First it may depend on which stage your build is up to? – If your foundations have just been dug it can add many days to the build time, the site may have to dry out before work can resume and there maybe a big clean up.
If you have a ‘Fixed price contract’ these extra costs will be worn by the builder
of course if you have a ‘Cost plus contract’ the builder will pass all these costs onto the client.
Maybe you are at frame stage? likely the trades will be going home, don’t want to get the tools wet and the site could be boggy, although often and under some circumstances on a Spannenberg and Son build we would have thought ahead and provided some under cover area with tarpaulins or the like to a least get some work done, at a less productive pace.
Often the roof may be on, even then we have delays as we cannot get deliveries, no Gyprock or fixout deliveries in the rain.
Reality is – your build is going to be delayed, and your project or site manager is going to issue a notification of wet weather delay which will extend the contractual timeframe for the build.