said, if you *want* to try to adapt a design like that for very cold winters, I would suggest the following:
-- install a raised wooden floor in the run for wintertime, high enough that you won't get rats living under it too badly. This will prevent you from having the nekkid ground serving as a humidity source (see below for reason). It would not be *too* hard to do this in a removable way so that they could still have the 'real' ground as the run floor in the warm months.
-- close in the run so that it effectively becomes indoor space for the winter. I would put plywood on the N/W side(s) and clear or translucent plastic (sheet or panels) on the S/E side(s), except that I would leave the top 12-18" or so of the S side open as a vent with a hinged flap to cover it so you can adjust how open it is on any given day. If you are enclosing the run this way, you really will do a lot better (in a very cold climate) to have that raised wooden floor I described above, otherwise you will find that enough moisture evaporates/sublimes from the ground, at least on sunnier warmer days, to create a serious humidity problem despite the vent. And that humidity will give your chickens a high risk of frostbite.
-- insulate the 'coop' (indoor) part real well, but maintain large, adjustably-closeable vents on the S side and also on the side facing into the run. You will often want it ventilated into the (itself-ventilated-to-outdoors) mostly boxed-in run area.
-- use very deep bedding, in the raised-floor run as well as in the 'house' portion.
-- then leave the popdoor from house to winterized run open as much as possible, certainly all daylight hours every day, to give them maximum indoor-type space to use.
Even so, it's still only 50 sq ft. That's only 10 sq ft apiece *total* for 5 chickens, only slightly more if it's just 3. My chickens all have 10-15 sq ft apiece indoors, and even with that they are really not thrilled about being on lockdown (in just that indoor space) when I'm away over the holidays -- they are not normally very outdoorsy but they are THRILLED to be allowed out into the runs again when I get back. With grumpier chickens you could easily get into social problems (picking, cannibalism), plus they just won't be as happy and relaxed as they could be.
Just my $0.02,
Pat