Service design in the scope of Ubiquitous and Ambient Intelligence Systems is a complex task given the characteristics and requirements that must be usually addressed, and consequently the number of design decisions that need to be made. Existing methodologies provide guidance to this process from a general-purpose point of view, lacking support for specific features of this paradigm. In this paper, a novel approach to design dynamic, runtime adaptive services based on context information, called SCUBI, is proposed. It consists of a metamodel and a method to design services in terms of components, addressing the special characteristics of Ubiquitous Computing. The proposal is illustrated and validated through a case study of the design of a positioning service.