Implement terrace without drainage in the best possible way

Hello everyone, so - the terrace issue has now become urgent and we have taken care of disposing of the slabs and the sand. Now we have noticed that there apparently is some drainage after all. The water (from the terrace and also from the garage roof) is directed into a small shaft located in front of the stairs to the garden. The shaft is covered with a grate. Inside this shaft there is a drain. We dug a little from there and found a pipe embedded in concrete, which runs towards the house and then down into the ground. After that, the trail is lost - unless we dig the ground meters deep now.

There are now two possibilities

a) there is a main pipe running under the house towards the street and the sewer. There is also a gutter / downspout for the drainage of the roof, which runs into the ground on the terrace. The suspicion now seems likely that the water is directed into a main pipe, right?

b) the water seeps into the ground. Somehow I can’t imagine that, since the basement walls, as mentioned, are relatively dry. If water seeped constantly in front of the basement wall, it should eventually push into the wall, I would guess.

What do you think? Could there have been such drainage with main pipes already in 1927? Does anyone know about houses from that time? If yes, how can I best get hold of plans? It was already quite an effort to obtain any plans and floor plans of the house at all. Drainage plans will surely be even harder to come by.

You could get someone to do a camera run through the pipe. Alternatively, you could introduce water with a garden hose and check at the transfer point if it arrives there. For dirty and rainwater, there will certainly be a corresponding shaft? With the garden hose, you could also experimentally determine with a flow measurement (available for 20-30 EUR) at which amount of water (per time) your drainage reaches its limits. Trial makes wise, right...

The camera is coming tomorrow and we will take a look at the pipe, I will report.

There is a shaft, at least for wastewater, in the laundry room. Here, the wastewater from the house was led via a downpipe and then under the basement floor through a concrete channel into an old, dilapidated soakaway shaft. This old soakaway shaft is the only handover point from house -> sewer known to me.

During the renovation, the old downpipe inside the house was then removed and the new downpipe was installed in a different place. From there, we laid a new main pipe under the basement floor, so the basement floor was broken up, KG pipe was laid in sand with a slope, and then concreted again. The new pipe also leads to a new inspection shaft, which we installed instead of the old soakaway pit and connected to the sewer. Long story short -> the old channel is decommissioned and any rainwater that was discharged here could theoretically no longer flow into the sewer.

Of course, there is the possibility that rainwater from the roof and terrace also flowed through the channel and ultimately through the old soakaway pit. Then a lot of water should now have accumulated in the old channel. I could verify this by a test drilling, if necessary.
I therefore hope that the roof and terrace were drained in another way, ideally via a separate main pipe - independent of the old, decommissioned channel.

Have you hired a company or are you doing it yourself? May I ask, how much does it cost?

This is done by a company here directly from the region.
Gross 160 € / hour. Duration for one line usually around 0.5 hours, if nothing needs to be milled or flushed.

Not cheap, but I want to have clarity here.

So, the optimal case has occurred. The drainage of the terrace is connected via the downpipe of the roof drainage to the main line, which runs under the house into the sewer. It was a matter of 30 minutes.

So I can simply lay the terrace with a slope towards the drain.