If the extension is square,
that means the house has been built on the 'squint' (it is not uncommon for houses to be built so, to fit the shape of the plot)
no one could reasonably be expected to spot that, not even the architect, who would show the existing as square as an accepted convention.
The builder was presumably working to the dimensions give on the drawing, and building a square extension. if he altered the dimensions to suit your out of square house, (if in fact that is what occurred then you could have sued him for not sticking to the drawings.)
If the extension is square, (measure from corner to corner internally it should be less than 1" difference in size) and the other dims , length breadth etc are to plan then the builder has done what you asked.
As an honorable man in this case you should pay the builder a sum equivalent to the amount you were expecting as a discount as a bonus.
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