Combining rainwater and soil downpipes.

[Brix]

I understand why rainwater and soil downpipes cannot be combined in modern buildings with separate water waste collection systems. But why must they be kept separate in older properties with a combined sewer. In this case, two 4" downpipes - one soil one rain -  run parallel to each other down the wall until they each empty into the same manhole about 12" from where they reach the ground. Everything is channeled from the manhole though another 4" pipe under the house, picking up from other outlets along the way, so capacity of the pipes surely can't be the issue?

Edited July 20, 2021 by Brix

[Mr Punter]

I understood this was because there may be future new stormwater sewers and you would have the opportunity of connecting to this separately.

[Brix]

I wondered if that were the thinking. Compared to separating out the drains under a terraced city house (so complex that it seem unlikely to happen in the property's lifetime) separating a couple of downpipes would be simple.

Edited July 20, 2021 by Brix