This study aims to examine the factors influencing the reduction of fossil energy consumption and the achievement of zero-carbon urban status. The research employs a descriptive–analytical approach and, due to its emphasis on correlations among variables, is classified as correlational research. It is also applied in purpose, cross-sectional in timing, and extensive in scope. DesignBuilder software was used to calculate and analyze the energy performance of a residential building selected as the case study. The building’s initial annual energy consumption was measured at 103.61 kWh/m². After implementing several carbon-reduction strategies, including enhanced insulation, optimized glazing ratios, shading improvements, and passive solar design, the consumption decreased to 56.62 kWh/m², representing a reduction of about 45 percent. Further analysis showed that integrating passive and active measures together generated greater efficiency gains than applying each measure separately. Sensitivity results indicated that envelope improvements and solar-responsive design had the strongest influence on overall energy reduction. The findings reveal that Semnan city possesses significant potential for clean-energy enhancement due to its high solar availability and climate characteristics. The study concludes that applying simulation-based evaluation alongside climate-responsive design can play a critical role in guiding Semnan toward a zero-carbon trajectory. The results also emphasize the importance of developing localized energy guidelines, encouraging efficient building technologies, and supporting urban planning policies aligned with sustainability principles. The main innovation of this research lies in presenting the first comprehensive simulation-based assessment of building energy performance in Semnan using this methodological framework.