Construction costs are currently skyrocketing

The roof and the two Aco gutters on the terrace run through a swirl filter into the cistern... That's the plan

Swirl filters help against medium and large dirt. In the water, there is very fine dirt. I would reconsider the plan.

Do you have that much rain at the ACO gutter? For me, it is purely an emergency gutter so that the house does not flood in case of heavy rain. If allowed, I would only direct it into the sewer.

Pretty sure is rich, but that's exactly the keyword I mean. I wouldn't have a peaceful retirement if I knew I had to move out of my home if I'm lucky enough to live to 98. :/

There are resources on this topic. I can recommend the article "Recognizing when bankruptcy threatens – is action needed during the withdrawal phase?" from "Finanzen? Erklärt!". If you realize that things are getting tight, you can take countermeasures and, for example, spend less money. Basically, you have the wrong risk perception here. It is not guaranteed that the statutory pension insurance (GRV) will still pay a pension you can live on in 30 years. It is not even guaranteed that the German state will still exist in its known form in 30 years. And these are not just mere thought experiments or conspiracy theories. You can already calculate now that the GRV will run into a massive deficit in the coming decades. Just because the GRV sends you a letter every year and makes a promise for the 2050s does not mean that this promise will also be kept.

Just so on the topic of chemicals in the cistern. It wouldn't be the first cistern to eventually go bad, the reasons for that are varied. You sometimes have to deal with pool chemicals or similar. Of course, you meticulously check the values before sealing it, obviously. So a cistern is neither destroyed by the red wine spilled on the terrace nor by that one drop of window cleaner that sneaked past the vacuum/mop.

I would just consider whether I even want this water from the ACO channel in the cistern. Even if you have absolutely ZERO protection from rain over the channel (which is a stupid idea anyway, because where rain gets, sun also gets, and the last thing I want in summer is an unprotected glass south-facing front), the yield amounts from this channel are quite manageable. If I had such a channel and were allowed to lead it into the sewer, I would do it immediately. We are not allowed to lead rule water into the sewer and I had already built a far-protruding balcony over the terrace doors, so I don't have a channel. No channel, no problem.

It's not about red wine or a drop of dish soap. But a whole bucket, that's a different story. I think the yield doesn't outweigh the risks.

There are resources on the topic. I can recommend the article "Recognizing when bankruptcy is imminent – is action needed during the withdrawal phase?" from "Finanzen? Erklärt!". If you realize that things are getting tight, you can take countermeasures and, for example, spend less money.

Basically, you have the wrong perception of risk here. It is not guaranteed that the GRV will still pay a pension in 30 years that you can live on. It is not even guaranteed that the German state will still exist in its known form in 30 years. And these are not just mere thought experiments or conspiracy theories. It is already possible to calculate that the GRV will run a massive deficit in the coming decades. Just because the GRV sends you a letter every year and makes you a promise for the 2050s does not mean that this promise will be kept.

I think we're talking past each other. I was not referring to our situation here, but still referring to the teacher from Australia.

Yes, but he argued that it would be the same here. Because the statutory pension is also not secure (despite Nobbi's promises). However, it is not quite as drastic here, as there are hardship regulations. So here you get (supplementary) social assistance, which also covers ongoing housing costs, if for example at 90 your saved-up money runs out and you receive no or too little statutory pension and you are not living in a 250m² villa, then you don't necessarily have to sell beforehand. Don't know if there are such regulations in Australia.