Yeah, it can work, but it's a lot tougher with the door at the end. Still, there are very narrow sinks that you can put on a side wall and squeeze by, and you can put the faucet to the side.

Powder Room Renewal - · More Info

Harris Powder Room · More Info

Lets get to the other clearance issue that will get you red tagged. What's the ceiling height? You have to have 80" in front of all of the fixtures. Without that, you've just got an awkward closet, not a half bath.

Its got the clearance, hence the 5'5'' length, otherwise the cloakroom would be the full length of the stairs. No problems there.

I read through the comments on both photos you posted of tiny under the stairs powder rooms. Both are 3' 2" wide. You're lacking a critical 8 inches of space to build your powder room. Most powder room designs are a minimum of three feet wide. Could you devise a way to enter from the side, sink at one end, toilet at the other? Otherwise, you'll not likely meet local codes, and the project might just be better of remaining a closet under the staircase.

Thanks for the replies guys. I'm scrapping the plans for the cloakroom seeing as we have an adjacent guest bath that can have two accesses. We don't have much guest anyways...