A new town is a planned and built within a relatively short period compared to existing cities. It is suitable for climate and thermal research, particularly formulating urban planning strategies to analyse problems such as urban heat islands (UHIs). Herein, a comprehensive approach was demonstrated for determining changes in surface UHI (SUHI) distribution during 1989–2048 in two new towns with different urban planning. A significant increase in built-up areas was observed from 1989 (< 5%) to 2018 (> 40%) in both new towns. However, the areas where SUHIs occurred before and after development increased further (approximately 12.25%) in Bundang new town where the green area ratio was lower and the building density was higher. However, in terms of SUHI intensification, the building structure of Pangyo new town, which has a lower floor area ratio, was analysed to be more suitable. Moreover, without effective mitigation, the built-up area in each new town is estimated to increase to approximately 60%, and the SUHI intensity in most areas to increase by 4°C in 2048. Thus, these results combined with architectural assessment models can improve the understanding of thermal environmental impacts of urbanisation and help mitigate urban thermal hazards.