If you have gains, yes you have to pay taxes on the gains. In either case, whether you have gains or no gains, you will need to file a federal tax on the property EACH YEAR. You are collecting rents on an apartment building. Since those rents are generating income, you will need to declare that income. For further information, I suggest that you speak with a CPA who specializes in real estate. Different parts of an apartment building can be depreciated on a different timeframe.