For new construction, work must commence within one year from the date of the letter and must be completed within 18 months from the date of the approval letter. Major modifications must be completed within 12 months from the date of the approval letter. Minor modification approvals are valid for 90 days and work must be completed within 90 days from the date of the letter. An extension to these deadlines may be requested by submitting a request to the ARB Administrator in writing prior to the expiration of the approval.