I have a Goodman Heat pump and air handler. The system is less than year old. I am wanting to put a 13 seer versus the 12 seer that I currently have. Do I need to replace the air handler as well. I would rather just replace the outside unit if possible.

Thanks in advance

A unit that is only one year old is not normally a replacement candidate to move up one one seer point. You will not get a good return on your investment. You will have purchased two compressors plus installation. You might save $25 a year in energy but look how long it will take to recoop your new expenses. I'd skip this idea, should have doen it last year.

Guess I should give reasoning behind it. This is a new house and I am already adding an 600sq feet to it. I am afraid that the 12 seer will not be sufficient. Affter reading an other advise it looks like I can go up to a 16 with out changing muchCurrently I have 2 12 seer units for 2200 square feet. I know I will be losing money and should have put a system in for expansion, nut hind site is 20/20.

In your original post you did not mention an addition to the sq.ft of the house. The seer rating refers to the efficiency of the unit and not necessarily the size of the home. Most units are rated 2 tons, 2 1/2 tons and so on. You may need a HVAC contractor to re-evaluate your home's size and needs before you commit to a cure.