If we ever build a house, it will harvest rainwater. We installed an 800 gallon tank behind our current house and love it. We use it to top off our cow trough pond and to water the garden and sometimes the grass. Truth be told, 800 gallons is not enough. We want more.
But how much more could we get with a new house? To help answer that question, the Texas Water Development Board has a helpful manual with ways to calculate and size a tank for your home.
The first thing to figure out is roof catchment. We're considering building a house in the two-story 2,500 square feet vicinity with a detached garage of about 400 square feet. Halving the 2,500 (to account for the two stories) and discounting possible "orphaned roofing", we have about 1,500 square feet of catchment to work with. Austin gets, on average, about 32 inches of rainfall a year. Now, there's nothing average about Austin, and there's sure nothing average about the rainfall here as well. We've had dry months (no rainfall) and gullywashers (15 inches in 30 days). The average monthly amounts that make up that average annual number range from 1.72 inches in July to 4.33 inches in May. We're fortunate to have two rainy seasons: May-June and then September-October. If we were planning to rely on rainwater as a sole source of water, I would use the median rainfall values, which are lower than the averages (and more reflective of central tendency, but we won't go there...). However, average is good enough for government work.
Based on that roof size and that rainfall, we should be able to capture, on average, 30,000 gallons a year (inches of rain divided by 12 [to convert inches to feet] multiplied by the catchment area in square feet multiplied by 7.5 [the approximate number of gallons per cubic foot]). Divided by 12 months, that equates to a storage estimate of about 2,500 gallons. This is probably on the low side because it would be ideal to have a larger nest egg of water going into the high demand summers. If we were relying on rainwater for all of our water needs, we would need to go through the detailed worksheet in the rainwater manual mentioned above (and have much more storage). But we're not. We're just looking for lawn and garden watering (and perhaps an occasional car wash).
It would be super cool to tap into rainwater for toilets and other uses such as showers and clothes washing, but some issues-that-cost-money arise. One is the need for a backflow preventer on the city-supplied water line. Water providers start getting sweaty-brow nervous when you use other sources of water in addition to theirs. Their primary concern is having water of unknown quality sucked into their system during a pressure loss. If that happens and your water has little wigglers in it, you could make your neighbors very sick or even kill them (now don't be getting any ideas...). There are ways of dealing with this potential siphoning, such as having an air break where the two sources of water potentially mix (having an air break means that there is always air between the city source and your source and there is never the potential of your water coming in direct contact with the outlet from the city source). But there's a lot of trust that has to occur between a rainwater collector and the water provider to rely solely on an airbreak. Hence the backflow preventor: No trust required. If you have to install a backflow preventor, you have to pay for it (upwards of $1,000) and pay for once-every-two-years testing by the city (upwards of $200 a test). Bringing the water indoors would also probably require some treatment such as filtering and a dash of UV to kill the little nasties. If you plan to drip irrigate, you may need to filter the water first as well to keep any sediments from clogging your drip emitters.
So how much would 2,500 gallons of storage run us? We're really partial to those gorgeous Texas Metal Cisterns metal cisterns: Shiny, metallic, and somewhat clean-lined modern. A 2,500 gallon tank runs $1,650. Still need piping to hook it up (and perhaps a sump pump?), but that's not too bad in the grand scheme of things. An additional 500 gallon tank for, say, the garage would cost another $645. Austin gives a $500 rebate for installing a city-approved rainwater harvesting system. And the State of Texas doesn't charge state sales tax on tanks and accoutremount purchased to install a system.
Rainwater harvesting in the city has to be more of a love affair than a financial-based decision. Water is cheap. Just to pay off the 2,500 gallon tank with saved city water will take over 622,000 gallons, enough water to meet our current lawn watering needs for more than 30 years (note that the economics of rainwater harvesting in the boonies if used as a sole source in lieu of a well are far more appealing). Nonetheless, it's a "right thing to do" sorta thing, it's fun ("Look! The tank is filling!!!"), and plants and ponds prefer rainwater.
So do we.