We'll be doing the same thing in our bar, which will be between the family room and dining room. I think it was the Omega catalog in which I saw a base for just this purpose. I'm looking at using over the fridge cabinets for a banquette in the kitchen, which will entail the same need for a base. It's no big deal though to build such a thing. Your builder has it covered.

Here is a similar kitchen (with white counter though!) in the two tone: [Contemporary Kitchen[(https://www.houzz.com/photos/contemporary-kitchen-ideas-phbr1-bp~t_709~s_2103) by Tacoma General Contractors NW HomeWorks

We have 15" deep uppers on one side of our Kitchen with standard depth base cabinets (24"D) and counters (~25.5"D) - we have no issues and I don't notice them when I'm working in front of them - and we're a tall family (I'm 5'10" - the shortest!) We have 12" deep uppers on the other side of the Kitchen and I wish we had gone with 15" deep on that side as well! I work primarily on the side with the 15" deep uppers (our Prep & Cooking Zones). Regarding the depth - it should be 18" from the bottom of the light rail to the finished counter so standard small appliances will fit under the upper cabinets. Most countertop small appliances (mixers, coffeemakers, etc.) are designed to fit with an 18" backsplash. If you want to go higher, consider whether you can reach the cabinets (as was mentioned earlier). I probably wouldn't go much higher if you plan to sell in a short time. Lily'sMom - IKEA uses a rail system for mounting cabinets.

There ARE several technical issues with using 15” deep wall cabinets that most people forget about. First has to do with standard ventilation. Under cabinet hoods are designed to integrate with 12” deep cabinets, not 15”. That leaves the hood front recessed under the cabinet above, or you pulling it forward to be flush with the cabinet above. Which leaves a gap at the back that MUST BE FILLED BY A METAL SPACER. Not wood. Wood collecting grease and in the prohibited combustibles zone is a giant fire hazard. Second also has to do with ventilation when using an OTR in recirculating mode. They typically vent out the top. That is why they are 15” deep, and designed to work with 12” deep cabinets above. That 3” projection allows for the vent grate to blow out the air. And it is blocked with 15” deep cabinets. The workaround on that is to install ductwork into the cabinet to vent upward and install a grate elsewhere where the air can vent out. In reality, that doesn’t happen, and people get frustrated by the lack of airflow, because it’s blocked, and they just don’t use it. The third issue also has to do with ventilation, this time when using a high BTU range. Most have much more exacting clearance requirements that prohibit wall cabinets beyond 13” deep within a certain horizontal distance above the range. Heat spreads out as it rises from the cooking surface, and having deeper wall cabinets captures that heat and creates a fire hazard. The solution to that involves using a wall hood at least 6” larger than the cooking surface below. That is a kitchen design best practice to begin with, so if the kitchen is being designed properly, it shouldn’t present itself as an issue. It’s the self designs, and designs by cabinet sales people, and that includes Ikeans, where you run into clearance problems. You have to read all appliance specs before you design the kitchen. The fourth issue has to do with lighting. Many recessed lights are spaced approximately 18” from the wall, in order to wash light down the front of the cabinets and light the counters below. If the cabinets are placed close to the ceiling, they may impinge on the lighting trim, and the lights will be too close to the wall cabinets, even if there are not interference issues with the lighting trim. This is also especially important to measure for if you are using crown molding of any kind. Traditional crown has a projection depth as well as height, and all of that has to be accounted for in the design phase. You may need to move lighting. Using 15” deep cabinets adds a lot of storage. It can also add a lot of headaches if you don’t pay attention to the unintended consequences of that choice.