Ohio Rev. Code § 5705.43: Improvements paid by special assessments.

In the case of an improvement, the cost of which is to be paid in whole or part by special assessments, a contract may be executed without an appropriation or certificate for that portion of the cost derived or to be derived from special assessments, provided that a resolution or ordinance authorizing such assessment and the bonds or notes to be issued in anticipation thereof has been passed in the manner provided by law, or that such contract is for the provision of engineering, legal, or other necessary professional services in connection with such improvement.