change to an 18" horizontal for your appliance garage

You can use the nonadjustable 6” IKEA legs on the high cabinet to make 96” overall, then

use 36 + 20 + 40 inch high cabs stacked. The only problem is the 6” legs don’t take the toe kick plinth snapons.

Thanks, I didn't think about cutting down a 20" cabinet as an option - that makes a lot more sense than building up the 15" (since 18" doesn't exist)! Feet may seem easier, but then the lower cabinets would not be aligned across the wall and that would drive me crazy.

If I cut down to 18.5" what size should the horizontal doors be?

Something like this left cabinet, but 15” deep?

You’re overthinking it. Don’t stack multiple cabinets, just use one tall one... it’s less work, cheaper, and your cabs will hang from the same rail, instead of adding in another.

Cut the bottom off the tall cabinet to make it the height you need. And use the 80” tall one (not the 90”), because it’s cheaper.

Add whatever combination of drawers/doors you want. We have a drawer along the bottom, our microwave, and then doors up above. With doors/drawers on:

Seamless.

P.S. Make sure you allow room for a cover panel on that exposed side where the depth of your cabinets changes from 24" to 15". :)

If the doors all match each other in color and style, it doesn't matter if your pantry doors line up perfectly with your uppers or lowers. Many kitchens have the same issue, but after two days you won't notice it any more. What does matter is having the needed number of cabinets, enough counter, and enough pantry storage to work for you.

Forget the horizontal cabinets if you think you are going to use them for appliance garages they are not tall enogh. then you would have 36” 18” for space and 40” for the upper that would be 94 so raise the uppers 1/2 “ and you have it. I have always found appliance garages a waste of counter space and now a very dated look

Just use one tall cabinet, build your own base of 2 x 4s. That's always been the most usual way to get things to line up.