I need to store about 20,000 litres of water for our off-grid house in Spain (to passivhaus standards). And since we're planning on building a basement anyway, I was wondering whether it will be a good idea to make use of it to store the water by basically walling off an area and creating an underground cistern. Now before you shout "Interseasonal store" , the realities of our water supply and demand are that we'll probably be using 300 litres/day and will be refilling those 300 litres daily from our 110m deep borehole. So in winter, we will likely be cooling the basement by adding water. The basement walls will be reinforced concrete (whether I include the cistern or not), and will have exterior insulation applied. It won't be considered a livable space and will mainly be used for storage/utilities. Insulation between basement and ground floor will be through panels located under the basement ceiling.
On the plus side having such a large body of water under the living room during the hot summers would help with cooling.
An alternative to having the cistern in the basement is to build it next to the house, but also underground which would be more costly.
Two Euro cents for your thoughts

Posted By: chuckeyAs you are saying that you can pump 300 litres/day in the winter and you have a buffer of 20,000 ~ 60 days, when do you need to actually use the buffer?

Potentially in summer, the performance of the borehole in summer is unknown at this point, as the house is off-grid and rural, I'd like a good water backup.

"Just walling off a section", I think that you will need to use reinforced concrete or hollow blocks with steel rods sunk into the concrete oversite to ensure the "wall" will not buckle with the water pressure. Of course it will need to have a waterproof coating and a drain cock.
I am not sure that I would want to drink water that has been sitting around in a cistern for two months without any further chemical treatment.
Frank

Yes, reinforced concrete all the way. Water will be treated. I was more concerned about the thermal aspects of having such a large body of water inside the building.