Separate water meter for garden to save on wastewater charges?

For me, it went through the [Gemeinde].

Do I have to plan the meter with the plumber right away or can I do it myself?

Ask at the municipality, they should know. Our neighbors were able to retrofit it (have it done).

Does it mean if I don't have an extra meter, I pay wastewater charges for the water I use in the garden?

Yes, that's how it is with us or will be. Wastewater costs 4€/cbm; what goes through the garden meter and seeps into the garden doesn't go into the sewer and is therefore deducted.

Hm I'm thinking....I'll probably only need it when the cistern is empty, because I use it to water my garden I guess.....and for the little amount I then take from the drinking water supply......

You have to consider whether it is worth it. If the cistern is properly sized, you can get pretty far with it. However, if the lawn is to be regularly watered, then that's a different matter. And I wouldn't want to fill a pool from the cistern now.

And I didn't want to fill a pool from the cistern now.

But you won't fill the pool via the garden meter either, right? At least in our bylaws that is explicitly excluded, as is cleaning vehicles in the parking spaces. That makes sense too, since the water doesn't infiltrate but goes into the sewer system... Is it different for you, or do you live by the motto "Dare to leave gaps" or "What the municipality doesn't know, ..." ;-)

So for us, this is not an issue. We currently have more of a paddling pool than a pool, but it is drained onto the lawn. Our neighbor has a large pool (without separate meters) and calls the municipality before filling it to inform them that he is filling his xx m³ pool, and then the amount is recorded as a deduction.