Sir: For over 12 years we lived in a cottage without mains water. The original supply seeped into a hole cut in the rock (and turned red in the summer with daphnae). I constructed an underground tank and fed rainwater into it from the roofs, through a simple sand filter and it served us well, until we moved.
The cost of an underground plastic tank and pump would add only a tiny percentage extra to new houses. At its simplest, it would provide for watering the garden and car washing or, with the the addition of one of the readily available filters, provide for all purposes.
Our present house has mains water. It reeks of chlorine and is often brown in colour. A filter is essential for the kitchen supply. Such is progress.