The City of Austin will calculate your drainage based on the following formula:
Monthly Drainage Charge = Monthly Base Rate x Impervious Cover x Adjustment Factor
Monthly Base Rate
The monthly base rate for Fiscal Year 2016 to 2017 is $0.00498 per square foot of impervious cover. This represents a slight decrease from the previous fiscal year.
The impervious cover for each property includes surfaces like rooftops, driveways, parking lots, walkways, patios, unpaved driveways and unpaved parking. The City has measured the impervious cover for each property through its aerial data and mapping software, supplemented with zoning and development records.
The adjustment factor is unique to each property and is based on the percent of impervious cover. It is calculated with the following formula:
Adjustment Factor = (1.5425 x Percent of Impervious Cover) + 0.1933