I would think the architect could give budget estimates including development and permitting costs as well as construction costs? 

Long Term                                        Short TermMortgageTaxesHOA FeesWaterGas/ElectricInsuranceProperty managmentLawn CareSnow RemovalPool CareInternet/CableFurnishingsMaintenanceBooking feesVacancy Permits/Licenses

Always pull permits for all work and never underestimate what can hide behind panelling and drop ceilings!

And as time permits, I hope you post some more photos and updates on your BRRRR property. 

In CA the taxable rate is assessed at time of sale, and that tax basis is used to determine the taxes until the property is either sold or has permitted improvements (there is an inflation-adjusted increase allowed, but the starting # is the FMV at time of sale).

But how do you know if an addition is permitted or not?

Closing dates are unknown per their Agents, largely because the area is such as massive building zone and they are held up in permitting. 

Permits are a huge pain.

Does anyone have any insight into the timeline and costs of getting a garage conversion permitted by the City?

David Greene points out you should look for population growth, job growth (and what kind of jobs), vacancy rate, average income, and building/construction permits.