During review of your proposal, the lead agency may identify possible adverse environmental impacts. If so, you and the agency can work together to identify ways to reduce the impacts, either through changes to the proposal or identification of mitigation measures. Mitigation measures are usually conditions placed on the permit or approval.
Mitigation is defined as (WAC197-11-768):
Repairing or restoring
Reducing or eliminating over time
Replacing, enhancing, or providing substitute resources
Monitoring the impact and taking appropriate corrective measures
Mitigation may involve almost anything, such as paying impact fees to local school districts, or changing the design of the project to avoid impacts to wetlands or other sensitive areas. Some mitigation may be required by city or county development regulations, or other local, state, or federal laws. Mitigation can also be based on information on adverse environmental impacts in the SEPA document.