I have added 600 sf. To a 1500 sf. House and need to upgrade my heat and a/c. I only have a 2.5 ton unit and it is only three years old. The repairman says it is best to go with a second 2.5 ton unit to take care of the new area. Is this the best way.

2.5 ton for 600 square foot is WAY oversize for most locations. Where do you live (state).

Houston, tx. We would be splitting up the house with the second unit not just the 600 sf.

That's 5 ton of cooling in 2,100 square foot. Have a real professional do a manual "J " on the entire structure to see exactly what you need. If to large a unit the humidity will not be drawn out of the air and it will be cold and clammy.