I've seen many retrofitted powder rooms under first floor stairs here in Toronto. Downtown 19th Century homes on small lots, some even built before indoor plumbing was installed.

IKEA has a splendid collection of fixtures and fittings designed just for these small spaces.

The restrictive issue may be the ceiling height. Usually either the switchback must occur high on the stair run,r the floor to floor height must be very high or the stair has a straight run without a switchback (I suspect most examples are of this type).

Here are the minimum IRC ceiling heights:
R305.1 Minimum height.
Habitable space, hallways, bathrooms, toilet rooms, laundry rooms and portions of basements containing these spaces shall have a ceiling height of not less than 7 feet (2134 mm).

Exceptions:
1. For rooms with sloped ceilings, at least 50 percent of the required floor area of the room must have a ceiling height of at least 7 feet (2134 mm) and no portion of the required floor area may have a ceiling height of less than 5 feet (1524 mm).
2. Bathrooms shall have a minimum ceiling height of 6 feet 8 inches (2032 mm) at the center of the front clearance area for fixtures as shown in Figure R307.1. The ceiling height above fixtures shall be such that the fixture is capable of being used for its intended purpose. A shower or tub equipped with a showerhead shall have a minimum ceiling height of 6 feet 8 inches (2032 mm) above a minimum area 30 inches (762 mm) by 30 inches (762 mm) at the showerhead.

Another item to note is that a coat closet holds more than just coats. We use ours for umbrellas, shoe basket, outdoor balls, seasonal décor...I used to keep the big Dyson vac in the coat closet at our old home. It may be nice to have as a closet even if you don't need it for coats :)

I hadn't thought of Ikea fixtures. I've walked through their showroom thinking, "Why would anyone want this teeny-tiny sink?" But I was thinking of them for a real bathroom . . . those tiny items would be fine for a powder room, which would only be used for washing hands.

Yes, I'd been sketching some drawings, and I'd already suspected that ceiling height would likely be the limiting factor. I'm sure the "first few feet" of the closet would be just fine . . . but I'm not sure whether we'd have space to set the toilet "back far enough" into the closet to allow for the necessary leg room in front of the toilet.

We have space for shoes, umbrellas, etc. planned -- loss of the closet near the front door really wouldn't hurt us.

Interesting thought. Probably won't work.

Your plan shows hardly any place for storage and I don't think there's a basement, so I'd think the closet would be very useful. Can you add a couple of feet or so across the front to allow room for a closet in the den, a closet in the hall, and the powder room?

Anne

not sure whether we'd have space to set the toilet "back far enough"

Wall-mounted water closets can reduce the footprint 8"-12" front to back by mounting the tank behind the wall.

Kohler wall-mounted w.c.

This post was edited by worthy on Mon, Nov 3, 14 at 9:07

Wall mount toilets are much easier to clean as well.

I've been on board with the idea of wall-mounted toilets for a while now -- simplicity in cleaning is one of the cornerstones of my plans!

However, I hadn't considered that it would mean less space is needed for the powder room. This could be the difference in whether this idea works or not!

No, we're not particularly short on storage. Our plan has a modest walk-in closet in the bedroom . . . and we're adjusting things in the center of the house for the laundry and pantry; it'll be a large L-shaped area. Upstairs we'll have a large linen closet and a walk-in attic. We're planning built-ins for storage in the master bedroom, the living room, and the den/office . . . currently being renamed the Library . . . but that hasn't stuck yet. Admittedly, I didn't draw the built-ins on the plans I shared, and that could've been misleading.

Anyway, right now our plan has a powder room and an under-stair closet. If we move the powder room INTO the under-stair closet, we gain the current powder room as closet. It's kind of like a balloon -- if we push on one end, the other pops up, but it's all the same quantity regardless of which end we push.

If we move the powder room to the under-stair area, we save a bit of money because it'd be "back to back" with the kitchen plumbing . . . and because it would eliminate one door.

We have a powder room under the stairs in my home that we are building. it is teeny! But it is a perfect use for that area :)