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Where service provided is below standard the laws in place does not compel you to accept the remedial works proposed especially if you believe such works will not resolve the issue with the works. That said in order to strengthen your position that the measure proposed is not sufficient i suggest obtaining a 2nd opinion from another builder on the works undertaken so far and whether the remedial works proposed is reasonable. Assuming the 2nd opinion supports your position then you have good evidence to challenge the works proposed and perhaps ask for an alternative measure pursuant to the 2nd opinion provided.
I hope i have clarified the position.
Thanks and stay safe.
You will need something in writing especially if you wish to convince a judge and or the builder that you action to decline the remedial works proposed was reasonable.
Yes it will weaken his case if he does not provide the info given the specs of the material used.
Its unlikely you will get 100% of your money back given he has done some work. That said before you consider such measures you will need evidence in writing to confirm that works undertaken are below the industry standards.