I have a similar drain clean out in my garage. I placed a row of IKEA cabinets on that wall. The GC cut a whole in the back of the cabinet to access the drain. You will need to remove the two lower drawers, but the drain is accessible if any issues come up.

I would rotate yours into the space under the stairs, as long as it doesn't interfere with the water heater.

If you’re putting something like a fridge there, that counts as “accessible” because you can roll it out of the way to get to it.

chispa: thanks, that's good to know.

Sophie - I am would like to put lower cabinets along the wall below the stairs, but if I cannot, the fridge could go there. My hope is to separate the fridge from the oven.

I wouldn't worry about that stack. Cover it and cutout the cabinet at the bottom for cleanout access.

Water heaters are more movable then people think. I'd consider working your design to recess the frig under the stairs to some degree. One option would have it so it was close to flush with the counter if you're adding cabinets to that wall A second would have it totally recessed if you're not putting cabs there. Both can improve your work zones and free up counter space.

You could put a tall skinny cupboard door there and use as a broom closet. Or put shelves in the upper half and use as a pantry.