S.D. Codified Laws § 36-18A-69

Any contract, written or oral, for engineering, architectural, land surveying, landscape architectural, petroleum release assessment, or petroleum release remediation services made by any person in violation of any provision of this chapter is unenforceable as to such services. It is a complete defense to any action to enforce payment for any services, if the party contracting for services proves that the person rendering or offering to render services was not at the time such services were offered or rendered, legally authorized to contract for such services.