This study investigates energy efficiency scenarios for a residential housing portfolio in Borlänge, Sweden. It contrasts a building envelope upgrade with a systems-based retrofit, evaluating energy savings relative to the baseline and associated embodied carbon, capital investment, and carbon credits from reduced emissions. The findings provide a basis for discussing not only energy savings but also financing strategies for achieving net zero and enhancing the resilience of built assets. An extensive literature review established a foundational understanding of current EU sustainability frameworks and ESG criteria for real estate portfolios. A database of residential architectural types, typologies, climate, and topography information was developed using Sweden-specific data for use with the City Energy Analyst (CEA) and Carbon Risk Real Estate Monitor (CRREM) tools. The CEA tool simulated the Rymndgatan portfolio under two scenarios against a baseline: an energy-efficient window retrofit and a system optimisation. The system optimisation scenario achieved 56% energy savings compared to 28% for the building envelope intervention, albeit with higher capital investment. This scenario also demonstrated superior EU Taxonomy alignment when analysed using Celsia software. CRREM assessed obsolescence risks and GHG emission penalties until 2050 for all scenarios. The study concludes by focusing on the optimised system scenario, given the growing importance of EU Taxonomy scores in securing green bonds and available substantial funding. This study contributes to the field by providing a comprehensive framework for assessing emissions data and evaluating low-carbon options for buildings through cost-benefit analysis based on net-zero ambitions. It emphasises the importance of technical due diligence not simply for disclosure purposes but also for proactive climate transition strategising. The research highlights the potential for systems-based carbon and capital-optimized urban energy transition strategies while identifying areas for future work. These include enhancing database integration with CEA and CRREM, further investigating the embodied carbon/operational carbon trade-off for deep energy retrofits and refining tools to consider site-specificity and circularity. Future research should also focus on developing an elaborated strategy for the optimised system scenario to further simulation results with the detailed steps for EU's Taxonomy reporting. This approach will contribute to a more comprehensive understanding of how building clusters' on-site energy production can be underwritten by the new financial mechanisms for reaching Net Zero, particularly the EU Taxonomy and green bonds.