When an insurance company has invested its funds in real estate pursuant to the authority granted by this article, it shall, within five years from the date upon which it has acquired the real estate, sell and dispose of the same unless within that period it has commenced the construction of one or more apartments, tenements, or dwelling houses thereon pursuant to the plans and specifications set forth in the project approved by the board of commissioners or other governing body of the city wherein the real estate is located, in accordance with the provisions of this article. The commissioner may, in his discretion, extend this period of five years for such further period as the circumstances of the particular case seem to him to require.