145.551.  Extension of time for payment or filing return or other document — interest to accrue, rate. — 1.  The director of revenue may grant a reasonable extension of time for payment of tax, or for filing any return, declaration, statement, or other document required by sections 145.011 to 145.995 on such terms and conditions as he may require.  No extension for filing any return, declaration, statement or document shall exceed six months.

  2.  If a taxpayer has been granted an extension of time for filing the federal estate tax return, the filing of a copy of the extension with the director of revenue shall automatically extend the due date of the tax return required by sections 145.011 to 145.995.

  3.  If a taxpayer has been granted an extension of time for paying any portion of the federal estate tax, the filing of a copy of the extension with the director of revenue shall automatically extend the time for the payment of the tax or a portion of the tax required by sections 145.011 to 145.995 to the applicable limitations specified in Internal Revenue Code Section 2011(c) or 2015.  The portion of the Missouri estate tax which is subject to deferral or payable in installments shall be determined by multiplying the total Missouri estate tax payable by a fraction, the numerator of which is the gross value of those assets of the decedent's estate having a taxable situs in Missouri which qualify for deferred or installment payment under Internal Revenue Code Section 6161, 6163, or 6166, and the denominator of which is the gross value of all assets of the decedent's estate having a taxable situs in Missouri.  For purposes of this section, the value of property shall be that determined for federal estate tax purposes.  Deferred payments and installment payments with interest shall be paid at the same time and in the same manner as payments of the federal estate tax are required to be made under the applicable sections of the Internal Revenue Code.  Acceleration of payments under this section shall occur under the same circumstances and in the same manner as provided in Internal Revenue Code Section 6166(g).  During such extension, interest shall accrue and become due annually on the Missouri estate tax at the same rate as provided in Section 6601 of Title 26, United States Code for the extension of federal estate taxes during the same period.  If more than one rate is applicable to amounts owed by a taxpayer under the federal estate tax during such period of extension, the same rates shall apply to the Missouri estate tax, and in the same proportion in which such rates are applicable to the amount owed under the federal estate tax attributable to assets having a taxable situs in Missouri.  After the period of extension for the payment of Missouri estate taxes has expired, interest shall accrue as provided in section 143.731.