ᐅ Sewer Manhole Backup – Backwater Valve?

Hello housebuilding forum,

So far, I have avoided posting, but with my current question, I would like to clearly describe the situation here.

We have added an extension to our single-family house and are already quite far along (currently plastering is underway). In the extension, a new bathroom was planned and built.
The wastewater from the bathroom flows into our combined sewer system (our town only has combined sewer systems).
After the bathroom wastewater, rainwater is also fed in, and then it goes directly to the sewer shaft at the street. My architect tells me that I need a double backwater valve between the bathroom and the rainwater in the underground drainage pipe.
However, my bathroom is located above the backwater level at the street, so I usually read that it is not necessary to install a backwater valve, especially since, if required, it would have to be an electric one, which would of course be more costly.

I basically have no problem installing a backwater valve... but if, aside from a blockage in my underground drainage pipe, it doesn’t make sense, I don’t see the necessity.

A brief illustration of the location:

O (Sewer shaft) Shaft cover is still 35cm (14 inches) below the finished floor level of the bathroom
|
|
|
|
________L (Rainwater inlet)
.................|
.................| (This is where my architect wants to install a double backwater valve)
.................L (Bathroom)

I hope my question is clear enough.

Regards, Anbauer2019