How to determine the new construction cost for a small building

[Jack Zhuang]

I'm looking at a small piece of land about 6000 sq ft in SoCal and put a new class B 6 unit condo on top of it. I think I can talk the seller to bring land sale's price down close to market avg. but I am having a hard time estimating the new construction cost. 

To add some context, there's currently an auto shop tenant sitting on the lot but the lot has the appropriate zoning and utility lines to support the construction of a small MF. In order to satisfy the parking requirement, I would most likely to build a 3 store building with ground floor dedicated to parking. The current foundation is poured concrete that I might have to remove and make new. I'm going to meet with an architect this week and figure out the scope of the project. 

But I also want to get some opinion on how to do an estimate on my own. Can anyone help?

- Jack