Before you decide how much a rooftop deck is worth, I would suggest that you find out if the deck can be built to code. Note, in Chicago many of these rooftop decks are actually illegal. City code requires that a building with 3, 4, 6 or 8 flats are required to have two exits for the rooftop to be legal. Yes, private rooftop decks are worth alot of money. A lower floor unit can be worth as much as $50,000 less than the top floor provided the top floor has its own private deck and it is stated in the property report.

Also, be aware that many of the new construction developers when selling the top floor unit will only build the deck after the unit closes as they are concerned about the city code. Or better yet, they will pay for the deck and suggest someone else build the deck for you. Note, how are you going to get off the deck should a fire start if there is only one exit.

Unless your buiding also features an elevator , then you will be the one using the roof deck the most. Ken's advice is solid, understand how the roof rights are spelled out in your condo decs and by-laws because along with that goes the majority of the responsibility/expense if issues happen due to problems caused by people being up there. The warrenty issue is important, most times they only are only good if the roof stays unchanged during the period of time it is good for.

I am with you Reese, it doesn't really sound like a win-win deal for you! I don't know for sure what you can do to stop it. I would suggest that you get into curiousity before you judge. Do a little research on the matter, encourage others to as well, and compare notes. Evaluate it with an open mind and then decide.

Reese,
I would refer to your condominium documents and seek the assistance of a Real Estate Attorney to advise you correctly. Often, if the top floor unit owns the Roof Rights, they can build a deck. However, they would need to adhere to local building codes and obtain the necessary permits required prior to construction. The roof structure may need to be reinforced to support a deck and the increased loads associated with it. This would all be at their expense and in addition would likely make any current roof warranty void. If the association is responsible for the roof then it would be the unit owners that would vote on such an addition based on the Condominium documents and pay according to % of ownership. The expense of retorfitting a building to support a deck and the cost of the deck itself along with obtaining plans and permits would appear to outweigh the benefits of individual owners on lower floors. Establish a budget for all the costs required and determine if it's an expense you're prepared to pay based on how often you may use it. If it can increase the marketability of your home by offering stunning views it may be worth doing. If not, enjoy the back deck you currently have.

Reese,
You asked for opinions, so I'll toss one your way. All your points seen valid and well thought out--ESPECIALLY the new roof warranty (check with the installer/manufacturer before you change anything!). Nevertheless, a rooftop deck, if a common amenity in your area, will add value to the entire building. But as you noted--proportionately. Instead of evenly splitting the cost, I suggest prorating the cost to each unit according to perceived benefit.... Best of luck!