The objective of a rain garden is to capture the first 1/2 inches of runoff from a rooftop or driveway. A rain garden should be about a fifth of the size of the rooftop or paved area draining to it and be deep enough to pool three inch of water. For instance, if you have 500 square feet of rooftop draining to a downspout, the garden should be 100 square feet (10 by 10) and 3 inches deep.
Rain gardens are intended to infiltrate rain water to the ground, not into your basement. Gardens should be located downhill and at least 15 feet away from your foundation (and your neighbor's)!
Rain gardens are designed to overflow during heavy rains. Make sure it will spill over to a safe distance from sidewalks, property lines or retaining walls.
The water in your garden should infiltrate into the soil within 24 hours of a rain. To test your soil, dig a hole 18 inches deep and fill it with water. Let the hole drain once, and then refill it. As the hole drains for the 2nd time, time how long it takes. If it takes more than 24 hours, hire a landscape contractor.
Chronically wet locations are not well suited for rain gardens since they don't infiltrate well. Native wetland plants such as ferns, iris, milkweed, and blazing star, will help absorb water in these locations.
Topsoil and compost, if needed. Use your judgment like you would for any perennial garden. Good soil and compost make all plants happy. Always mulch to reduce weeds and evaporation during dry weather.
If you direct downspout water to the garden via pipe, have it surface above the three inch pool so water and decaying material don't clog the pipe. This may mean digging your garden deeper. However, you may have enough slope to have it enter the rain garden on the uphill side, yet still be above the 3 inch pool.
Expect to pay $2 per square foot if you do it all yourself, $5 to $8 if you hire a landscaper to dig, $15 if you hire out the design and construction. Prices vary widely.
Expect to water regularly for the first season to reduce mortality and speed root development. After the first year, native plants need only be watered during very dry weather. Weeding is also most critical in the first season or two. After that, weed should compete poorly and will likely appear at the edges.