Ind. Code § 8-1-2-23
The commission shall keep itself informed of all new construction, extensions and additions to the property of such public utility and shall prescribe the necessary forms, regulations and instructions to the officers and employees of such public utility for the keeping of construction accounts which shall clearly distinguish all operating expenses and new construction. Unless a public utility shall obtain the approval by the commission of any expenditure exceeding ten thousand dollars ($10,000) for an extension, construction, addition or improvement of its plant and equipment, the commission shall not, in any proceeding involving the rates of such utility, consider the property acquired by such expenditures as a part of the rate base, unless in such proceeding the utility shall show that such property is in fact used and useful in the public service; Provided, That the commission in its discretion may authorize the expenditure for such purpose of a less amount than shown in such estimate.
IC 8-1-2-23