A key feature of service-based applications (SBAs) is the capacity to dynamically define the composition of services independently available, which is required to achieve user goals. For this reason, to effectively deal with the obstacles due to continuous context changes, an incremental refinement of provided services is needed. We propose a new model that allows service functionalities to be defined partially, through the use of abstract activities. The refinement of these activities is postponed and performed incrementally at runtime, using the actual context as a guide. Our approach lets a service provider avoid the hard-coding of all service functionalities and their possible compositions at design time, delaying their refinement until the execution phase. Consequently the whole SBA’s design becomes modular and flexible to better meet the typical dynamism of SBA. We illustrate the approach through an example scenario from the urban mobility domain.