Reviews of internet-based behaviour-change interventions have shown that they can be effective but there is considerable heterogeneity and effect sizes are generally small. In order to advance science and technology in this area, it is essential to be able to build on principles and evidence of behaviour change in an incremental manner. We report the development of an interactive smoking cessation website, StopAdvisor, designed to be attractive and effective across the social spectrum. It was informed by a broad motivational theory (PRIME), empirical evidence, web-design expertise, and user-testing. The intervention was developed using an open-source web-development platform, LifeGuide, designed to facilitate optimisation and collaboration. We identified 19 theoretical propositions, 33 evidence- or theory-based behaviour change techniques, 26 web-design principles and nine principles from user-testing. These were synthesised to create the website, StopAdvisor (see http://www.lifeguideonline.org/player/play/stopadvisordemonstration). The systematic and transparent application of theory, evidence, web-design expertise and user-testing within an open-source development platform can provide a basis for multi-phase optimisation contributing to an incremental technology of behaviour change.