Garden irrigation with cistern

You can never have enough water for lawns and plants, if more is possible, then order bigger

Sure, more is always possible. Back then, I had about 5ccm basins concreted under the outside stairs and painted inside with sealing slurry. Of course, it depends on the roof area and whether you even catch enough water with it, and it depends on the individual use. I lived there for 30 years, 900 sqm of building land with meadow and some plants. It was never empty, and if it had ever been, I would have topped it up with tap water. The decisive factor is solely your usage, because I had it that way all those years, and for "normal" use it was never too little. In any case, I would keep it simple, otherwise it quickly becomes a savings bank.

We have to provide 2m3 as a retention capacity per 100sqm of sealed surface area from the municipality (I don't even know yet what counts towards that). Our property has 560sqm and the house about 95sqm of floor space + garage 24sqm.

As mentioned above, we still need to research or seek advice on this. The cistern prices themselves seem to be reasonable. approx. €2500 for 10m3. (I don't have an overview of earthworks yet). And domestic waterworks (which can also be installed in the garden, if I understand correctly) don't cost a fortune either.

By the way, we want to use the cistern purely for the garden.

Can you additionally connect the tap water line to these domestic waterworks?

We had a house water system from the hardware store back then, I think 80 euros. It only has to do a simple job. In winter, the water has to be drained from the device because of frost. I built a box around it and insulated it a bit because I didn’t want to always have to remember. and the suction pipe in. That is, of course, a price...... basically, you only need some kind of underground container, then an inlet and overflow. I wouldn’t invest too much money in it, because it doesn’t work better that way. With DIY work, you could concrete something yourself; it doesn’t have to be a masterpiece. When connected to the house water pipe it gets more complicated, which is why I would keep it separate.