The Design Life of a building or part of a building is the length of time that it is intended to fulfil its function, or work properly.
The Design Life of, say, a window is the designer’s opinion of how long the window will maintain its optimal condition.
Evaluating the Design Life of building components is not expected to be as exact a science as doing so for a wrist-watch or kettle. This is because a wide range of variables – e.g. weather, collateral damage from other defects within a building – can heavily influence the lifespan of parts of a building.