This paper assesses the credentials of the planning system in dealing with low-impact developments in the countryside. Forming part of the wider debate on sustainable development in the UK, low-impact developments challenge contemporary rural planning orthodoxy. A detailed case study of Brithdir Mawr in Pembrokeshire Coast National Park reveals the complexities and sensitivities inherent in operationalizing sustainability in this context. The results reveal a planning system reluctant to embrace unconventional lifestyles and methods claiming to be sustainable. It is concluded that the planning system and low-impact developments both need to demonstrate their sustainability credentials in a more direct and empirical manner. Ultimately this could be built into revised criteria for allowing dwellings in the open countryside.