As indicated above you cannot (ever) split a contract and sell the portions. If you have two separate contracts, original and add-on, you can sell either one of those because each is legally a separate real estate interest from the other. When you purchase under a contract, you are a sold a percentage interest in the real estate that cannot be divided. In other words, it would be like owning a condominium and trying to sell the bathroom. As a matter of real estate law, it cannot be done.