I have install a few cisterns on small island around my area because they don't have water. Cistern is a common thing in many rural area in the world. The water is fine to drink right after a rain fall, but it will quickly become non potable once bacterias grow due to fallen leaves and germs in the air.
Stored rain water can be use for non potable purposes such as gardening & flushing toilets, however to make it potable filters must be install and trees leaves must be keep out of the cistern.
Stage 1 -- multimedia filter (anthracite [crush coal], fine sand [quartz], coarse sand, gravel)
Stage 2 -- 2, 3, 4, 5, stage carbon filter or RO system.
Slide a UV light in between stage 1 & 2 if you still worry.
Capture water run off is illegal in many places because of the cross contamination fear, such as leaving a garden hose in the rain water barrel and open the hose bib a the other end. Because, there is a change of siphoning or back water contamination due to negative pressure in city water line (broken pipe, or pipe is being work on).
Local code must be check to see what type of check valve, hose bib vacuum breaker, double check valve, pressure vacuum breaker, or reduce pressure back flow assembly that you must install on water line that may come into contact with the cistern/storage water.
There are plastic storage tanks design for this purpose; 500, 1000, 1500, 2000, and 3000 gallons.
My grand parents farm house have a cistern of 20000 litters (5291 US gallon for the metric impaired), that would last 3 months for 4 people with strict water conservation if there were absolutely no rainfall.