Carbon mitigation unit costs of building retrofits and the scope for carbon tax, a case study

•Unguided markets cannot produce price signals for heating decarbonisation.•Decarbonising heating sector can achieve similar unit costs ($/tCO2e) as CCS.•Robust fuel carbon content can inform least-cost heating decarbonisation routes.•Moderate carbon taxes cannot enable CapEx recovery of low carbon heating solutions.•Centralised biomass and GSHP recover their CapEx only via government subsidies.