The top of the stairs is west/left/by the upstairs master bedroom. Flipping the stair direction is not worth it, as it really doesn't work downstairs. "However... I would not go through the effort and expense of building a home without an en-suite bath where I intended to have the master for the foreseeable future." - so much this. I'm really questioning this part of the project because of this issue. DH thinks I'm nuts, and doesn't care at all about en-suite baths. He thinks its crazy to try and make it work, and that losing closet or loft space to have an en-suite bath is not worth the trade-off. I admit, making an en-suite is proving to require some convoluted layouts, all of which require sacrificing usable space for circulation space.

Forgot to add -- we can add windows on the "bottom" wall (same wall as the bedroom windows).

Are the dimensions you've indicated finish dimensions? If not, you are going to have a pretty small shower -- If you don't need the tub (or won't use it much and have access to another tub in the house), I'd consider just putting in a larger shower and no tub, or go with a smaller tub and increase the width of the shower enclosure -- in that case the wall of the shower would no longer line up with the wall of the toilet enclosure, so the one door option would not work w/configuration shown.

I wholeheartedly agree with adding a bathroom. My own goal is to convert four bedrooms and two baths to three bedrooms and three private baths. I am interested in your post to learn things. I like your proposed layout with the vanity as focal point. This positioning allows you to have a full base cabinet if you want, without shrinking the space visually. And in a master bath, you likely need the afforded storage. Of course a floating vanity will make it look like you have more space, even if you stick the bathroom scale under it. Maybe make it the same tone as the wall to help it blend in though and not jump out at you. It would help the entire space visually to color coordinate the bathroom with the bedroom. To add more storage, I will suggest a couple ideas that worked for me in my existing small bathrooms. Keeping the vanity counter clear will help make it look spacious, especially since it is the focal point of the room. Have 2-3 storage shelves over the toilet for baskets with your makeup, extra tp, lotion, extra roll of paper towels, whatever. I also put a basket on the tank to store even more stuff, so choose a tank with a flat top. Another is to have a trio of shelves in the shower on the wall opposite the showerhead. They hold so much in-shower stuff. I used stick on wire ones I got on Amazon and love the way everything dries and doesn't get moldy or soapy. I imagine you already planned for a medicine-cabinet mirror for more storage. You may be stuck with overhead lighting, but maybe you are young and beautiful and don't have my wrinkles. :-) Wrinkle are why we made side-lighting. I haven't seen side-lit medicine cabinets, but they would be worth looking for. The great thing about small bathrooms is everything is within reach or close enough. So, though it might not appear to make sense to have stacking towel racks for drying used towels across from your toilet, they are close enough and really the only wall space available, especially with a pocket door. I imagine before a shower I would fold them and place them on the vanity to grab when I was done, then hang them back to dry. I'd also close the toilet lid before flushing though.  Some people feel like a small space feels cheap and therefore depressing. I don't, but I like the idea of compensating with high-end components. A top-end toilet with a heated bidet seat and skirted base, for example, will make your space feel luxurious. Or a wall-hung toilet? Luxurious towels are nice too. You can make your towel racks heated. Have a great lighted vent. Get radiant-heated flooring. Get a heated anti-fog mirror. Color the room with dark jewel tones and have lots of artificial lighting. Try canned ceiling lights in the shower ceiling, vanity lights, an overhead vent with light, and even some lighting in the toilet area. Pack that room with luxury and it won't feel small. Whatever dark color you use for the shower, continue it out beyond the glass on your back wall to prevent choppiness. Not everything in the room has to be dark. You can still have a white toilet, white towels, a white counter if you want. Just tie it in with the bedroom to mentally make a bigger space.  Speaking of the bedroom… The bathroom light will shine on the bed. Can I suggest turning the bed with the headboard at the window so the light doesn't shine on sleepers? This may make you cringe, but it affords you advantages. Currently it creates wasted space by the windows. If the head of the bed is at the window you now have additional useful space. For example, you have room on both sides for floor to ceiling shelving functioning as uber-nightstands. You have room at the foot of your bed for a bench to sit on while you don socks and underwear, AND if it is a storage bench, that's where you can store clean replacement towels near the entry of a bathroom too small for storing extra towels. It also creates space for a pair of dressers on the walls near the closet. I agree that you probably could use more clothing storage, but don't see space for it, so maybe room for two dressers is the next best thing and now you have room by your closet for two dressers instead of one. If you agree there are reasons to move the head of the bed to the window, the next concern is how to deal with the blocked light. The answer is more lighting. You can have light-blocking draperies for night and open them during day. You can get a slatted see-through headboard. You can use mirrored-shelf etageres for your nightstands to bounce light up or mirror the wall around your bed to create visual space you can see through the shelves or etageres. You can install mirrored french-door-look gridded closet doors to approximate a walk-out balcony. You will want to make the closet wall a beautiful thing to look at, since it is now opposite your headboard. Maybe a tall fake potted plant with an up-light by the dresser would help. Or if it's all too fake, an up-lit meaningful tall sculpture and reeded or textured closet doors instead of mirrors. Maybe a lot of dimmable canned lights in the ceiling, like stars in a night sky. A small room can be very luxurious so you don't feel cheated. I hope some of these ideas have helped.