Letting rainwater infiltrate - cost-effective option?

To store water, it would be worth greening the carport roof, permeable paving is also a good idea. Although with the soil, that actually doesn't help much. You would need to put a 6m clay layer underneath, then it will seep away - I would look for a tree that can cope with the soil, it will root through and loosen the soil, thus allowing better infiltration.

Cistern is not suitable for our individual case.

Yes, of course!

We installed a concrete soakaway shaft

Exactly that is the optimal cheap solution if there is otherwise no space. It also withstands the authorities: size is calculated, the earthworks contractor digs it in for you and connects the downpipes. It cost us 3-5000 back then.

What would speak against two rainwater barrels (patterned, so not the green ones), possibly equipped with a faucet, so that you can use the water with a hose for lawn irrigation from time to time after it has rained for a day?

They only collect locally, do not hold during a real rain event due to too little capacity. Also not recognized.

Stupid question, are you not allowed to discharge the rainwater into the sewer system?

Stupid question, aren’t you allowed to discharge the stormwater into the sewer system?

Why should you do that? You have to pay for that, don’t you? And besides, it unnecessarily burdens the wastewater treatment plants.

That will probably be forbidden like with us. We also have a [Sickerschacht]. One of the neighbors has nevertheless connected it to the sewer. Let's see how long that goes well.

If you have the option, it's better to pay a few euros (it's not much for us) and have peace of mind. It's not about normal rain showers, it's about heavy rain, preferably when the ground can't absorb any water.

you are not allowed to? In our new development area that's currently being built, retention space must be provided, my 5 cubic meters. The rest can be drained. This is because the sewer system downhill can't handle the rain. There, it feels like the manhole covers have always been pushed up during heavy rain.