Rainwater cistern yes or no and which material?

We are currently planning a single-family house. I am undecided regarding a concrete cistern. (Concrete because of durability). The plot is about 800 sqm in size. The cistern is intended to serve garden irrigation. (Although not much is to be planted in the garden). How large should the cistern be at a minimum? What costs should I expect? Is the purchase worthwhile? How is the water extraction done? Can the cistern be connected to the outdoor faucet?

Do you have to build a cistern? Isn't a well perhaps a (cheaper) alternative (provided the groundwater level allows it) - here in NRW many cisterns have completely run dry in recent weeks, so the garden had to be watered with expensive tap water.

No, we don’t have to build a cistern! How do I know if a well is allowed? What costs should I expect?

It is best to ask your municipality about a well. It doesn’t work everywhere either; there must be enough groundwater available. If you are building somewhere on a hillside with a nice view, the chances are poor; if you are in the valley near a flowing watercourse, the chances are rather good. But as I said, check with your municipality to what extent there are regulations or maximum extraction amounts. They might also refer you to your district authority, depending on responsibility.

But if you use a cistern for garden irrigation and you have to have a hole dug anyway, then better go one size up, especially with cisterns made of concrete rings.

You wrote that a cistern is not mandatory. Are you referring ONLY to the cistern?
For example, with us, a suitable rainwater infiltration measure is mandatory. A soakaway pit, Regule, pond, or also a cistern.
You might want to check again whether something of this kind is necessary. Because costs will come up for you anyway – from my point of view, choosing the cistern would make sense, as you use the rainwater and don’t have to pay for tap water + wastewater.
Edit: By the way, there are already threads in this forum on this topic where you can get more detailed information.

Does a cistern ever make sense if it is not required?

From a certain water consumption upwards, one can have the sewage costs for outdoor water waived here. Then the m³ costs only 2.xx€ instead of 6-7€. The neighbors used about 20-30m³ last year.
Now you can compare the purchase costs + interest costs against that.