Can interior designers provide construction administration, and is it their role to ensure that contractors build within the local and national building codes?

Yes, interior designers can provide construction observance and administration services on behalf of their clients. However, unless the submitted plans conform to local and national building codes the local building officials will not approve them, for permit purposes and therefore cannot be used for construction. Once construction starts, only the local building inspector ensures that work conforms to all codes, not the designer, not the architect, not the engineer, not even the contractor.