A cemetery authority shall start construction of that section of a mausoleum or bank of below-ground crypts in which sales, contracts for sale, reservations for sale, or agreements for sale are being made, within five years after the date of the first sale or when 75 percent of the mausoleum or below-ground crypts have been sold and the purchase price has been received, whichever occurs first. The construction shall be completed within six years after the date of the first sale made. Extensions for completion, not to exceed one year, may be granted by the board for good cause shown. If the units have not been completely constructed at the time of need or the time specified herein, unless otherwise specified in the preneed contract, all monies paid shall be refunded upon request, plus interest earned thereon if deposited by the cemetery authority in an escrow or trust fund, and if not so deposited in an escrow or trust fund earning interest, then plus interest in an amount equal to the interest or discount which would have been earned thereon had the funds been invested in United States Treasury Bills having a 90-day maturity.