Can a California landlord introduce a new fee mid-tenancy?

Generally no, on a fixed-term lease. The lease locks the rent and any specified fees for the term; introducing a new fee (parking, pet, trash, key replacement) without tenant agreement is a unilateral lease modification, not enforceable. On a month-to-month tenancy in California, the landlord can add a new fee with the same advance notice required for a rent increase (California requires 30 days notice for rent increases of 10% or less and 90 days notice for increases above 10%. AB 1482 caps annual increases at the lower of 5% + local CPI or 10% flat, with exemptions for natural-person single-family owners and buildings under 15 years old.). Once the notice period passes, the tenant either accepts the new fee by continuing to pay or terminates the tenancy. Some fees are statutorily controlled regardless of the lease: late fees (late fees must be a reasonable estimate of actual damages, no specific cap, but excessive late fees are unenforceable under Cal. Civ. Code 1671), application fees, and fees that exceed the actual cost of the service. A "new" fee that effectively re-categorizes a previously-bundled cost is treated as a rent increase.

Source: Cal. Civ. Code 827(b)