Legal Question in Real Estate Law in California

There are two factors involved in your question - zoning and your lease.

Start with your lease. It almost certainly has something in it about subletting, occupancy, etc. That, and any arrangements/agreements you can make with your landlord will decide that part of it. Without either your landlord's consent or compliance with the lease or both, it would not be legal.

The second issue is whether the in-law is a legal unit. If it is, one wonders why your landlord would not be renting it out separately. In any case, you would have to find out if the in-law can be rented as a separate unit under whatever zoning and other planning and use codes apply to the parcel. You have to check with City Hall on that.