53 Pa. C.S. § 8424
(a) Minimum time periods for taxpayer response.--
(1) The taxpayer shall have at least 30 calendar days from the mailing date to respond to requests for information by a local taxing authority. The local taxing authority shall grant additional reasonable extensions upon application for good cause.
(2) The local taxing authority shall notify the taxpayer of the procedures to obtain an extension in its initial request.
(3) A local taxing authority shall take no lawful action against a taxpayer for the tax year in question until the expiration of the applicable response period, including extensions.
(b) Requests for prior year returns.--
(1) Except as provided in paragraph (2), an initial inquiry by a local taxing authority regarding a taxpayer's compliance with any eligible tax may include taxes required to be paid or tax returns required to be filed no more than three years prior to the mailing date of the notice.
(2) A local taxing authority may make a subsequent request for a tax return or supporting information if, after the initial request, the local taxing authority determines that the taxpayer failed to file a tax return, underreported income or failed to pay a tax for one or more of the tax periods covered by the initial request.
This subsection shall not apply if the local taxing authority has sufficient information to indicate that the taxpayer failed to file a required return or pay an eligible tax which was due more than three years prior to the date of the notice.
(c) Use of Federal tax information.--A local taxing authority may require a taxpayer to provide copies of the taxpayer's Federal individual income tax return if the local taxing authority can demonstrate that the Federal tax information is reasonably necessary for the enforcement or collection of an eligible tax and the information is not available from other available sources or the Department of Revenue.
53 Pa.C.S. § 8424