The part against the defendant is reversed, and that part of the case is remanded to the Seoul High Court.

1. Regarding ground of appeal No. 1

A. In a case where the operator of a public project (hereinafter “project operator”) supplies a housing site, etc. (hereinafter “special supply”) to a person subject to relocation measures by means of the relocation measures, and sets the sales price of a re-resident’s housing site at a certain amount at a discount from the development cost of the housing site, whether the sale price exceeds the “amount obtained by deducting the installation cost of basic living facilities from the development cost of the housing site” and the scope of the cost included therein should be determined.

(see, e.g., Supreme Court Decision 2014Da8997, Oct. 14, 2015). Moreover, even if the sales price actually increased due to the fact that a person subject to the relocation measures has paid additional interest in arrears due to the overdue payment, it is merely a cause attributable to the person subject to the relocation measures, and the project operator cannot be deemed as unjust enrichment.

(See Supreme Court Decision 2014Da35365, 35272, 35389 Decided November 27, 2014, etc.) B.

For the reasons indicated in its holding, the court below acknowledged that the overdue interest corresponding to the "sale price calculated by deducting the basic facilities installation cost from the sale price of this case" among the overdue interest paid by the plaintiffs who purchased a detached housing site by special supply from the defendant who is the business operator under the sales contract of this case cannot be unjust enrichment. (2) The amount of unjust enrichment by each sales contract of this case should be calculated based on the amount obtained by deducting the legitimate sales price from the actual sale price paid. (3) The specific amount is "total payment amount including interest rate - the supply area - the legitimate sales price per square meter - legitimate overdue interest."