How much to keep aside for capital gains tax?

[John Dersoe]

I'm selling in NY. 2fam, I lived in one unit and rented the other. My acct says when I bought the house for 560k 14 years ago, and sell it for 825k, it will be split into two sales.. and on the investment unit is what I will have to keep some money aside for the eventual capital gains tax when I file next year. Is there a way to lessen the amount? I've been reading that if you have made improvements over the ownership of the house, that you will lower your tax burden significantly. I must have put 100k into this house since owning it... any ballpark numbers as to what I should set aside to pay my taxes so I don't get hit with a big surprise?