Development Question for property in escrow

[Brandon Stapper]

I have acquired over 10 properties in the past 5 years so I am semi familiar with real estate but I have never built from the ground up.  

I have a property under contract in San Diego.  It already has a 4plex with great renters.  The lot size is 24k sqft and pretty flat.  I went to the city and they said I can build an additional 4 units on the property.

I am trying to figure out my true cost of building/fees/architect/civil engineer/etc.  I am also considering splitting the property and selling off the lot.  

Does anyone have experience with this and what steps I should take to make sure this is a good investment before I close?

Here is the property > https://www.redfin.com/CA/San-Diego/3461-Martin-Ave-92113/home/114014465