Cal. Civ. Code § 910-2-3-4

Sec. 4. (a) It shall be unlawful for any person to refuse to permit, at the expense of a handicapped person, reasonable modifications of existing premises, occupied or to be occupied by a handicapped person, if the proposed modifications may be necessary to afford the handicapped person full enjoyment of the premises of a dwelling. In the case of a rental, the landlord may, where it is reasonable to do so, condition permission for a modification on the renter agreeing to restore the interior of the premises to the condition that existed before the modification, reasonable wear and tear excepted. The landlord may not increase for handicapped persons any customarily required security deposit. However, where it is necessary in order to ensure with reasonable certainty that funds will be available to pay for restoration at the end of the tenancy, the landlord may negotiate as part of such a restoration agreement a provision requiring that the tenant pay into an interest bearing escrow account, over a reasonable period, a reasonable amount of money not to exceed the cost of restoration. The interest in any such account shall accrue to the benefit of the tenant.
(b) A landlord may condition permission for a modification on the renter providing a reasonable description of the proposed modifications as well as reasonable assurances that the work will be done in a workmanlike manner and that any required building permits will be obtained.
(c) The application of subsection (a) may be illustrated by the following examples:
(1) A tenant with a handicap asks his or her landlord for permission to install grab bars in the bathroom at his or her own expense. It is necessary to reinforce the walls with blocking between studs in order to affix the grab bars. It is unlawful for the landlord to refuse to permit the tenant, at the tenant's own expense, to make the modifications necessary to add the grab bars. However, the landlord may condition permission for the modification on the tenant agreeing to restore the bathroom to the condition that existed before the modification, reasonable wear and tear excepted. It would be reasonable for the landlord to require the tenant to remove the grab bars at the end of the tenancy. The landlord may also reasonably require that the wall to which the grab bars are to be attached be repaired and restored to its original condition, reasonable wear and tear excepted. However, it would be unreasonable for the landlord to require the tenant to remove the blocking, since the reinforced walls will not interfere in any way with the landlord's or the next tenant's use and enjoyment of the premises and may be needed by some future tenant.
(2) An applicant for rental housing has a child who uses a wheelchair. The bathroom door in the dwelling unit is too narrow to permit the wheelchair to pass. The applicant asks the landlord for permission to widen the doorway at the applicant's own expense. It is unlawful for the landlord to refuse to permit the applicant to make the modification. Further, the landlord may not, in usual circumstances, condition permission for the modification on the applicant paying for the doorway to be narrowed at the end of the lease because a wider doorway will not interfere with the landlord's or the next tenant's use and enjoyment of the premises.