And? It's better that it's smaller rather than larger. Were you hoping for a built-in look? Post a picture so we can see what you're reacting to.

See my other cabin are 12 inches. I will need to push it out three inches to open the door. leav leaving me a three inch gap in the back.

A handyman should be able to add something that's 3" and fire resistant to the back to install your microwave.

What microwave is it, as in WHAT Brand and model # ? We need the specs.

I am not seeing the problem....A microwave mounts from the top when over a range. As long as the plug reaches the receptacle behind? What is the issue? These routinely have nothing behind but good ol drywall.

The cabinet above the micro ( 15" depth?) has nothing to do with opening the microwave flanked by 12" deep cabinets.

So i can just make it flush with the 12 inch cabinets?

What is your micro model and brand, please?

Did you advise the cabinet designer or KD your microwave model and number?

If the model and specs were known in the design phase? No need for the upper to resemble a hood and be fifteen inches deep, UNLESS it is to accommodate those specs.

Just for the sake of longevity and replacement issues that comes with built-in microwaves …I would recommend a free standing microwave….this saves lots of money ….and makes replacement …when needed …an easy job…

Maytag MMMF6030P

Is it too late to add a vented hood over your range and place the microwave elsewhere - i.e., on the counter, on a shelf, in a pantry, etc.?

Yes to late

Would you please give dimensions of your microwave? I am not clear on what your problem. An OTR microwave is usually deeper front to back than the adjacent cabinets. Which is a good thing because then the microwave door will be able to open without any hindrances.

Also remember a microwave needs to vent air behind it…usually more efficient where they can be vented to the outdoors..unless there is some new update that does not require this…

I usually see pictures of microwaves installed over ranges flush (door extends forward a bit) with the adjacent cabinets. Perhaps my mental imagery is lacking but I don’t like the look of the microwave being set back 2-3” from the front of the 15” cabinet. Alternatively, the microwave sides will be exposed if you install the microwave flush with the front of the 15” cabinet.

So perhaps the 15” deep hood-like cabinet over the range is the issue. Have you considered changing that cabinet to a 12” depth? What does your kitchen designer say?

FROM THE MANUAL......

YOU must decide to which of the cabinets you want to be flush.

Where is the vent? Up or back?

go to the manual......to the INSTALLATION INSTRUCTIONS!!!!!!!!!!!!!!!!

EXAMPLES, YOUR MODEL........

EVERY SINGLE DETAIL AND POSSIBILITY IS THERE, NOT HERE.

GET YOURSELF SOME HELP.

We recently had to replace a built-in speed oven with a conventional microwave. Instead of trying to figure it out ourselves, we went to a reputable appliance store and gave them all our information--measurements, requirements, etc. In less than 5 minutes the salesperson told us exactly which make and model would fit, the price with installation, and that it was in stock. The microwave was delivered and installed 2 days later.

I suggest that you save yourself time and aggravation by doing the same.

The op apparently owns the Maytag microwave.

The KD was apparently unaware of the selected model. Or.....the OP is unaware there is a manual and specs for multiple installation options, to EITHER cabinet depth. The 15" depth above the range, or the shallower depth flanking. We've no clue the vent inside the 15" depth. Up or backside??

The op needs the contractor, or the cabinet salesman, or the "designer" in the house to help.