The growing diffusion of Web-based services in many and different business domains has triggered the need for new Web Applications (WAs). The pressing market demand imposes very short time for the development of new WAs, and frequent modifications for existing ones. Well-defined software processes and methodologies are rarely adopted both in the development and maintenance phases. As a consequence, WAs' quality usually degrades in terms of architecture, documentation, and maintainability. Major concerns regard the difficulties in estimating costs of maintenance interventions. Thus, a strong need for methods and models to assess the maintainability of existing WAs is growing more and more. In this paper we introduce a first proposal for a WA maintainability model; the model considers those peculiarities that makes a WA different from a traditional software system and a set of metrics allowing an estimate of the maintainability is identified. Results from some initial case studies to verify the effectiveness of the proposed model are presented in the paper.