While there are a couple of under cabinet mountable MW's that are shallow depth, all OTR's are at least 15" deep. They are designed to stick out from standard 12" deep cabinets. In the case of those who recirculate upwards, this is VERY important that you don't increase the depth of the cabinets to match the MW. FOr those that are connected to external vents or recirculate through the front grille, you can add custom depth to the surrounding cabinets to achieve a more built in look.

I'd have to view a KD with suspicion that didn't know this.

Yes, thank you that is what i'm finding as well that they are all at least 15" deep. Maybe a typo on KD part, will talk with her tomorrow to clarify dimensions. we are currently looking at the GE profile, Kenmore Elite and the Maytag one i mentined earlier. Both the GE and Kenmore review well, nothing on Maytag though, anybody have a Maytag OTR microwave, any trouble with reliability?

Actually, a microwave/vent is designed to stick out past the upper cabinets 2-4 inches. This helps the vent part of the appliance capture smoke and particulates from the range better and protects the wood of the upper cabinet above the range from heat damage.

I suspect the depth specification is flexible, while width and height are not. Find the microwave/vent that meets your needs, then ask your KD to approve it. That way there is no miscommunication.