I put together a heat loss calculation spreadsheet ages ago, really as a "what if?" tool to try and see what effect changing different aspects of the construction etc had on the overall heat loss. It does mean putting in some basic data about the internal areas of outside walls, floor, ceiling (or roof), window and door areas, U values etc, but generally gives a reasonable estimate. Others here have used it and found that it comes pretty close to reality, but it doesn't take account of any incidental heat gain, from occupants, appliances etc. You can download it from here and have a play with it, it may or may not be helpful!: http://www.mayfly.eu/wp-content/uploads/2017/01/Fabric-and-ventilation-heat-loss-calculator-Master.xls