ADAAG 4.1.6(1) Alterations.
(j) EXCEPTION: In alteration work, if compliance with 4.1.6 is technically infeasible, the alteration shall provide accessibility to the maximum extent feasible. Any elements or features of the building or facility that are being altered and can be made accessible shall be made accessible within the scope of the alteration.

Technically infeasible. Means, with respect to an alteration of a building or facility, that it has little likelihood of being accomplished because [...] site constraints prohibit modification or addition of elements, spaces, or features that are in full and strict compliance with the minimum requirements for new construction and which are necessary to provide accessibility.

“What does ‘maximum extent feasible’ mean?
Occasionally, the nature of a facility makes it impossible to comply with all of the alterations standards. In such a case, features must only be made accessible to the extent that it is technically feasible to do so. The fact that adding accessibility features during an alteration may increase costs does not mean compliance is technically infeasible. Cost is not to be considered...”
--DOJ Title II Technical Assistance Manual

In downtown Auburn, AL a complying passageway was constructed along building facades at a sidewalk elevation that eliminated steps at each shop entrance. The sidewalk run at the curb was retained, with landscape elements separating the two. Ramps connected the two levels at several points along the sidewalk.