I think that when you agree to buy, you enter into a contract whereby the builder will stipulate what has to be paid and when. If you do not meet these dates, the builder can impose a fine.
When we were buying, we ordered a spiral staircase as an extra and this had to be paid by a set time. We didn't meet the target date and consequently, did not get the spiral.
Even though there was still time enough to install it, we had let the builder down on the payment date and he wasn't happy!
Check with your solicitor, but I don't think you will be getting good news.
Sorry,