There aren't any known guidelines specifically for letterboxing, but they have developed guidelines for geocaching which would likely be applied to letterboxes as well. Most of it is the regular common sense stuff, but read the full details at Geocaching in State Park.

The Midpeninsula Regional Open Space District has Geocaching guidelines that letterboxers are expected to follow. It's unclear how we are supposed to list letterboxes on the geocaching.com website like the rules require, however. If you plan to plant a letterbox within this area, it would be a good idea to talk with someone in authority about your plant first. This area includes the following parks:

• Bear Creek Redwoods
• Coal Creek
• El Corte de Madera Creek
• El Sereno Foothills
• Fremont Older
• La Honda Creek
• Long Ridge
• Los Trancos
• Miramontes Ridge
• Monte Bello
• Picchetti Ranch
• Pulgas Ridge
• Purisima Creek Redwoods
• Rancho San Antonio
• Ravenswood
• Russian Ridge
• St. Joseph's Hill
• Saratoga Gap
• Sierra Azul
• Skyline Ridge
• Stevens Creek Shoreline Nature
• Teague Hill
• Thornewood
• Tunitas Creek
• Windy Hill