Del. Code tit. 9, § 8321

Whenever a board of assessment or Department or Office of Finance shall propose to alter or change any assessment by increasing the same, or make a new assessment other than a general, supplemental, or annual assessment, it shall, before such alteration, change or new assessment is made, give 10 days’ notice in writing to the owner of the property affected thereby, and if such owner cannot be found within the county by reasonable inquiry, then to the person in possession of the property, or to the person in whose custody the same may be.