Impervious cover is any type of surface that doesn’t absorb rainfall. Surfaces such as stone, rooftops, patios, driveways, sidewalks, roadways, parking lots, and some decks are considered impervious cover. Uncovered wooden decks count as 50% impervious cover.
City of Austin see below.
Impervious cover does not include sidewalks or paving in the public right-of-way or water features such as ponds, fountains, and water detention basins, porous pavement. Access ramps used for the disabled can be exempted if they meet the requirements of Section 25-8-63(C)(10) of the Land Development Code. You can meet with a reviewer to discuss the details if you believe you might be eligible.
Dependent on zoning designation, a certain percentage of impervious cover is permitted on a property. A reviewer can help you calculate your impervious cover, but if you would like to do it yourself, download this Building and Impervious Cover Calculation Aid spreadsheet.
If you have the features listed above, The City of Austin may be able to lower the impervious cover on your account, which will in turn lower your bill. Please call 512-494-9400 and ask about an administrative review of your drainage charge.
A more complete definition of impervious cover is found in Section 25-8-63 of the Austin City Code.