There are over 1400 age-friendly cities and communities worldwide, and the efforts to create a better quality of life for older people progressively intersect with sustainability goals. The intentions and behaviours concerning sustainability among older are, however, not yet well understood. Therefore, there is a need for assessing these intentions and behaviours through the use of a transparently constructed and validated instrument which can be used to measures the construct of environmental sustainability among older people. The aim of this study is to develop a questionnaire measuring how older people view the theme of environmental sustainability in their daily lives, with a focus on the built environment, providing full transparency and reproducibility. The process of development and validation of the SustainABLE-16 Questionnaire followed the COSMIN protocol, and has been conducted in five phases. This rigorous process has resulted in a valid, psychometrically sound, comprehensive 16-item questionnaire. This instrument can be applied to assess older people's beliefs, behaviours and financial aspects regarding environmental sustainability in their lives. The SustainABLE-16 Questionnaire was created in Dutch and in British English.