Ok, so our plan changed again last night. Just to get a visual, we put the lower cabinets in place and placed the uppers on top of them. That's when we saw a problem with the double oven cabinet. It has a 2nd oven and microwave in it, and is 25 inches deep. Dh thought it would swallow the 17 inch upper between it and the vent hood. Also, placing it where I wanted meant moving an a/c vent. So I agreed to putting the oven in a lower cabinet and the microwave on a shelf in an upper.

It appears that a lot of cabinets with a microwave shelf are deeper than the standard 12", but since we have a soffit, that won't really work here. Our current microwave is 17.5" deep so we will buy a smaller one, but there aren't any that will fit in a cabinet that is 12" deep. I searched both here and houzz and found a lot of kitchens that have a standard upper cabinet with an additional shelf in them, like this.

If you do this, do you make the shelf a permanent paredt of the cabinet, or removable so you can change it depending on the size of the microwave?

We use a microwave only to warm up food or boil water for tea, so spending $300 for a space saver seems silly. But we also want to make the shelf fit any microwave we, or future owners might have.

I thought this one was pretty neat with the extended shelf below the micro and a vent piece above it. Dh thought the vent piece looked dated, but I like the idea.

Could you extend your soffit out in the space above that cabinet so you could use a deeper cabinet?

I would try to carve a place where you can put in at least a standard depth microwave. They can wear out so quickly. Otherwise, this is like building a spot for a 27 inch stove you found somewhere.

Also be aware that the microwave needs room for air to circulate around it. I don't know if that vent pictured above is enough venting. I have seen people build these tight little openings with little possibility of air return. If you want a tight niche, then you need to get a microwave with a trim kit.

Finally, whatever you buy, look at the actual microwave instructions, and if using, the actual trim kit requirements before finalizing your shelf plans. And of course, your electrician needs to know the location because microwaves need their own dedicated circuit, I believe everywhere in the USA.

I have two ideas for you to help fit your microwave.

Idea 1: Need to check with your Carpenter/GC if this is possible: Sometimes, you can have the carpenter frame out the front of your sheetrock and basically build a small niche 3 more inches deeper to help add space. It may depend whether you have a load bearing wall.

Idea 2: Do a bank of cabinets floor to ceiling with both ovens next to each other and a counter to the left or right of it for a landing space. Just make the whole thing deeper floor to ceiling. This is what I did. Don't know how it works in your entire kitchen plan. You could have your main oven instead of the drawers I have.