If you work in:
AK, AZ, CA, CO, GU, HI, ID, IL, IN, IA, KS, MI, MN, MO, MP, MT, NE, NV, ND, OH, OR, SD, UT, WA, WI, or WY
and your H1 is sponsored by a private company, your should file the h1 (togetherwith h4) extension to:
H-1B Extensions:
USCIS
California Service Center
ATTN: H-1B Extensions
P.O. Box 10129
Laguna Niguel, CA 92607-1012
See: