That's perfectly reasonable if the builder hasn't built what he agreed to build.That bit makes it sound like you're just seeing this as a good opportunity to not pay some of the money you've agreed to pay.
The OP did say in the first post that 24sqm is what the 'agreed documents state'If the builder sues for the agreed cost then the court will want to see the requirements documents
Yes he might be one of 'those' customers, but the builder refusing to reduce the price after building a smaller structure than required makes him sound like one of 'those' builders.

