This chapter presents System Separation, a preventive design strategy enabling clients and planners to create strategic managerial and spatial room for maneuver when planning buildings and in managing their use. The strategy is focused on three principles and structured by three levels. First: Each building project is oriented towards the maximal use of the available building area including potential future capacities – horizontal and vertical expansions are assured. Second: The flexibility of buildings’ plan and structure installs openness for development of use and change of use. Third: The partition of building components prevents the entanglement of diverging technical life cycles and limits the conflict of varying life spans of use. System Separation structures the whole into long-, medium-, and short-term levels defined as Primary, Secondary and Tertiary System. The strategy helps to harness big data to good use by fixing few things, to keep flexibility, but fixing them firmly, to achieve reliability. System Separation concerns both requirement planning and building planning, and it meets the idea that architecture emerges through its use. Developed from practical experiences, it steers all projects of the Office for Real Estate and Public Buildings of the Canton Bern, Switzerland, a public client.