This "technology" has existed for 30 years, but it has never become widely adopted. This is because it has not been financially attractive so far (only then do such technologies unfortunately become established). Anyone who builds a cistern just for that reason should think carefully, as it costs a relatively large amount of money (from €2000 upwards). Anyone who wants to build one anyway - which I would always do for a garden - simply takes a slightly larger one, which does not cost much more and only needs filters and a pump, as well as connections inside the house. You just have to keep in mind that the inlet filters take up space and thus the cistern sits deeper, meaning it becomes more expensive.
Every cistern needs an overflow, so if it is not connected to the sewage network, a soakaway pit, called a "Rigole," is required. This also costs money (a large, deep hole filled with gravel) and must be approved. It also requires permeable soil and usually needs a throttle inside the cistern (therefore the cistern must be even larger). All in all, it is also an expensive undertaking. Otherwise, you cannot avoid the rainwater fee, but you can "only" save on drinking water fees.
Those who connect to the toilet and possibly the washing machine must accept a few disadvantages:
- Investment costs
- Filter cleaning (must be done regularly, otherwise the cistern will be empty or the pump broken)
- Water is not free of microorganisms, which can cause deposits and/or odor
- Cistern cleaning (otherwise even the best filters do not help)
- Older cisterns often leaked, resulting in less water storage being available and not enough for household needs. Today's monolithic (i.e., one-piece) ones have this problem less often