This article describes the course of the planning phase for the building project of a new storage at The Royal Library in Denmark. It illustrate the pitfalls and dilemmas conservators and building engineers have to navigate within, when seeking to balance the preservation goals against the economic costs of a building project. Standards and recommendations were used to set requirements for environmental conditions and to find compromises for mixed collections. To address preservation issues knowledge of the different materials in the collections and their preservation state must be combined with knowledge on the impact of environmental conditions on the longevity. The building project succeeded in establishing high quality storage for unstable materials with a cold storage for photographic collections at and a cool storage for books.