Reasonable would be to put in window unit for this area. The trade off would be more noise than other options. They make window units that are capable of heating as well as cooling, but those will cost more than cooling only.

There is not likely what you consider a reasonable way forward outside of the window unit option.

(Note: the window unit you select will need to be properly sized for the area in which you are trying to control temperature. If undersized too much it will not keep up with the heating or cooling load being placed on it.)

Part of this maybe related to this loft being part of a RE flip, in which the flipper added additional space to the existing townhome so as to make more money on the flip. Then they just ran ducting to this area from the existing HVAC system.

Because your post is short on information I can only guess here.

Your HVAC system may have a problem unrelated to any RE induced non-sense. Again I only know what you tell me.

Location: Severe climates cost more to solve comfort problems. Again your post is vague.

I service the Katy, Texas area. Texas Licensed HVAC contractor 24 years experience. Austin Air Companie (IN KATY, TEXAS)

This is a common problem with a third story townhouse loft. You have no attic above the loft and the insulation is probably marginal. It is basic design flaw. If the the HVAC system is on the first floor, or worst yet the basement, temperature differential can be 10 degrees on a hot sunny day.

The window air conditioner is the best low cost solution for cooling. The unit can be removed in the winter when it is not needed. I would think the temperature differential in the winter is much less. On a bight sunny day it may even be warmer in the loft than the first floor due to the sun beating down on a dark roof.

Is there a part of the loft open to a lower floor? A ceiling fan would help move hot air down. Otherwise, the window unit may be the only other quick fix.

Of course, installing a minisplit unit for $$$$ would also work. Would another floor on the same end of the house also benefit from having one?

So our 2nd floor doesn’t have enough return air which should be fixed soon. My second floor is pretty warm in the summer but adding another grille should help that

theres a smallish like 5000 btu AC in there now. It really doesn’t do much of anything

And mike is correct. The winter is maybe a tad cooler up there but it’s very comfortable. Far from too cold.