| King County | ||
| Zone 1 | Zone 3 | Zone 5 |
| Bellevue | Renton Regional Fire Authority | City of Seattle |
| Redmond | North Highline Fire District - Merged with Burien | |
| Kirkland | Skyway | |
| Woodinville Fire and Rescue | Burien (KCFD#2) | |
| Bothell | Valley Regional Fire Authority | |
| Northshore | Tukwila | |
| Shoreline | South King Fire & Rescue | |
| Eastside Fire & Rescue | ||
| Mercer Island | Maple Valley - Merged with Puget Sound RFA | |
| Mountain View Fire/Rescue | ||
| Black Diamond | ||
The fire departments of King County are in the process of implementing a county wide numbering system. For many years, mutual aid agreements have been in place for one department to provide assistance to another. King County is divided into three Mutual Aid zones. Those zones are identified in the table above. Each zone has its own dispatching organization. NorCom dispatches for Zone 1. Zone 3 is dispatched by ValleyCom. Seattle Fire has its own Fire Alarm Center.
For years, when a unit from one zone enters another to provide mutual aid, a cross-zone identifier is used. For example, if Ladder 11 from Seattle responds into Zone 3, they will place the cross-zone identifier 50 in front of their unit number, becoming Ladder 50-11. Implementing the new numbering system, the zone number will preceed the current unit number. For example, Renton's Engine 11 will become Engine 311. Seattle numbers won't change.
As of December 2019, it appears that all of King County has migrated to the new number system. Zone 1 was complete in early 2019, while Zone 3 switched in the fourth quarter of 2019.