Can a New York 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 New York, the landlord can add a new fee with the same advance notice required for a rent increase (New York uses a tenure-based notice ladder at RPL 226-c: 30 days for tenancies under 1 year, 60 days for 1-2 years, 90 days for 2+ years. Only triggers if the increase exceeds 5% or the landlord does not renew. Good Cause Eviction (Part HH 2024) caps increases at CPI + 5% or 10% max in NYC and opt-in municipalities, with exemptions for small owner-occupied buildings and new construction.). 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 ($50 or 5% of monthly rent, whichever is less, under HSTPA 2019), 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: RPL 226-c (HSTPA 2019)