Hi Bryan, congratulations on getting your offer accepted - not all that easy in this market. What island is your duplex on?
And here in Hawaii, adding sq. ft. to a property generally a great thing to do - but I agree with Rick, check out comps, and see if the extra space will give you a decent return on your money.
And want to highlight a few important things:
* Although it is usually takes a long time, I highly recommend you get all necessary permits. If you do an addition, the appraiser will not give you any value for it if it is not permitted. Plus, when it comes to sell, many potential buyers like to see that any remodeling work was permitted.
* At least on O'ahu, quality contractors are quite busy here right now.
* It is pretty challenging to find some materials right now. Recently, I've seen some fix/flip opportunities be put back on the market because the originally investor doesn't want to hold the property until materials arrive on the island to finish the job.
Wish you well, and if I can be of value to you in any way, contact me at any time.