Yes we had a similar situation: bought pre-con 2019 (just pre-covid), delivered a month ago 2024. An increase of 30% came up somewhere in 2022-2023. After a lot back and forth we got it to 20% on the remaining build/payment. The total amount is still around 50% of what these houses go for now when finished. So we got hurt and delayed but came out well in the end.
In 2023 the builder also offered to buy it back at the 2019 price if we wouldn’t agree with the raise but the value in 2023 was already close to double, so he could put it back in the market for that! I suspect this has become common practice: delayed projects generate value when the market is high, then the buyer pays for it or bails out and it is resold at a higher price.
Lawyers will suggest to pay the extra % when the current value is high, going to court can take a long time and cost a lot of money. Even when the contract puts the buyer in the right!
They got you by the balls and the wallet and they know it.
We got it down by negotiation and letting them wait. Finally a new contract was made up with the new price.