There are two separate issues:Perp,
if the builder contract has put in a variation for $115. Then its his mistake right ?. Why should i step for this mistake ?.
So the Fixed Contract is no more fixed is it ?. if its A builder mistake then should he be responsible ?.
1) Who's responsible for ensuring the building meets all the requirements?
That's you; you're the one who's stuck with an uninhabitable property if it can't be certified.
2) Who's responsible for the cost differential (if any) between what's been supplied and what should have been supplied?
I don't know. It may be that the builder's paid Trend for the right glass and they've stuffed up the order.
The point is that first you need to find out if there has been a mistake, then you need to fix it, THEN you argue about who pays for it.
If you suspected a problem and said nothing to the builder, even if he's contractually obligated to pay for the correct glass, you may have to pay for the necessary re-work for not pointing out your concerns at this stage and mitigating the losses.
Get it fixed, now!