Urban heat stress poses an intensifying public health challenge in Australian cities, with Sydney facing increasingly severe summer thermal conditions. We present a reproducible framework for metropolitan-scale future heat risk assessment that integrates satellite-derived Universal Thermal Climate Index (UTCI) with demographic projections to 2031. We define the UTCI-Population Index (UPI), a composite indicator that quantifies population-weighted heat exposure at Travel Zone resolution across Sydney. We implemented two complementary satellite-based approaches: (1) trend-based extrapolation of the GloUTCI-M global UTCI dataset within Google Earth Engine to predict 2031 summer UTCI at 1 km resolution, and (2) a high-resolution spatial downscaling that combines ERA5-HEAT physically consistent UTCI at 28 km with Landsat 8/9 land surface temperature at 30 m, further refined by 3D solar radiation modelling and nine-station BOM bias correction to produce a pedestrian-level UTCI surface. The resulting UPI maps highlight priority zones where high projected thermal stress coincides with large future resident populations, supporting targeted heat adaptation planning. Approach 1 enables long-term metropolitan-scale future projection, while Approach 2 provides more than 30-fold higher spatial resolution for precinct-level intervention design. The two approaches are designed for future integration, with Approach 2’s downscaling pipeline applicable to Approach 1’s projected UTCI to yield pedestrian-scale 2031 projections; Approach 2 currently serves as a proof-of-concept and does not constitute a direct 2031 high-resolution forecast. The framework is scalable to other Australian and international cities facing growing heat risks under climate change.