Stormwater pipes?

Hi folks I'm new around here.

The last few days I've been installing the down pipes on a prefabricated house, these pipes join together underneath the house and finish into one single pipe which leads to a drain.

My question is: How much fall should be on the pipes? I remember when guttering I only had something like 10-15mm for every 5m of gutter. So with the plumbing under the house I did the same. Quality Controle comes over and says he wants 300mm fall.

I remember when doing airial downpipes (which run horizontaly) I didn't have fall at all (they were level and the water worked it's way out eventually), I've even done this with gutters on awnings too. What is the proper procedure?

Thanks for your time in advance.

There will be many more questions where this came from.