Building drainage drainage plan

Tomorrow, so it is a quite standard city villa with an uninhabited attic with a hipped roof. Above the utility room is the bathroom with a tub, a shower, and a toilet. Below that in the utility room is a washing machine and the condensate drain of the boiler. Guest WC and kitchen as seen in the pictures. I would like to know if the over-roof ventilation on the downpipe is sufficient so that everything runs through one pipe as I have drawn it. Regards

Above the utility room is the bathroom with a bathtub, a shower, and a toilet. Below that, in the utility room, a washing machine and the condensate drain of the boiler. Guest toilet and kitchen as shown in the pictures.

Okay, you had your chance. I now wish you a happy Pentecost and am off.

Your civil engineer wants to choose the safe option and your installer the somewhat simpler one. As a layperson, I see the advantage in two independent lines, as the second line is routed separately away from the house. If one gets clogged, you don't have to go to the neighbor to shower. Unless the clog is behind the junction of the pipes. Both will work IMHO.