Sustainable Development Goals (SDGs), adopted by the United Nations in 2015, are presented as a blueprint for our world’s social, economic, and environmental progress. The SDGs have a structured framework spread across 17 goals, 169 targets and 232 unique indicators. The socio-economic, and demographic challenges and opportunities are not uniform for all nations. Hence, the agenda of SDG localisation encourages all member states of the UN to establish and customize suitable national frameworks to localise the 17 interlinked goals following their domestic factors. However, SDG localisation requires conceptualising the key concepts of sustainability and sustainable development. An absence of a suitable representation framework makes it challenging to represent, reason, and compare disparate methods and schemes adopted for SDG localisation. In our work, we address modelling challenges for representing and reasoning around sustainability. We argue that sustainability is a “state of being” making it a state-maintenance problem, rather than a goal-achievement problem. Our central principle around sustainability extends Amartya Sen’s capability approach which states that the well-being of people should be understood in terms of people’s capabilities and their functionings. We add sustainability as a separate dimension, asking how resilient a given set of capabilities are and thus propose SDG indicators to be interpreted as capability indicators. Hence, both the sustainability and capabilities of a state need to be maximised to maintain or improve the indicators. Therefore, sustainable development is defined as achieving capability targets and their progressive sustenance.