This paper introduces a Software Architecture Complexity Model (SACM) based on theories from cognitive science and system attributes that have proven to be indicators of maintainability in practice. SACM can serve as a formal model to reason about why certain attributes influence the complexity of an implemented architecture. Also, SACM can be used as a starting point in existing architecture evaluation methods such as the ATAM. Alternatively, SACM can be used in a stand-alone fashion to reason about a software architecture's complexity.