@Cold Canadian That kind of a modification would require a building permit, which I don't think would be easy to get. And it would end up costing quite a bit. The build itself would be easy, there's already a concrete porch with a proper roof there, I would just need to build walls. But it would look a bit weird on the house, and as we're not planning on living here for that long, I don't think it would be worth the effort. Switching the inner door to open in the other direction would be easier, but it would interfere with the bathroom door. There are loads of little tweaks to the plan that might have made it more functional, but it's mostly pretty functional. Our laundry room is completely separated, and I quite like it, you can hide the mess of laundry away from the living space.

Here's our floor plan. The bathroom door actually opens in the opposite direction. And between the sauna/laundry compartment and our apartment there's actually our neighbor's apartment, and their mirrored sauna compartment.
Beer, since I'm using a plastic box as the inside of the little bunny house, I'm going to be able to wipe it clean pretty easily. I'll be able to even hose it down if needed. I might build it so that I can just lift the inside out of it's shell for easy cleaning. Thanks for the input, I hadn't thought of doing it like that before.
Here's our floor plan. The bathroom door actually opens in the opposite direction. And between the sauna/laundry compartment and our apartment there's actually our neighbor's apartment, and their mirrored sauna compartment.
Beer, since I'm using a plastic box as the inside of the little bunny house, I'm going to be able to wipe it clean pretty easily. I'll be able to even hose it down if needed. I might build it so that I can just lift the inside out of it's shell for easy cleaning. Thanks for the input, I hadn't thought of doing it like that before.