Here are some mentions of RR2 zoning:
1. General Plan/Zoning
County zoning: RR2 (2du/acre, 20,000 sq. ft. min. lot size)
2. The parcel is zoned RR2 - Rural Residential, which permits the development of a second dwelling at more than 30% but no more than 50% the size of the main dwelling unit subject to a Minor Use Permit, pursuant to Section 6156(x) of the Zoning Ordinance.
Apparently there are other permissible uses, such as group homes.
You can find info on how to learn more at this site: http://www.sdcounty.ca.gov/dplu/docs/444.pdf