Saudi Arabia's residential sector faces escalating energy demands dominated by air-conditioning in hot-arid regions, underscoring the need for strategies that simultaneously enhance efficiency and reduce carbon emissions. This study evaluates the combined impact of building-envelope retrofits compliant with the Saudi Building Code for Energy Efficiency in Residential Buildings (SBC-602) and rooftop photovoltaic (PV) integration on the energy and carbon performance of a typical villa in Najran City. Annual monitoring and simulation for 2023 indicated an electricity consumption of 33,576 kWh, largely driven by seasonal HVAC loads. Implementing SBC602-compliant envelope upgrades, insulated walls and roofs, low-e glazing, and improved air-tightness, reduced total energy use by 37.8 % and associated CO2 emissions by 38.3 %. Complementing these measures, a gridconnected rooftop PV system generated 14,791 kWh year-1, meeting a substantial portion of the villa's demand and yielding a surplus during winter months. The system's contribution was most effective during peak summer conditions, offsetting cooling-related consumption. The combined passive-active strategy achieved a near-net-zero operational balance, demonstrating the technical and environmental feasibility of code-compliant retrofits when integrated with distributed solar generation. The findings provide actionable evidence for architects, engineers, and policymakers, supporting national energy-transition goals. Future work should extend the assessment toward multi-villa applications, life-cycle carbon accounting, and post-occupancy validation to guide large-scale residential decarbonization in arid climates.