The ADA places the legal obligation to remove barriers or provide auxiliary aids and services on both the landlord and the tenant. The landlord and the tenant may decide by lease who will actually make the changes and provide the aids and services, but both remain legally responsible.
For additional information, take a look at the following resources:
Fact Sheet: Effective Communication
FAQ: If I am a business or non-profit organization leasing building space from a building owner and the parking lot is not compliant with the 2010 ADA Standards, am I obligated to address this or is it solely the responsibility of the building owner?