Rainwater collection involves a storage tank whose input is from the gutters (typically with a device called a "roof washer" that eliminates the first bit of dirty water coming off the roof), and whose output is to a pump of some sort and and overflow line.  These systems are much more akin to the water system used on a farm or ranch than city water, although the result is still water under pressure.

The first question is how much water to store?  The idea is to store enough to make it through the entire dry season - about three months.  Toilet flushing is a constant, and we figured about five flushes per day per person (which is probably lower than the average) at 1.6 gallons per flush for three people or about  24 gallons.  If we put in dual-flush toilets (at .8 gal for most flushes), we could probably reduce this to about 16 gallons a day.  Using the bigger number and multiplying by ninety days gives 2160 gallons!  Yard watering is harder to judge, since it depends on how much we're willing to let the plants wilt (ignoring that for the first two summers after planting, even drought tolerant plants need water to guarantee survival).  If we had 3000 sq. ft. of lawn instead it would take 1" of water per week, or 3000 cu in or 20.8 cu ft. or about 166 gallons; times 12 weeks gives about 2000 gallons.  Combined the two are a little over 4000 gallons, so anything that size or bigger would meet our needs.  Substituting drought tolerant plants ought to reduce the water needed to at least half, if not to a quarter of that needed for a lawn, so even 3000 gallons should be adequate for us.  In many situations, the actual size is determined by how much space is available, budget and what products are locally available, especially on small city lots.

Water takes up a lot of space.  Every 1000 gallons takes up about 133 cubic feet (alternatively 100 cubic feet of water is about 748 gallons).  A typical 2'x3' bedroom closet (8' high) is only 48 cubic feet, but you often don't want something that high, so to get the small volume you need a bigger footprint. To get five thousand gallons, you need a tank about the size of a small bedroom.

Tanks come in two varieties: above ground or buried.  An above ground tank is strong enough to hold the water, but not strong enough to be buried.  Typical commercially available above ground tanks are made of either polyethylene plastic or steel, and are mostly round, which requires more ground space than a square tank of the same size (unless you can somehow make it so that the site of the tank isn't trying to fit a round peg in a square hole). On a small city lot, above ground tanks take up a lot of space and tend to stick out like a sore thumb.  The only other alternative for an above ground tank is to put it in the basement or crawl space under the house, provided that you can fit it in there.  In some ways this is better than in the yard, because it keep the tank out of the sun, and so reduces the potential for mold growth.  In other ways, having the tank below the house is a recipe for disaster due to the possibility of increased moisture creating a mold problem.

For buried tanks, you have three reasonable options: buy a plastic tank designed to be buried, use a concrete septic tank, and have a concrete tank custom made.  No matter what option is taken, one issue with a buried tank is that they take up a lot of room, so you will need a big hole, and so a buried tank is often best done when you're excavating anyhow.  If you use a plastic tank, you will typically have six inches or so (check this) of soil on top of the tank.  For a septic tank or custom made tank they can be just below the surface and have either soil or a patio on top.  The tank can be below the house also, but if it is, care should be taken to make sure the tank is well sealed to prevent evaporation causing a moisture problem in the house.  In particular, the access hatch must seal tightly in this case.

Unlike plastic or metal, concrete tanks leak, albeit slowly since they are not highly pervious, and so should be sealed with something to keep them water tight (otherwise you tank will have a slow leak all summer!).   There are various sealants that can be applied ("paint on") to the concrete, as well as various liner materials.  We looked heavily into pond liners, because they are quite cheap and durable (you can get a big one for about $300), but it was never clear how to attach a liner to a square concrete tank.

In all tanks you will need at least four holes (maybe five):

1. An access hatch that a person can easily get in to clean the tank.  This is typically about two feet in diameter.  The hatch doesn't need to seal tight unless the tank is under the house.  In all cases the hatch should be high enough above the ground to keep gunk from running into the tank.
2. A hole to fill the tank (big enough to hold the water from the hardest rain coming from all the gutters it is tied to.
3. A hole for the pipe to the pump.  The hole can be very high on the tank wall with the inlet pipe extending down to close to the bottom of the tank (since gunk collects at the bottom, its best to stay away from it).
4. An overflow hole for when it rains but the tank is full. If the overflow goes to the sewer, some kind of trap for sewer vent gases needs to be installed.
5. You will also need a vent.  If the access hatch is not sealed it would probably act as a vent, although plumbing code tends to want a vent with an opening of as many square inches as the drain.

Once the tank location and material is chosen, you will need a pump and a pressure tank, both of which are commonly used by anyone with a well.    The pressure tank hold a small amount of water (40 to 80 gallons would be typical) at a preset pressure (40-60PSI would be typical), so that the pipe coming out of the tank has water at pressure similar to city water. From that point on all standard water using devices will work as normal.

The two options we considered for location are either in the basement or buried under a patio somewhere.  The high cost of plastic tanks (about $1/gallon or more, with the best cost per gallon being tanks in the 1000 to 1750 range) and the fact that they are not commonly available locally made them less appealing than they would otherwise have been.  Septic tanks are cheap per gallon (about 60 cents), but are short a hole to pump out of and so it needs to be added at some additional cost.  The biggest septic tank we could find was 1750 gallons, so to get our desired storage, we'd need three of them.  While we considered a concrete tank of some sort in the basement, it wasn't clear what to make the cover out of and how to seal the tank tight.   In addition, the basement limited how tall the tank could be because you still need an access hatch on top of the tank (there might have been a way to make a hatch door in the floor above the tank, then the tank could have been taller, but that is probably a bad idea).   None of our considerations really mattered, because the building department didn't like the idea of the tank in the house at all, so rather than argue, we just decided to put the tank under a patio outside.  Since Seattle rarely has a deep freeze,  we shouldn't have any problems with ice. 

Depending on how much water your tank holds, and how much you use over the course of the summer, you may run out of water in your tank and need a backup, which of course is going to be city water, but this is where regulations make doing so difficult.

The city health department presents two hurdles to using rainwater.  First, they don't want any pipe carrying city water to come in contact with your rainwater, for fear of contamination, and second, any pipe in the house that carries non-potable water (of which rainwater is considered) must be clearly marked as so, as well as any device that uses it.  While we understand their concerns are real, it is not our experience that people drink out of toilets, and when you're fixing one, you don't tend to treat the water in the tank as being clean (even though it is filled with drinking water, its usually filled with rust, and other gunk, and is pretty unappealing).   But we won't argue with this, and so whenever we finally get approval, our toilets will all have "non-potable water" signs posted on them.  Hose bibs are another matter, and its probably best not to make non-potable hose bibs unless you can clearly make sure no one will drink from them.

The real difficulty with the city, isn't with labeling, but with avoiding mixing city water with rainwater, because even designing in check valves and reverse flow protectors, the city simply wouldn't allow it.  We expect to sit down with the various city departments in the first quarter of 2004 to come up with a scheme that meet their requirements and is still "homeowner friendly" enough to make the system usable.  The city of Portland, Oregon already has a rainwater tank code, but it is very stringent.  In general commercial buildings are also already allowed to have rainwater tanks, because they tend to have standard maintenance schedules, which appeases the health departments concern about failing check valves causing contamination of city water.

To see what we actually built, see the construction notes.