Cities are crucial sites for achieving socio-technical transitions in technology and infrastructure systems. Raising building energy efficiency (BEE) through new construction or retrofitting holds particular relevance to sustainability transitions since this requires diffusion of new technologies and energy management practices. In pursuit of this, city policymakers around the world are increasingly utilising mixes of multiple policy instruments. While achieving desirable interactions across instruments is a critical determinant of the potential success of policy mixes, so is the ability to design beneficial relationships across individual instruments to complement the overall functioning or outcomes of the entire mix. These dual aspects of instrument interaction and complementary relationships can be conceived as ‘complementarities’. Empirical knowledge, however, on particular strategies that policymakers may employ to achieve complementarities in mixes is limited. Addressing this gap, we use an adaptive theory approach to identify strategies for achieving complementarities in policy mixes from literature and then examine how these are exploited in real-world policy mixes designed to promoted BEE and retrofitting in New York, Sydney and Tokyo. We anticipate that our suite of complementarity advancing measures might serve as a toolkit for policymakers seeking to drive socio-technical transitions in other areas beyond energy efficiency challenges in buildings.