First off, even if you have a tough drainage problem, use a dry well only as a last resort. Ideally the ground around your home should slope between 1/2 and 1 in. per foot away from the foundation for at least the first 6 ft. If you don't have this much slope, the standard 3-ft. extensions for downspouts probably won't do much good. Water from a severe storm can really add up fast—a 1,000-sq.-ft. roof will shed about 620 gallons of water during a 1-in. rain, or about 103 gallons per downspout if you have six downspouts. That's a lot of water dumped right next to the basement.
First, try longer (6-ft.) downspout extensions. Make them removable so you can move them when you mow your lawn.
The next best solution is to install an underground drain using plastic drainage pipe. Basically, it's the drainage pipe portion of the illustration that runs at a downward slope until it reaches daylight, rather than the dry well. The only catch with this system on a flat lot is whether you actually have a low area where the drainage pipe can come out. And, of course, it takes a lot of digging!
As a last resort, consider running the underground pipe to a dry well that's at least 20 ft. from the foundation. This will hold much of the water until it can naturally seep into the surrounding soil.
Keep in mind the limitations of a dry well. You probably can't size it large enough to hold more than a portion of a heavy rainfall. If your subsoil is heavy clay, drainage will be slow. And once the dry well fills, the water will fill up the drainage trench back toward the house and potentially cause more leakage. An overflow tube added to the dry well (not shown) or a slotted grate on the catch basin will help relieve this problem.
To begin, find the downspout nearest the spot where water enters your basement and install a 6-ft.downspout extension and elbow. Center a small catch basin at that spot. Then mark the dry well location and drainage pipe path and dig the trench and dry well hole. Make the trench about 12 in. wide to contain the 4-in. pipe plus 3 to 4 in. of surrounding crushed rock (1-1/2 to 2 in. diameter). Slope the trench downhill at least 1-1/2 in. every 10 ft. Make the dry well hole about 8 in. wider in diameter than the plastic well to allow space for at least 3 to 4 in. of crushed rock.
We purchased a sump liner with heavy-duty lid for our dry well. Before installing it, remove the cutout for the 4-in. tubing and drill 1-in. holes every 8 to 12 in. so water will escape and percolate into the surrounding soil profile. Wrap landscape fabric completely around the sump liner and pull a fabric sock over the tubing to keep out silt (both products are available at home centers).
Finally, install all components and backfill with rock. Close the holes and trench with dirt. Don't forget to use screens on your gutters and a screen inside the catch basin to keep leaves and debris out of the tubing and dry well.