Not sure if your property lies within the City limits of Modesto or outside the City limits and therefore under the jurisdiction of Stanislaus County.
If within the City, the R-1 district allows for second homes on parcels under certain circumstances, and those rights and conditions are defined in the City's municipal code at Section 10.3.217.
Sounds like you may outside the City limits in Stanislaud County, but the County has zoning designations of R-A ('rural residential) and R-1 (single family residential). If you are RA, then the permitted uses are covered under Section 21.24.020 of the County's Code. That section gives you right to place on dwelling unit, but also gives the right, if certain conditions are met, to have second dwelling unit.
Both zoning codes are available on-line for review (see link below to County Code), and you should review the zoning code and the enumerated conditions to see if you qualify, or you can contact an attorney to discuss your options.
Hope this was helpful to get you started.
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Generally, R-1 zoning means 1 unit per acre development. However, when you review the code provided above look for a couple of options that might work 1) see if they allow accessory dwelling units (these are generally smaller "mother in-law" units allowed as an accessory use to a single family home - you would have to decide who would get the larger unit in this case), or 2) if condominimizing or similar multi-family structures are allowed (this might mean you don't get two separate structures, but instead have space for a couple of families with separate entances and amenities in a single structure.