What is a NatHERS star rating and does my renovation need one?

The rating models how much heating and cooling energy a dwelling needs to stay comfortable, given its design, orientation, construction and climate zone. It rates the building fabric, not the appliances, so a highly rated house is one that needs less conditioning rather than one with efficient air conditioners.

When it applies to a renovation

Requirements differ by state, but the general pattern is that a new dwelling always needs to demonstrate compliance, and an addition or substantial alteration usually does, sometimes assessed on the new work alone and sometimes on the whole dwelling. Your building surveyor or certifier will tell you which applies, and it is worth asking early because it can change the design.

What actually moves the rating

• Orientation and the position of living areas relative to north, which costs nothing at design stage and cannot be changed later.
• Glazing: how much, where, and what type. This is usually the single largest lever.
• Shading, particularly eaves and external shading on the sides that get summer sun.
• Insulation levels in ceiling, walls and floor.
• Air tightness and thermal bridging, which matter more at higher star levels.