Va. Code Ann. § 58.1-101

Where before the expiration of the time prescribed for the assessment of any tax imposed pursuant to this title and assessable by the Department both the Tax Commissioner and the taxpayer have consented in writing to its assessment after such time, the tax may be assessed at any time prior to the expiration of the period agreed upon. The period so agreed upon may be extended by subsequent agreements in writing made before the expiration of the period previously agreed upon.