All land use and building permit applications for new structures or additions to existing structures are required by county ordinance (the Land Use Element, Estero, South Bay Planning Area) to have drainage plan approval before the permit can be issued. The drainage plan must provide for the retention of storm water runoff on the same site as the building. This requirement applies to projects within the area shown on the attached map , unless the County Engineer determines that the building site will neither experience nor create drainage problems. Drainage plans must be prepared and will be processed as required by Sections 23.05.040 through 23.05.050 of the Coastal Zone Land Use Ordinance.

When reviewing drainage plans submitted by applicants, the Engineering Department will use the following guidelines for site drainage and for sizing and locating retention basins:

1. Drainage basin location. The minimum setback for drainage retention basins from a sewage effluent leaching pit or disposal field shall be a minimum of five feet horizontal.
2. Drainage basin capacity. For single family residences, duplexes and triplexes, the basin in cubic feet may be determined by dividing the total square footage of impervious surfacing on the lot by 7. For others, the basin shall be engineered.
3. Drainage basin design and construction specifications. Where required, drainage retention basins shall be designed and constructed in accordance with the following standards. 

• Drainage retaining facilities shall be physically "formed" structures such as: 1) leach trenchers, or (2) recessed planter areas with redwood side walls.
• Recessed planters and drainage basins shall not exceed one foot in depth, unless fenced.
• The required capacity may be provided by more than one basin, provided there is sufficient contributing area for each to function.
• Rock filled basins will be allowed 25% of the volume of an open pit.

4. Driveway runoff. Where it is impractical to direct driveway runoff to a basin, it may be allowed to leave the site provided an equal amount of runoff is retained from other surfaces.
5. Other drainage. Engineered plans may be required for any other drainage situation, such as when the site is in the path of historic flow from off-site.
6. Site grading. Final grading of lots shall be in conformance with Chapter 70 of the Uniform Building Code and Sections Code and Section 23.05.020 through 23.05.036 of the Coast Zone Land Use Ordinance.

Addition to existing buildings. With permits for additions to existing buildings, if the impervious area being added is more than 25% of the existing impervious area of the site, a basin shall be provided to accept runoff for the total impervious area on the site, including existing buildings, structures, and charade