Depends on what state you're in. Building techniques differed from the east, to south, to west, etc.... In that were in CA, I'd guess mid to late 1800s. Check the nails in the wood: square nails went out of vogue in the late 1890s. So if it's got square nails, it's probably post 1900. Of course bear in mind that there could be round nails AND square nails both in a structure, if later rennovations occured.