If I decide later to start a construction business (actively building and selling new spec houses), assuming this is taxed as ordinary income with social security, etc and treating the homes as inventory - if you build a house, then sell it, then spend that money building a second one, but don't sell it until the next year, then sell it, then build another, can you effectively push off the taxes as you are rolling the sale proceeds into new inventory costs / construction costs?  Or do you have to pay the tax on all gains regardless of whether you spend that money on building a new home?  I thought with ordinary businesses you can subtract expenses from revenue and pay tax on the end result allowing you to keep reinvesting into the business?  I'm not talking about 1031 exchange here. 

Thanks,