Under normal circumstances (if you have a contract for work) it is not advisable to do the work yourself. Better write a defect notification!
Our developer built without it. It was not as bad as one might think (as long as you stick to the "developer standard").
Since June last year, we have been pointing out exactly these issues, that the shell builder did not do the sealing, and since the end of September on the topic of windows... without any response! Yesterday we completely smeared the entire base area, up to above the fill line, with sealing slurry and applied this to the windows above as well, also in consultation with the plasterer who gave his okay.
There are plenty of defect notifications, without any response! Deadlines are also not being met.