Without proper air flow and insulation from the finished space, heat is escaping to the roof top surface.

In particular, remove all bushes around the outdoor HVAC condenser unit...they'll get overgrown, restrict airflow to the condenser, deposit debris into the condenser, and ultimately make it less efficient.2.

Sometimes a house will have a damper on duct work to limit air flow to section that may not need as much heating and cooling,  that would need to be check also in this situation to insure it has not closed more somehow.

Over the years the kitchen was moved to the back of the house with no air flow so you could smell the grease from the moment you walked in the door.

Inspector says the home was just not getting proper air flow ? 

During that time it was very humid and there were no Windows opened or air flow.

I typically go with a split system to make sure there is good air flow throughout the new space and also get the most on your appraisal for the additional SF.

Make sure there are no gaps that could let cold air in, but be sure to have proper ventilation to achieve necessary air flow.

Just make sure your insulation contractors don't back the perimeter of the attic with insulation and block the air flow.

Also some form of crawlspace vents can be used to provide air flow.