If you sell before you have both owned and lived in the apartment as your principal residence, you will be liable for capital gains tax. Two years or more, and you may exclude $250,000 of gain if you're single. This is discussed in IRS Publication 523. In particular, there is an example given where a woman lives with her parents in a house for 6 years. She then buys it from them and lives in it for a further year. She has met the use criterion, but not the ownership criterion, and is liable for the tax if she sells.