House connection sewage

Hello everyone, I hope I am in the right subforum?! I am planning to buy a property with two houses on it. The main house (residential house) is properly connected to the sewage system. The wastewater drainage works according to gravity behind the property, as there is a slope towards the back. From there, it is pumped up to the street by a lifting station. This lifting station is owned by the local water association, which also takes care of maintenance etc. The reason for this setup is to be seen historically.
Now, the problem is the second house. It is currently used as a retail store and has no sewage connection and therefore unfortunately no sanitary facilities. And in that sense, it is very difficult to use or rent it otherwise. There is only a garden water connection with a sink.
Now this is supposed to change and a complete wet room with sink, toilet, and shower is to be installed.
How is this best carried out? The house is located directly on the street. According to research, a direct connection to the sewer would only be possible through the water association and would accordingly be very expensive. So, could one simply connect to the existing sewage connection of the residential house?! Would that be feasible? If yes, where? Somewhere in the middle or in the house or at the inspection shaft? What costs can be expected and is a whole new watertightness test/approval necessary?
Thanks for the help.
Best regards
Pschy

I don't have anything on the topic, but I think the username is great ;) I immediately went to the bookshelf to see if it's spelled the same way.

Also had to smile and check on the phone if the spelling is correct (yes, it is now also available as an app). ☺

:) Thanks. Then just look up the [Steinlaus] right away! But is there anything to contribute on the subject?

Isn't there a possibility to connect one house to the heavy-duty drainage of the other?

I recommend a sketch! Top view and sensible section with height details and distance (roughly, accurate to 50 cm, hand sketch, scan/photograph, insert here).