If the clutter on your property says one unit isn't enough, but your wallet disagrees, supplement your main unit with one of the less expensive, smaller models. There are three main types of such units, all of which go by descriptive names. "Corner sheds" are made to fit precisely into a corner and run about 3 feet tall by 3 feet wide by 2 feet deep. Meanwhile, either a "vertical shed" (5 x 3 x 4) or a "horizontal shed" (3 x 5 x 4) may be just the handy outdoor storage building to fill your need for additional space.
Become acquainted with zoning ordinances, deed restrictions and building codes. Depending on your area, you may need a building permit. Building codes may determine how far storage units must set back from property lines and what materials are acceptable for siding.