Idea about increasing cash flow on the existing triplex in San Diego

[Wai Chan]

I have just purchased a triplex (two 1b/1b and one 2b/1b, 1700 sq ft in total)  on a 7000sq ft land in San Diego (City Height). The zoning allow me build up to 5 units (1500sq ft max) on this land. I don't think demolish everything and rebuild 5 units is cost effective now so I plan to build two studios or 1b/1b on the empty land and redevelop other building in the future. I anticipate the rent is ~US1000 for 1b/1b. I have no idea how much would it cost to build a studio or 1b/1b. Any idea? What else can I do to increase cash flow as I don't want to waste the empty land. Thanks!