Water demand garden/year --> Is a cistern worthwhile?

The OP is building an outdoor water tap anyway, so a system separator is required. Somehow the cistern must also be able to be filled. Sure, you could also run a hose from the house and leave it for hours. I don't know anyone who would consider something like that. Therefore, a classic outdoor water tap and this or a branch for the refilling are used.

ah then I probably misunderstood that.

I thought he already had a regular drinking water tap on the facade and wanted to draw rainwater from this tap at the same time. Filling the cistern exclusively with rainwater.

To avoid any subsequent installation effort, I suggested the closed rainwater circuit with submerged pump and additional rainwater outdoor tap column. If the cistern is empty, then just use the drinking water tap for irrigation and wait for the next rain. That was my thought.

So I am installing an outdoor water tap, which is currently supplied with drinking water.

The idea from therefore sounds very interesting to me, and I will now look into it.

You would need a cistern, a pump, and an outdoor fitting / water column. The pump requires power from the house (through an empty conduit) and the hose to the outlet point also needs an empty conduit. Is that correct so far? That sounds feasible to me. Thanks for the suggestion.

And how do you fill the cistern when it is empty? For some things, you prefer to use fresh water instead of water from the cistern. Planning without an external extraction point is saving in the wrong place.

I would keep the outdoor water outlet (faucet) on the house, it is already planned anyway, and if the cistern is empty I could connect the hose there and draw fresh water. Or am I mistaken? The cistern would of course have to be connected to the gutters via pipes so that it fills up completely.

exactly. However, you might consider burying a line directly from the tap to the cistern. It might cost you about 20€. Either switch at the tap or screw a 2-way distributor onto the tap. That way you can easily fill the cistern. Later, there will probably be a desire for automatic irrigation. Then it is easier if you always draw from the cistern. The water line from the tap to the cistern and from the cistern to your withdrawal point does not require an empty conduit. Just PE-HD pipe.