Can a Texas 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 Texas, the landlord can add a new fee with the same advance notice required for a rent increase (Texas has no statewide statutory advance notice for rent increases. Lease terms govern. Default practice is 30 days written notice on a month-to-month tenancy, matching the Tex. Prop. Code 91.001 termination rule. No state rent cap, and local rent control is preempted.). 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 (12% of monthly rent for buildings of 4 or fewer units, 10% for larger buildings, under Tex. Prop. Code 92.019), 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: Lease-based; Tex. Prop. Code 91.001 (30-day month-to-month termination default)