Colorado Revised Statutes Title 37. Water and Irrigation § 37-96.5-103. Small-capacity rooftop precipitation collection permitted

(a) No more than two rain barrels with a combined storage capacity of one hundred ten gallons or less are utilized;

(b) Precipitation is collected from the rooftop of a building that is used primarily as a single-family residence or a multi-family residence with four or fewer units;

(c) The collected precipitation is used for outdoor purposes including irrigation of lawns and gardens; and

(d) The collected precipitation is used on the residential property on which the precipitation is collected.