I think that is a creative and practical use of that space.

I like that. Are you putting shelves in the 12" cabinets? Or pull-ours? The only consideration I would make would be if it would make more sense to have a full-height broom closet or pull-out pantry in that slot. Does IKEA make an 88" pull-out pantry that is only 12" wide?

No, IKEA only had 15" wide pullouts which we return for two 12" wide ones. I really like the idea of two separate smaller pullouts. I had seen in years back somewhere on this forum and it stuck with me. Each cabinet will have a single door that pulls out with maybe 3 rationell shelves attached. Our space is 80" high. DH had to raise to header to accomodate that. It'll be used as a supplemental pantry, maybe one drawer/shelf will be a charging station. I'll store my brooms/mops in my large laundry room pantry. I actually have a perfect space for that in the closet where the air handler is

OK. That makes sense. The only thing I would warn you about is how little space you'll actually have in the pull-outs/drawers for a 12" cabinet. I just measured our 15" drawers (we have no 12" drawers) and the "flat" space on the bottom of the drawer (not including the sloped drawer sides) is 10" wide. So I would think that on the 12" drawer, you'd be looking at a flat interior surface of only 7" width. Depending on what you plan to store in those cabinets, you may be better off just using shelves so you can get the whole interior width of 10-1/2".

As for the area above the two 12" cabinets, you'll need some sort of a cabinet body in there, I would think. You could always get another 12" cabinet and cut the top down. We didn't have to do any hacks like that, but that way you'd have something to affix the door to and would have an enclosed cabinet rather than two walls, the top of one cabinet and the bottom of another.

Also, how are you mounting those cabinets? The base cabinets aren't set up to take the wall mounting rail.

The cabinets will be mounted like any base cabinet. He'll gang them together somehow. Whatever he rigs up will usually be overkill because he's always cautious and particular. We were just saying that we'll pick up another box and he'll cut it down height wise. I'm sure he'll screw into the studs in the wall behind and probably connect to oven cab? He can make me a dowel thing for a tray cabinet and the door will be made by Scherrs. Can't wait to complete the cab thing so I can order my fronts. I hope I'm happy with my decision to go with slab on all the drawer/door fronts.

I think the pullouts are more important for the narrow cabs because they are narrow. It would be harder to reach anything in the back of a narrow cab if the shelf were fixed.

You can cut down the 15" for width. Use confirmats or screws meant for particle board in place of ikea's assembly hardware.

You could also think about using the 12" deep version without pullouts. The math hurts my head this morning, but a 7" x 20" drawer v. an 10.5 x 11" shelf is something like 140 square inches of storage per drawer v. 116 per shelf. Shelves are cheaper, adjustable, you can stack stuff and see what's inside the higher ones a little better before the step stool.

Instead of dowels for the tray storage you might look at IKEA for their metal tray storage dividers. I looked on the website but couldn't find them, but they are big wire "U"s that just screw to the shelf or bottom of cabinet. Might be easier and take up less room in an already narrow space. Last time I was at IKEA they were with all of the other kitchen cabinet organizing stuff.