The gain would be long-term capital gain. An example, if the cost of the property was $500,000, disregard the cost of land for this example, after 15 years the remaining basis would be $228,000, if the property were sold for $600,000 there would be a long-term capital gain of $372,000 at your capital gain rate.

You would owe tax on long term capital gains which is the difference between the sales price (minus sales expenses) and your basis. Your basis is the original cost plus any capital additions minus depreciation. Depreciation is recaptured as ordinary income. If you didn't know this already, figuring it is not going to be a DIY project so hire a pro. It's cold, but another option is don't sell and let your kids inherit it then they can have a stepped up basis that isn't taxed (unless you are worth north of $5.5 Million) and start over.