@MARK MONTANO, a pro forma is often used to pitch a project to investors. Especially for investors, you should always present a "best case," "worst case," and "most likely case" scenario, so they can properly assess risk. Worst case will usually include lease up rates about 75% less than the market. If I think I can fill up 10 units a month until I reach stabilized occupancy, then my worst case might project only 7 units a month.
For brand new construction on unknown dirt, a worst case scenario might include earth moving expenses 50% more than anticipated. That is because you might run into a really deep patch of top soil that has to be removed and replaced with clay. That can be very expensive. Those are usually the only two surprises that will affect a worst case scenario, assuming you've gotten good numbers for other construction expenses and have a realistic schedule for completion of construction and issuance of a certificate of occupancy.
Before doing a pro forma for new apartment construction, make sure you have good engineering ahead of time. Engineering expenses can be surprisingly expensive, especially if local authorities will require some type of water retention system to compensate for the fact that large amounts of soil with vegetation, that used to absorb rainfall, will now be covered with concrete and asphalt. Installing utilities is also a very expensive, and surprising, item. It's not just the cost of putting in the lines, but the tap fee imposed by the water works for simply tapping into their lines, and other such expenses.
Also, you will need architectural drawings before doing your pro forma, because things like firewalls can make a huge difference in construction costs. An architect can help you make decisions like increasing the set back line so you need firewalls only between units, and not along the rear walls that are too close to the property boundary.