The stormwater assessment is calculated for City parcels based on the square footage of impervious area on the property. The impervious area includes, but is not limited to, building structures, pavement, driveways, sidewalks, etc. Each standard impervious unit of 500 square feet is assessed at a rate of $1.92 per month. The annual stormwater fee for the upcoming calendar year is billed on the tax roll as a non-ad valorem assessment.