Develop the property yourself

So I can forget all of that anyway because I am hardly allowed to do anything myself?

Yep - except digging holes - then you can save 150€ on 700€,... But it's annoying if the sewer (usually at the very bottom) is already in place - it was closed up and then electricity comes - then you need to know how deep you are allowed to dig,... If the lines are on the property, you still have to pay the connection fee. So it doesn't help you at the moment, except that you might have to pay twice.

Is it also not allowed to connect to the sewer yourself if it already runs past the property (to the neighbor) and there is a branch installed there?

I also need someone from Wednesday to dig 25 meters, 1.75 meters deep to the [Kanalanschluss].

Hello,
here is my experience. We were allowed to lay the sewer connection (so-called house connection shaft) ourselves from the street to the property (undeveloped). In this process, we also laid the water connection to the property. For this, an employee from the city came on short notice and officially approved it. We cut the asphalt of the street ourselves, broke out the asphalt layer with the excavator, dug up to the utility lines, buried a shaft made of concrete rings on our property (sewer), and properly closed everything again. When the sewer and water pipes had to be brought into the house, we dug up again and laid the pipes to the house. Electricity/phone/cable connections were only laid after the house was already standing. The responsible companies coordinated and did all that themselves. However, we did not need construction power but were allowed to connect via the neighbor.
Maybe this helps you.
Regards
MeisterBob

Are you not allowed to connect yourself to the sewer if it already runs past the property (to the neighbor) and there is a branch installed there?

No, you are not allowed! The municipality can dictate where you have to connect your wastewater / gray water. Maybe your neighbor paid for the sewer himself and doesn’t want you to connect there? Maybe the sewer is too small for multiple connections? Maybe the main sewer is already full and you have to connect to another line? Things like that are regulated by the waterworks / sewage treatment plant here. You are not allowed to do anything initially. And if you pay as much for the inspection as MeisterBob does for the entire connection, you might as well have it done properly.