Had a family friend contact me about "adding on" to his exsisting deck. Its a little guy on the back side of his pre-fab house in subburbia. The deck is currently around 15ft(w) x 8ft(l), he wants us to take the deck to 21ft(w) x 15ft(l).
He wants to upgrade the deck planks from his current ones to all new cedar. I was thinking that I will simply add new footers (4) and tie into his exsisting joists to extend the deck. The only decks I've done in the past were based on hourly wages - my old billing method

Deck building is not something I am extremely excited about, but I can build them - I just finished two at another job.
I assume the entire deck (footers), joists (pt), deck planks (cedar), stairs (cedar), and railing (cedar) will take 3 days. $3382 (I thought this might be kind of low)
I took another persons suggestion on the boards from Iowa and did the square feet by $15 per sq. and got $4725 (that seemed a bit high, but maybe in the end thats right)
What would you folks suggest on a proper bid? And I getting close or under selling myself?