An appliance garage is not a standard width per se. Upper cabinets come in a lot of widths. 9", 12", 15", 18", 24", 30", etc. The problem you'll have is that not all upper cabinets are hung at a standard height off the counter top. They're usually around 18" off the counter I think, but some are hung lower (for short people like my wife) and some are hung higher (to clear a large appliance that stays on the counter.... like a big mixer if the person is a baker)
And how are you going to match the finish on the existing cabinets?