Surface Water Assessment

The Surface Water Assessment is for stormwater services in unincorporated Pinellas County to address flooding, water quality and the operation of County-owned stormwater systems.

The assessment is based on each property’s impervious area, including all hard surfaces that don’t absorb water, such as parking lots and buildings. Impervious areas increase the amount of stormwater runoff, which is the cause of much of our flooding and pollution problems.

Your Truth in Millage (TRIM) notice contains the assessment on your property for the upcoming fiscal year (October 1 – September 30). Read the Frequently Asked Questions for an explanation of how the funding affects surface water management.

What is my assessment?

Look up your current surface water assessment.

This assessment does not apply to city residents, though 65% of the total Pinellas County population pay their municipality a fee for these services.

Adjustment and Mitigation Credit Policy

Pinellas County has developed an Adjustment and Mitigation Credit Policy that provides details on adjustments and credits. This manual is intended as a guide for customers through the application process. Applications are accepted year-round. 

Credits and adjustments will be prorated on the current year’s tax bill based on the date of approval. Request a copy of the policy by emailing

Surface Water Assessment Adjustments 

Adjustments are changes to correct your impervious square footage, either due to a change on the property, such as removing a deck, the presence of pervious pavement, or to a correction made by the Property Appraiser’s Office on the parcel’s square footage information. All parcels’ impervious area information is either obtained through the Property Appraiser’s Office or through measurements of digital aerial photography. 

Property owners who think there may be an error in their assessment should also use this process for corrections.

Apply for an adjustment:

Surface Water Assessment Mitigation Credit

Mitigation credits may be offered for properties that can demonstrate through engineering plans and calculations that they reduce the burden to the County system (such as ponds or other stormwater features). Stormwater management systems are required to be properly maintained and in compliance with site plans and permits. A Certified Professional Engineer Licensed in the State of Florida is required.

Apply for mitigation credit:

Please call (727) 464-8759 for assistance with applications. If you would like to request a copy of our Mitigation Credit Calculator and/or Cost-Benefit Analysis Calculators, email

Residential Assessments

The Surface Water Assessment is based on the estimated ground-level square footage or “footprint” of a house. This includes areas such as a garage or patio. It does not include additional stories.

  • Small home: $72.76 per year / 0.6 ERU / Range: 200- 1,575 sq. ft.
  • Medium Home: $121.27 per year / 1 ERU / Range: 1,576 – 4,367 sq. ft.
  • Large Home: $278.92 / 2.3 ERU / Range: 4,368 – 10,000 sq. ft.

A typical Pinellas County single-family detached home or individual mobile home will be assessed as 1 ERU (Equivalent Residential Unit). This is equivalent to 2,339 sq. ft., which is the median impervious area for an unincorporated, single-family detached home.

Other Properties

Extra-large properties (10,000 sq. ft.), condominiums, multi-family homes and commercial properties, which include businesses and churches, are calculated based on the total impervious area of the property, including buildings and parking lots. Impervious areas are hard surfaces that prevent or inhibit stormwater from being absorbed into the soil.

Commercial Property Assessments

infographic of commercial property examples for surface water assessment

Commercial property assessments are calculated based on the total impervious area of the property (by digitized aerial photography), including buildings and parking lots. Impervious areas are hard surfaces that prevent or inhibit stormwater from being absorbed into the soil.

Properties with onsite stormwater management facilities may be eligible for a reduction in their fee of up to 75%.

Assessment examples:

  • Gas Station: $958.03 per year / 7.9 ERU / 18,504 sq. ft.
  • Office Building: $2,073.72 per year / 17.1 ERU / 40,050 sq. ft.
  • Supermarket: $9,580.33 per year / 79 ERU / 184,858 sq. ft.

Why Do We Need This Assessment?

Unincorporated Pinellas County faces a number of challenges in maintaining a large, aging stormwater system. Failing pipes, flooding and poor water quality are just a few of the issues that the funding from a stormwater assessment can help resolve.

Failing Pipes

Unincorporated Pinellas County has 24 miles of metal stormwater pipes that are over 35 years old and are dangerously near the brink of failure. Collapse of any section of the stormwater drainage system can impact roadways, drinking water pipes, sanitary sewer pipes, electrical and communications cables, as well as private property.


Unincorporated Pinellas County maintains more than 323 miles of ditches and 390 miles of pipes. The County’s network of drainage ditches and canals is maintained on a 25-year cycle. Some of these waterways are so clogged up with debris and sediment that the flow of water is significantly reduced.

Poor Water Quality

Over 70% of the lakes, streams and bays in Pinellas County fall below water quality standards. The State of Florida and the EPA require that standards are met or Pinellas County could receive significant fines and penalties. Poor water quality also has a significant impact on our quality of life and our tourism-based economy.

Preventative Management Programs

With sustainable funding, Pinellas County can implement cost-effective preventative management programs to:

  • Repair the failing metal pipe network in 10 years.
  • Establish a 10-year maintenance cycle for the open drainage network to increase flood protection for neighborhoods and improve water quality downstream.
  • Avoid costly fines and penalties by implementing measures to reduce water pollution.

Additional Resources

Surface Water Assessment Master Utility Ordinance