Calculating deprecation for my carriage house - cost basis?

Situation:

 

Bought house for $375000 in 2016

(property is zoned for 2)

 

I built a carriage house in 2018 for about $180000. This is rented 100% of the time. The main house I live in, it's not rented. 

 

County assessor says the land is worth $61000 (weird because smaller lots sell for $300,000 , but whatever) 

 

Now I want to calculate depreciation. 

 

This situation is a bit unique because I live in one portion and rent another portion. So what is my original cost that I use for depreciation ? The $375 I originally paid? The 180K I paid to build the carriage house? Add them both together? 

 

Thanks in advance.