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What is the purpose of a master drainage plan?

A master drainage plan (MDP) is intended to prevent significant adverse impacts to the natural and human-made drainage system, both on- and off-site. A MDP is a comprehensive drainage control plan usually prepared during the Large Site Drainage Review process.

What activities require a master drainage plan?

A master drainage plan is required if:

  • The project is a designated Urban Planned Development (UPD) on the King County Comprehensive Plan Land Use Map, OR
  • The project, in full build-out, results in 50 acres or more of new impervious surface within a single sub-basin or multiple sub-basins that are hydraulically connected across sub-basin boundaries, OR
  • The project is on a site of 50 acres or more (including opens space, critical areas, and growth reserve) within the recharge area of a sole-source aquifer as designated by the EPA and depicted as such on the "Areas Highly Susceptible to Groundwater Contamination Map" adopted as part of the King County Comprehensive Plan.

A MDP typically involves detailed sub-basin specific hydrologic data collection and modeling, however, a limited scope or special study may be appropriate for particular projects.

How much will it cost?

See the Permitting Department's current fee schedule for an estimate.

Where can I get application materials?

  • Refer to the Master Drainage Plans Packet
  • Request an application packet by mail, by calling the Permitting Department customer service at 206-296-6600 or
  • Pick up an application packet at the Permitting Department in Snoqualmie (see map and directions).

How do I submit my application?

See information on pre-applications as a pre-application meeting is required.

Notes / Comments

For more information, contact the Permitting Department at 206-296-6600.

Legal authority

Chapter 9.04* of the King County Code

Administered by

The Department of Permitting and Environmental Review. For questions, please contact the customer service at 206-296-6600.