The immunity principle espoused by the Barney’s Furniture court (concluding that the city could not be held liable in damages for floods resulting from a gradually increasing functional incapacity of the sewer system) is codified as the planning and design immunity provision of the TCA. See N.J.S.A. 59:4-6. The Act’s plan or design immunity is granted because such decisions are “an example of the type of highly discretionary governmental activity which the courts have recognized should not be subject to the threat of tort liability.” N.J.S.A. 59:4-6 cmt. Thus, under Barney’s Furniture, as well as under the Tort Claims Act, a public entity may establish plan or design immunity for its original construction of a drainage system. Once it does, “no subsequent event or change of condition shall render a public entity liable on the theory that the existing plan or design of public property constitutes a dangerous condition.” N.J.S.A. 59:4-6 cmt.

Although plan or design immunity does not depend upon any showing of the reasonableness of the design, nor can it be lost by changed circumstances, N.J.S.A. 59:4-6 cmt, it does not suffice for the public entity to show that works were constructed under a permit. For, although liability cannot be based on the inadequacy of the design or plan, immunity from liability for an independent affirmative act (such as the claimed discharge of high amounts of phosphates and nutrients) is afforded in the first instance only for an approved feature of the plan or design. Therefore, a fair reading of the TCA’s planning and design immunity provision compels the conclusion that the prerequisite fact which must be proved in order for the public entity’s burden of proof to be deemed to have been successfully carried is that the specific design or plan detail alleged to constitute the dangerous condition was the subject of prior governmental approval or in conformity with prior approved standards. Moreover, it is important to note that the public entity bears both the burden of pleading the affirmative defense and the burden of proof. Ibid.