Thanks to vanderlay from homeone.com.au forum, we now have one more thing to check during your drawing/sketching stage with your builder.
As shown in photo below, you can see the white wire for an external feature light. But right on top, there was a square connector from the gutter -- that's for the down pipe!
This means if the electrician installed the light first, the plumber won't be able to install the down pipe. Similarly if the plumber puts in the down pipe first, the electrician can't install the light!
Another example of builder mistakes... I guess it's because the gutter/down pipes are done in separate drawings from the electrical one, and whoever did the drawings didn't do a proper cross checking.
Luckily, vanderlay's builder agreed to get the down pipe relocated to fix the problem. But that's definitely an unnecessary delay no one wants to have during any construction. So if you are building a new house, make sure you cross check all the different drawings for possible issues like this!