Here is an email I just sent to a friend about rain tank info in SD, Its pretty informative so I thought I'd share...
San Diego offers a rebate for rain tanks 50 gallons (or more). They will pay up to $75 a tank, up to 4 tanks per household. This means that you can harvest 200 gallons min. of rain for free! A 60 gallon rain tank is about the size of a wine barrel to give you an idea. Tanks come in all different shapes and sizes. You can get something recycled (like an old olive barrel), something new (mostly plastic for durability), some are food-grade plastic so you can use the water for cooking, some are metal, some narrow and tall, others big and short, etc. You can have a tank that gravity feeds to a hose (run hose downhill to hand water), or dig it into the ground and use a sump pump. You can even hook it up to an irrigation system if you get a large enough tank.
Every 1" of rain yields 600 gallons per every 1000 sf of roof space. San Diego averages about 10" or rain per year. So if you have a 1000 sf home (roof space) then you could potentially gather 6000 gallons per year.
Just to give you an idea on price. A 1000 gal tank is around $700 delivered (not installed) and is about 6' diameter and 5' tall. Its pretty big. You can cover them with edible vines or make them look cool, but thats where a little design comes in.
I hope this helps. I can also hook you up with a amazing installation crew and even help design your garden around rainwater if you want. There are also ways to store water in the soil using mulch basins, etc.
Talk to you soon!