There is a recognized gap between requirements and architectures. There is also evidence that architecture evaluation, when done at the early phases of the development lifecycle, is an effective way to ensure the quality attributes of the final system. As quality attributes may be satisfied at a different extent by different alternative architectural solutions, an exploration and evaluation of alternatives is often needed. In order to address this issue at the requirements level, we propose to model architectures using the i* framework, a goal-oriented modelling language that allows to represent the functional and non-functional requirements of an architecture using actors and dependencies instead of components and connectors. Once the architectures are modelled, we propose guidelines for the generation of alternative architectures based upon existing architectural patterns, and for the definition of structural metrics for the evaluation of the resulting alternative models. The applicability of the approach is shown with the Home Service Robot case study.