The water is relatvely clean, and can be used in the house (supply water to the toilet and/or laundry), wash the car, and water the garden without treatment.
It is vitally important, however, to properly plan your rainwater harvesting:
How often does it rain, when does it rain, and how much rain do you receive?
If you live in San Diego, or San Franciso, you may regularly go without rain for 6 - 9 months at a time.
Even in a location such as Melbourne, Australia, where rain is regular for about 8 months of the year, the 4 dry months are in summer, when water is needed in the garden. Over a million raintanks have been installed in this one city. Most of them only hold 500 - 1,000 gallons of water, and most were installed by homeowners who had little concept of how much water is required to irrigate a garden. A tank in Melbourne needs to be at least 10,000 gallons to guarantee water for the garden.
Water tanks can take a LOT of room. If you were to rely on rainwater for your garden in San Diego, 9 months worth of garden water would be around 27,000 gallons. Slimline tanks are space effective, but most only hold 500 gallons. How will you make 500 gallons last over 9 months?
Over options such as underground tanks, and bladders that fit under decks can be used, although at significantly higher cost.
The most important decsion is making sure the size of the water storage is enough for your planned need.
If you are storing water to wash the car, 500 gallons can last for many months.
If you are planning to use the rainwater on the garden, 1,000 sqft of garden space will need around 2,500 gallons per month over summer (in San Diego). Therefore that 500 gallon tank will last about 1 week.
The best time to your rainwater is during your wet season. Having it sit on your lot, full while it is raining does not save any more water!
It doesnt make sense to water your garden heavily during your wet season, or wash the car. The only place people tend consumer water during the wet season is inside the house - personal consumption, showers / baths, replenishing the toilet, and in the laundry.
Most jurisdictions will not allow personal consumption of untreated harvested rainwater, nor allow it to be used for showers / bathroom handbasins.
Washing machines and the toilet can be supplied with harvested rainwater, and they are prolific water users (second only to showers / baths).
Carefully set a budget and determine what your return on investment will be:
Tanks and Cisterns can be expensive by the time site preparation and installation costs are included.
2 x Slimline rain tanks, plus site levelling, concrete bed preparation, installation, downspout connection, laundry / toilet connection (dual rainwater and potable water supply) can easily cost in excess of $6,000.
If you area only receives rainfall for 6 months of the year, the total amount of water saved is probably going to be around 2,700 gallons per person, per year.
A graywater system, in the same climate, is probably going to save around 9,000 gallons per person, per year, yet at only a third of the cost of the 2 slimline tanks.
Room to spare:
If you have room (at least 1/2 acre) large tanks will not occupy too much valuable land. Large round tanks are much cheaper (gallong for gallon price) to purchase and install than small plastic slimline tanks. In this case, large tanks can be used to irrigate the garden (still best to use dripperline irrigation to make the most of the water).
Regular rainfall throughout the year:
If you receive regular rainfall, then the tank does not need to be very large, significnatly reducing supply and installation costs.
This is an ideal scenario, and assumes rainfall for at least 6 months of the year.
Install a slimline tank, about 500 gallons, and reserve the water for use only in the laundry and the toilet. It will save water in those locations for about 7 months of the year.
Install a graywater pumping and irrigation system for the garden. It will save water normally used on the garden for about 8 months of the year.
If the amount of graywater is not quite enough for the size of the garden, consider connecting the raintank overflow (with a surge protector) to the graywater system so that excess water can build up in the garden during spring rains, allowng the graywater system to have a head start over summer.
By redesigning your garden, and external hardscape area, much rainfall can be reained on the property. While this will not help plants to be watered over a dry summer, it will reduce the anoubt of water flowing into the stormwater system.
Using gravels (or similar) instead of asphalt will allow rainwater to enter the soil where it falls.
Swayles (mini basins) collect water temporarliy, and allow it to sink into the soil over a few days. While this does not help over a dry period, swayles will promote growth during a wet season. Plant choice is critical - plants must be able to adapt to lengthy wet and dry spells.