Zoning question from sfr to multi-units in san diego

[David Morrison]

I am currently trying to buy a property in San Diego.  The property is a 3 bedroom 2 bathroom house and it has 2500 square foot on a .33 acre of land.

The county records indicate it is zoned as a R7.

I believe R7 means medium density residential. 7-14 units per acre and is intended for wide range density.  But the property is on a .33 acre of land.  

I would like to add two more units to this property.  Making the property a house plus the duplex.  So making the property a tri-plex (house plus 2 one bedroom one bathroom unit)  

My questions are:

1.  What would my challenges be?

2.  Any companies in San Diego to help me make this happen?

3.  cost per unit?

I own several houses in San Diego but never tried to add any units and looking for any advise.