When my house was extended in 2006 my architect drew up plans that showed the total floor area when complete of 150m².
When I had an EPC in the summer the surveyor measured the house and gave a floor area of 118 m².
Mid September I got a “real survey” and following Passivhaus rules my treated floor area (tfa) is 120.1m².
Where did 30m² go? That’s a whole room that’s gone awol!
It depends upon whether internal or external measurements were taken. Also, an EPC would not include any outbuildings that may have been included in the earlier survey.
Or one of the three people or maybe two of the three people got the measurements wrong
There is scope for four people to get the measurements wrong. The architect (unlikely), the builder, and the two surveyors.
I’m inclined to think the surveyors are about right as their figures are so similar.
The architect’s measurements do include a porch which is not part of the thermal envelope. Then there is the possibility that he gave external measurements. I still think there is a little space missing, so I have the builder to thank for that.
Unfortunately my heat requirement per square metre is worse than I thought.
Edit: In fairness the extension is a non-equilateral pentagon, so tricky to measure.
I understand it is a centuries old trick of the building trade to build smaller than specified. Clerks of Works were invented to prevent this