28 CFR Part 35 Title II Notice of Proposed Rulemaking (NPRM) (published 2008)

(2) Exception for structural impracticability.

(i) Full compliance with the requirements of this section is not required where a public entity can demonstrate that it is structurally impracticable to meet the requirements. Full compliance will be considered structurally impracticable only in those rare circumstances when the unique characteristics of terrain prevent the incorporation of accessibility features.

(ii) If full compliance with this section would be structurally impracticable, compliance with this section is required to the extent that it is not structurally impracticable.  In that case, any portion of the facility that can be made accessible shall be made accessible to the extent that it is not structurally impracticable.