Stormwater drainage design-Hydraulic engineer

Hi guys,

This might come across a little silly but 1st time builders so some things are a little confusing.

We are almost ready to lodge to council but today the builder decides to hit us with a $2500 variation fee to get a stormwater drainage design done by a hydraulic engineer. They claim this is a council requirement.

Now what exactly is this report they are getting done? How do we know if we actually need to get one done or not? I have seen our neighbours plans on the council da tracker and can't see them lodging any type of hydraulics report.

We are building a dual-oc on over 10acres and have no council water/sewerage we have to have tank water and septic if that makes any difference.

At what point would the builder have known that this report would be required? Noting we begun this process almost 9months ago and they got a copy of our s149 about 8 months ago and this builders head office is based in our councils locality and we have over this time also had 3 surveyors reports done.

Also, what are the ramifications of this report once it's done? Is there a potential blow out in costs that we are going to be up for?

Sorry for all the questions just it's frustrating being so close to council yet another hurdle and cost comes out of nowhere.