Jim, I still have a couple of questions.
In general there is no requirement for an accessible means of egress to be created in existing buildings. Getting out isn't the issue. However, getting IN is an issue. Access into the building is a concern - both from a building code and federal accessibility perspective.
You mentioned a renovation for an "old" building. Is it designated as historic? If it is designated as historic, then the landing may not be required if it would interfere with the historic character of the building as determined by the historic preservation officer; but, access would need to be provided at another entry into the building. So, regardless, there is a requirement under the building code to provide an accessible entry (either 3411.8.1 of the 2009 IBC or 605.1.1 of the 2009 IEBC), and as barrier removal under Section 36.304 of the ADA law itself.
The question about whose responsibility it is for the outside sidewalk is an important one and it varies depending on where you are in the country. each state and municipality can address this in a different manner. In principle, if the sidewalk outside the building is in the public right of way, it is the municipality's responsibility. that being said, there are often laws in place to shift that responsibility to the building owner. For example, in New York City, the sidewalk must be designed to meet the city's criteria for accessible design but the design, installation and maintenance is up to the owner. The city is clear about whose responsibility it is to accomplish building access - the owner's. In the village I lived in a couple of decades ago, the Village was responsible for maintaining the sidewalks but design and installation was up to the owner. In some of the suburbs where I live now, the city will dictate how the sidewalk is to be designed and must approve that (through zoning) before the sidewalk is installed.
So, the IBC, IEBC and federal ADA require access into the building. Whose responsibility it is to address the design outside the front door is a question that needs to be addressed locally. And, as Mark mentioned, it should have been a contractual item covered in at least one of the contracts for the project. Usually the architect gets hung out to dry for anything that's missed.