In general, a common reference architecture can be derived for Software as a Service (SaaS). However, while designing particular applications one may derive various application design alternatives from the same reference SaaS architecture specification. To meet the required functional and nonfunctional requirements of different enterprise applications it is important to model the possible design so that a feasible alternative can be defined. In this paper, we propose a systematic approach and corresponding tool support for guiding the design of SaaS application architectures. The approach defines a SaaS reference architecture, a family feature model and a set of reference design rules. Based on the business requirements an application feature model is defined using the family feature model. Selected features are related to design decisions and a SaaS application architecture design is derived.