(1) A Washington return must be filed if the gross estate equals or exceeds the applicable exclusion amount.
(2) If a Washington return is required as provided in subsection (1) of this section:
(a) A person required to file a federal return must file with the department on or before the date the federal return is required to be filed, including any extension of time for filing under subsection (4) or (6) of this section, a Washington return for the tax due under this chapter.
(b) If no federal return is required to be filed, a taxpayer shall file with the department on or before the date a federal return would have been required to be filed, including any extension of time for filing under subsection (5) or (6) of this section, a Washington return for the tax due under this chapter.
(3) A Washington return delivered to the department by United States mail is considered to have been received by the department on the date of the United States postmark stamped on the cover in which the return is mailed, if the postmark date is within the time allowed for filing the Washington return, including extensions.
(4) In addition to the Washington return required to be filed in subsection (2) of this section, a person must file with the department on or before the date the federal return is or would have been required to be filed all supporting documentation for completed Washington return schedules, and, if a federal return has been filed, a copy of the federal return. If the person required to file the federal return has obtained an extension of time for filing the federal return, the person must file the Washington return within the same time period and in the same manner as provided for the federal return. A copy of the federal extension must be filed with the department on or before the date the Washington return is due, not including any extension of time for filing, or within thirty days of issuance, whichever is later.
(5) A person may obtain an extension of time for filing the Washington return as provided by rule of the department, if the person is required to file a Washington return under subsection (2) of this section, but is not required to file a federal return.
(6) During a state of emergency declared under RCW
43.06.010(12), the department, on its own motion or at the request of any taxpayer affected by the emergency, may extend the time for filing a Washington return under this section as the department deems proper.