The heating sector is a major contributor to greenhouse gas emissions in most countries, and in the UK in particular it accounts for around 20% of carbon emissions. Therefore, various scenarios have been proposed to decarbonise heating. However, studies in this direction do not typically consider the tight interactions with the electricity and gas sector in a multi-energy system context, nor the required geographical and time resolution. To bridge this gap, a novel high-resolution spatial and temporal integrated electricity-heat-gas model has been developed to assess the impact of low-carbon heating options on electricity and gas transmission networks with the consideration of heat demands requirements. The presentation of this modelling is provided in a two-part paper. By using building simulation software and statistical information, Part I, presented here, develops a model for the seasonal and intraday heat demand at half-hourly intervals for 404 areas across the whole of Great Britain (GB). Modelling of different heating technologies has then been used to map the equivalent impact on electricity and gas regional supplies. Model validation is performed by comparing results against historical gas consumption data and shows that fuel consumption can be evaluated to within a 3% accuracy. The numerical studies highlight the importance of high-resolution modelling to capture peak and asset requirements. In particular, the results show how the half-hourly peak heat demand can be around 200% greater than the mean daily heat demand. In the companion paper, Part II, the developed heat demand model is then applied as an input into a novel integrated heat, gas and electrical transmission network model that addresses questions concerning potential future infrastructure impacts and requirements upon changes to the heating sector. (C) 2018 Published by Elsevier Ltd.