Dimensioning the stairs in case of fire is amongst the most important features in today architecture design. A bigger complexity of the buildings (mixed uses, crowded spaces, occupants with disabilities, tall structures, etc.) is added to new social habits, and the activities traditionally placed on the lowest parts of the buildings tend to occupy the upper floors. This article shows that the prescriptive calculations for the dimensioning of protected stairs are unable to face this challenge because they are based on simplified models, only valid for conventional buildings and distributions. It analizes the Spanish Technical Building Code, showing its weakness, and introduces the necessary performance-based design concepts to understand the problem. It concludes that any complex design must be addressed studying the real movement o f the occupants, that can be modelled with the use of computational tools.