Implement terrace without drainage in the best possible way

Hello,

we want to renovate our approximately 20 sqm terrace. The old terrace slabs have already been removed – underneath there is a thick layer of sand (about 7-8 cm), below that gravel and then the soil, with no drainage or anything similar.
As can be seen in the picture, the terrace of our semi-detached house lies between the house and the garden on one side. On the side, towards the neighboring house, also just barely visible in the picture, is directly the flat roof of our garage.
Everything is built very, very tight here, which complicates the situation. There is practically no possibility to install drainage and lead it past the house towards the sewer/street.
What surprises me, however: Our basement walls, which lie in the soil on the terrace side, are dry. So there seems to be some kind of sealing, but it must be at least 50 years old.

As an absolute layman in the field, I now have several questions and hope you can roughly help me.

[*]Is the above-mentioned structure of the terrace subsoil still up to date? Or should I better excavate and replace the material / supplement it with other material?
[*]Would you take the opportunity, when making a mess in the garden anyway, to reseal the basement wall from the outside? Or should I rather not touch the sealing, given the dry walls?
[*]Would it be sufficient for drainage (also considering increasing heavy rainfall etc.) to lay the terrace with a slope away from the house and install a channel with infiltration into the ground towards the garden side? I am skeptical about that.
[*]Are there any other drainage options you know of?

I hope the explanation is not too complicated. I am of course happy to provide further information if needed.

Thanks in advance!

- Today, mostly permeable pavement is used, which is then laid on gravel 2/5 for better infiltration and jointed with gravel 1/3. Laying it on sand is basically also possible, but much more water flows off on the surface.
- What does the current slope look like? And where does this water infiltrate? Where does the water flow off when there is significantly more during heavy rain than can infiltrate? Over the garage?
- I can't say anything about the basement waterproofing.
- What is shown on the right in the picture? Is there a slope? If so, does it go up or down? A site plan with a rough elevation profile would be very helpful.

- Nowadays, permeable paving is usually used, which is then laid on 2/5 gravel for better infiltration and grouted with 1/3 gravel. Laying it on sand is basically also possible, but then a lot more water flows off on the surface
- What does the current slope look like? And where does this water infiltrate? Where does the water drain off if significantly more comes during heavy rain than can infiltrate? Over the garage?
- I can't say anything about the basement waterproofing.
- What is on the right in the picture? Is there a slope? If yes, does it go up or down? A site plan with a rough elevation profile would be quite helpful

Thank you for your answer!

I currently do not have a site plan with an elevation profile. But yes, on the right, beyond the wall, is our garden. There is quite a steep slope there, I would estimate about 10%. It is all lawn and flower beds, and it should basically remain that way. Nothing additional should be sealed, so I assume that water infiltrates here without problems, even during heavy rain (it has always worked like that in the past).

Where does the water currently infiltrate? That is a very, very good question. I cannot fully explain it myself.
Next to our patio door is the downpipe from the gutter. It just goes into the ground. I cannot imagine that it is connected to the sewer or anything like that (not even indirectly).

The rest of the water, i.e. rain, seems to have simply always infiltrated into the ground. I can only speculate about this because I have not dug yet, but I am about 95% sure that there is no drainage installed there. According to the previous owner, there have never been any water problems. Also not during heavy rain - no flooded basement, no flooded kitchen, no standing water on the terrace.

There is no drainage over the garage. The garage roof is slightly higher than the terrace level, so drainage via this route is not possible.

The question now is how to implement it optimally if I start digging anyway. And that without possible drainage around the house to the street and thus to the sewer. I basically just want to implement the best possible drainage. I am aware that it will never be optimal. The thought that comes to my mind is possibly working with a soakaway pit.

There is quite a slope, I would estimate about 10%.

In which direction? That is, downward from the terrace or upward?

In which direction? So from the terrace downwards or upwards?

Upwards - so starting from the terrace, ascending.

Then you either have very permeable soil if this has gone so well over the years, or there is a drainage system underneath.