In some cases people have been known to add a tank preceding their water heater. This is sometimes termed a 'tempering tank'. In effect your adding storage capacity that is maintained in good condition by way of throughput of the water used in the house.

This additional tank can be either a water heater, sometimes these are available cheap from local lumbers as people upgrade to larger or more efficient units, or a simple uninsulated galvanized steel model commonly available through supply outlets catering to farmers and cattlemen. Simple galvanized water tanks are pretty cheap so a 150 to 400 gallon capacity might not be out of the question.

This additional tank capacity can feed the entire house or just a single tap. The more you feed with it the faster the contents are turned over. If it makes more sense to you you can feed a single tap and install a manual valve to isolate the tank. The contents of the tank could be either piped into or fed manually into whatever you wish. Every three months you could use the water to wash the cars or water the garden. Isolating the tank means more work but it also eliminates any chance of the water being contaminated even if the municipal water supply was polluted.