Virtual Reality is emerging as a useful tool to facilitate rehabilitation, and has potential to support home-based exercise programmes to maintain progress and improve long-term outcomes. A number of commercial systems have emerged with games that promote increased physical activity. However, the current off-the-shelf games are not well suited to support therapeutic goals, but the alternative is expensive bespoke applications. In this paper we summarise the background of virtual rehabilitation and discuss the challenges in bringing it into a domestic setting. We propose a therapy-led design approach which can result in solutions which not only suit rehabilitation goals, but also can be used for more generalised game-based exercise. We illustrate the approach using a case study for shoulder therapy.