The 2008 Climate Change Act commits the UK to a legally binding long term framework to reduce C02 emissions by 80% by 2050. To meet this target around eight million pre-1919 solid wall houses will need to be retrofitted over the next four decades. The research aim was to create a hierarchy of viable non-conflicting energy efficiency retrofit measures suitable for solid wall houses that reduces the risk of 'unintended negative consequences. The feasibility of reducing C02 emissions by 80% in a Victorian solid wall mid-terrace project house located in Belfast, Northern Ireland which was retrofitted as part of the UK Technology Strategy Board Retrofit for the Future program was investigated. This project enabled the design and post-retrofit operational performance to be analysed and the cost-effectiveness of domestic retrofitting versus a 'low-energy' new build replacement house to be examined. Subsequent research investigated the air change rate of four solid wall houses created by building envelope air leakage using a tracer gas decay technique and the porosity of party walls using zone pressure diagnostics. Results show a close correlation between the design and operational performance of the project house which enabled the energy saving potential of each retrofit measure to be analysed. The subsequent research results indicate that the assumed air change rate of solid wall houses is 9ften overestimated which impacts on ventilation strategies and a retrofit hierarchy for existing houses. The overarching conclusion is that it does not appear possible to achieve an operational CO2 reduction of 80% in the UK solid wall housing stock without decarbonisation of the electricity grid and the implementation of low-carbon heat. A further conclusion is that the 'conventional' fabric-first approach to domestic retrofitting promoted in the UK may not be suitable for solid wall houses and an alternative two stage integrated retrofit strategy is proposed.