The value will be zero or negative without a permit. As for it's value, I'd recommend you do it more for yourself than the future value. Roof decks in small condos rarely pay for themselves. Itnwill help a place to sell more readily or get higher rent but doesn't always mean more profit
Second. Accessibility. Is it off the living room? If not, it's value falls. Off the kitchen is ok too. If you need to go up a spiral stair or go through a bedroom think twice unless you know you will get the enjoyment.
As a note' depending upon your neightbors and condo rules the permit may cost more than the deck. Especially in a condo building you need to get permission from the executive board. They will likely need architectural drawings.... more