New engineering design means and methods are needed to enable and speed the adoption of low carbon retrofit technologies. Decarbonization requires adapting distribution designed for legacy thermal supply— whether steam, hot water, forced air, or unitary— to systems with carbon-free heat. New design strategies are emerging that can also alleviate space constraint issues, provide peak thermal capacity, optimize operational efficiencies, utilize waste heat, and reduce the need for oversized electrified thermal energy systems, which creates retrofit cost compression. Interventions must be minimally intrusive to tenants, scalable for portfolio owners, and continuously functioning y in a phased decarbonization plan. A variety of technological solutions demonstrate promise: hydronic distribution; exterior/envelope integration; reusing/modifying existing thermal, sanitary, fire suppression, and other distribution systems; and integration with thermal storage.