Can a North Carolina 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 North Carolina, the landlord can add a new fee with the same advance notice required for a rent increase (North Carolina has no statewide advance notice specific to rent increases. NCGS 42-14 sets 1-month notice for year-to-year termination; 30-day month-to-month notice is the default practice for rent changes. No state rent cap.). 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 (the greater of 5% of monthly rent or $15, under NCGS 42-46), 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: NCGS 42-14 (30-day month-to-month termination default)