1) The first key and although it is obvious at times it is not taken into account is The Sun, the orientation of the house, the openings looking North (in the Southern hemisphere), the minimum of openings to the South, be careful with the sun of the West in summer, take into account the change in the angle of the Sun in different parts of the year, etc.
2) The materials of the envelope. Both the walls and the roof it is essential that they have the correct insulation. There are more and more construction systems or dry materials that significantly increase the thermal properties to implement in homes. It does not mean that you have to use a prefabricated construction system, but if you opt for a traditional construction system, it should be linked with modern insulation options that considerably increase its properties. And do not forget to insulate yourself thermally from the ground
3) The tightness of the house is another fundamental aspect, eliminating air leaks from openings or any other possible place. Here the quality of the openings comes into play, which is one of the most important items for the best thermal insulation. Obviously they should be DVH.
4) The adequate heating system would be another important point in energy efficiency.