Pation permitted but not added in the square footage?

[Pandu Chimata]

I am in the process of purchasing an investment property for rental purposes. The listing says 1350 SQ foot area with 3 bedrooms and 2 bath. 

On visiting the property, the patio is closed with walls and windows. In city permits, they have permit to close th patio. The square footage still shows 1350 only. The assessor record also says 1350 and says the porch has 250 sq foot.

What does this mean - does this mean the patio converstion is permitted ? 

If that is the case, why the same is not added to the assessor data? Is it just to save the tax? or somethin else? I want to make sure I don't get any legal/code/permit issues in the future?

Please advise.