This is an excellent question. While appraisers CAN use comps from other neighborhoods, they won't ignore sales in your neighborhood. The really only go outside the neighborhood when there are no good comps within your subdivision - and even then they are "weighted" less than neighborhood sales.
OVER improving can be dangerous, especially if you are getting a loan for the improvement/addition.
If you are adding a bedroom and a common area, that could be OK and at least set you apart from other properties in the neighborhood when you do go to sell. You never want to be the "best" property in the subdivision.
I know you prefer not to move, but I took a quick look at your other options instead of improving your current home - just to give you an idea:
At a glance, there are 8 sales (3 are foreclosures) in the neighborhood since March of 05 - the regular market sales - not foreclosures - range from 131,000 to 167,000. Some are 3 bed and some are 4.
There are 28 current listings under 250K that are 4 bedrooms or more IN the same school district and 97 available properties in 30064 under 250K.
If you would like to get in touch, I would be happy to provide you with more specific recent sale data to aid you in your decision.
Good luck to you - you've already taken a big step by asking the right questions to the right people rather than just getting started on an addition - I'm sure you will get some other great responses here!