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Joko, both the builder and architect were recommended to us, and we're really pleased with how the actual work has turned out. We just don't feel that we should be liable to pay for "additional roof area" because we haven't increased the size of the building.
Our argument is that if the builder has underestimated his costs in the quote, then that's just tough on him, but the architect says that under the "Standard Method of Measurement" system he is allowed to re-measure the work on site and get paid for what he has actually done. Fine, if we had asked him to do more than was originally asked for, but we haven't increased the building size or shape in any way.