possibility of building on if FAR is maximized?

Yes, if an adjacent property has unused (and unsold) development rights, you can buy them.

To figure out what you can do where, you'd need to put together a legal and architectural team, the development process in NYC being what it is. If there were easy ways around it, everybody'd do it.

Lots of articles on the subject of air rights given the likes of One57 and 432 Park. For example: http://www.nytimes.com/2013/02/24/realestate/the-great-race-for-manhattan-air-rights.html

Are you sure the building has tapped out available FAR? Maybe it's worth speaking with an architect who is well versed in these manners before going nuts.

Yeah, our architect says there isn't any left. Just wondering if buying adjacent air rights is only for big development projects or if it's every doable for a small project.

Here's a little one, where an owner on West 38th bought unused rights from an adjoining owner on West 39th: http://a836-acris.nyc.gov/DS/DocumentSearch/DocumentDetail?doc_id=2013101100357004

The research and paperwork are the same whether it's big or little, so it might turn out that adding 500 ft² wouldn't be worth doing.

You also have to work around landmarks. Nobody is going to do it for 500 ft., but in for a penny, in for a pound, and why not do several thousand feet? Note a new roof, stairs that comply with code, possibly sprinkler, electrical, plumbing, and elevator will all have to be extended.

To get the process started:
1) Negotiate air rights purchase w/ neighbor. If they won't do it then stop here. If they will then move to next step. You start here b.c. this is the only free step.
2) Hire zoning analyst and surveyor to figure out square footage avail. for neighbor. Est. Cost is 10-20k for this.
3) Hire attorney to draft and execute purchase. Est cost is 15-20k.
4) Hire architect to draw up plans. Cost can be in 10's to 100's of thousands of dollars.
5) Build!