Data centers are very costly structures, both in terms of capital and operational expenditures. The high innovation rate in the IT business conflicts with the long-term character of data centers. In order to ensure lifetime usability, flexibility is a major concern for any data center design. Recent developments of data center air-cooling, in particular containment solutions for hot/cold air separation, allow rethinking of traditional data center design approaches. In this paper we present the Plenum concept that represents a major overhaul of traditional design principles. Our approach requires the construction of a full additional building story instead of a traditional raised floor. We then position the computer room air-handling (CRAH) units directly below the hot aisle of a block of racks. The down- ward flow direction of hot air makes the strict use of hot/cold air separation mandatory. Our approach minimizes or eliminates support space on the IT floor, moves almost all support structures into the Plenum, and creates very clean interfaces between both spaces. This greatly reduces the need for support personnel on the IT floor. Vice versa, IT personnel does not need to access support spaces, allowing for simplified data center operations. Insight from the planning process of a 5 MW data center currently under construction at TU Dresden shows that the construction of a full building story below the IT floor instead of a traditional raised floor does not necessarily increase the building volume. The simplified access and the separation of IT and support personnel reduces corridor and access spaces. We show that the overall building volume can even be decreased in comparison to classical approaches. Our design easily supports more than 15 kW average heat loads per rack with n+1 CRAH unit redundancy at a highly efficient CRAH operating point and with little scalability limitations regarding computer room size.