All improvements on real estate which become substantially completed between January 1 and June 30 in any year shall be assessed for taxes. Such improvements shall be subject to taxation for the full taxable year beginning on July 1 at the regular tax rate levied for State and County purposes for the year. All improvements on real estate which become substantially completed between July 1 and December 31 of any year shall be assessed for taxes and such improvements shall be subject to taxation in the year at one-half (1/2) of the regular tax rate levied for State and County purposes for the year. In the case of buildings under construction the term "substantially completed" shall mean when the interior walls are finished.