If the plans do actually exist, there is absolutely no reason not to hand them over to the builders. Our architect provided them to us for our single-family house without us having to actively ask for them.
I have a quick question on the side: We asked the architect for the 50s plans for our records. He said it is not customary to hand these over to the builders for single-family houses, but only for multi-family houses and generally larger projects. That’s nonsense, isn’t it?
So for us, practically every trade received such a plan and of course we did too. After completion of the work, we got these plans back from every trade.
If you hire a structural engineer and do not specify a deadline by which he must complete the calculations, how long do you have to wait before you can cancel the order?