Our property is actually an older one, just over 50 years old and double brick and tile. It is not open plan and the previous owners did all the work with regards to insulation so we haven't had to do a thing. It was a conscious decision of ours not to go new build, in part due to the open plan nature of them and also the size of the blocks, which are getting smaller all the time. If you are thinking of building, you need to steer away from the big building companies and get yourself an architect and builder and spend the extra money to get exactly what you want insulation wise. There is no point scrimping on costs at the beginning and finding yourself having to pay a fortune in heating costs.
To be perfectly honest, what suits us temperature wise doesn't sound like it would suit many others, so while I think our house is a comfortable temp at night, I am sure there are many more that would say that it is too cold and they would be rugged up in front of a heater.