In this case, substantial improvement means 50% of the value of the property. However, substantial improvement is applied differently when applied to other kinds of "projects," such as pure land development or starting a new business. What "project" did you have in mind? Again, the criteria can change depending on what you're doing. For real estate, if you're buying then renovating, for example, you'll want to be sure you hit the 50% mark. Regarding the 30-month timeframe, there is a penalty regime.