For valuation purposes or appraisal purposes, the non-permitted addition is not included in the GLA (gross livable area) square footage of the property. Therefore it is not included in the determination of overall value. However, it may be included in the appraisal as an addition to the property IF the comparables also have such (permitted) additions. If included it would normally get valued as additional storage (minimum value) regardless of the use, condition, or quality of construction. It may also reduce the appraised value of the property overall because is was not permitted, and the reduction in overall value would be due to "cost to cure". In other words the cost to remove the addition and restore the property to the original structure.
If you were to purchase the property you may have to have a building inspector inspect the addition, for code compliance and have it added to the assessor's records which could increase property taxes, as a condition for financing approval.