Do you want fish? If so, build your pond at least 24 inches deep. That depth keeps the pond from freezing in winter or overheating in summer. In extreme northern areas, the minimum depth should be 3 feet.
Will you need a fence? In some areas, local codes mandate that yards with ponds deeper than 18 inches be surrounded by a fence with a locking gate to keep out unsupervised children.
How will it be refilled? Ponds must be topped off periodically to replace water lost through evaporation or splashing. You can do the job manually with a garden hose or have an auto-fill valve connected to a buried water line. When using city water, protect your fish by adding a dechlorinator directly to the pond.
What about the leftover dirt? Digging even a small pond will create a large pile of soil. A hired installer should get rid of it for you, but if you dig your own hole, use the soil to raise the grade around the pond or to build a waterfall.
Where's the power? A weatherproof GFCI outlet to power the pump should be located at least 10 feet from the pond. The electrical cable leading to that outlet needs to be buried at least 18 inches deep.
Rather than slope the sides of a pond right down to the bottom, make a shelf about 18 inches wide and 18 inches below the water's surface all around the pond's edge. This shelf serves as a platform for plants and a convenient step for anyone who falls in.
—Demi Fortuna, August Moon Designs, Stony Brook, N.Y.