Nice design! I have a similar fondness for compact homes, with one or two hearthlings living with all the amenities in a single building.
I find mezzanine levels like the one you’ve created to be a wonderful solution for increasing the usable space inside a single storey. Given that one storey is 5 blocks tall, there’s almost enough room for two hearthlings to stand one atop the other already… but if you add a pitched roof, you can easily get the extra 2 blocks of height required to have two levels with a single block separating them. It’s a bit…“cosy”, but it doesn’t break the immersion since older cottages generally did have lower ceilings.
Using crates for your staircase was very clever; I normally pack the space under the staircases with crates (or at least as much as the hearthlings can comfortably reach) but your more direct option dramatically increases the amount of accessible storage.