The Iowa Utilities Board announced Friday that it has issued a proposed decision conditionally approving a proposed 650-megawatt natural gas-powered power plant to be built in Marshalltown.

The decision gives conditional approval for an electric generation certificate for Interstate Power and Light Co., an Alliant Energy Corp. subsidiary, to construct its proposed Marshalltown Generating Station. The order establishes an overall cost cap of $920 million on the project, including all necessary transmission and other related costs.

 The board’s decision provides for an 11 percent rate of return on equity on the project. The company had originally proposed an 11.25 percent return on equity. The project was given a depreciation lifetime of 35 years for the company to recover its investment cost in the project.

Requests for advance ratemaking principles enable utilities to establish regulatory treatment for a generation plant in advance of construction, but such cases do not immediately impact customer rates.

The utility company will have 30 days from the date of the order to notify the Board whether it accepts the ratemaking principles. The generating station would go into service in 2017.