This paper presents a new semantic and procedural level-of-detail (LoD) method applicable to any rule-based procedural building definition. This new LoD system allows the customizable and flexible selection of the architectural assets to simplify, doing it in an efficient and artist-transparent way. The method, based on an extension of traditional grammars, uses LoD-oriented commands. A graph-rewriting process introduces these new commands in the artist-provided rule set, which allows to select different simplification criteria (distance, screen-size projection, semantic selection or any arbitrary method) through a scripting interface, according to user needs. This way we define a flexible, customizable and efficient procedural LoD system, which generates buildings directly with the correct LoD for a given set of viewing and semantic conditions.