Depreciation basis for room rental in owner occupied house

We own our own home and have lived in it since 1985. 

this year (2019), we rented out a single bedroom in our house for the entire year, say 20% of total square footage 

what is the basis for depreciation?

is it the price we paid in 1985 + improvements?

is it zero, since we’ve lived in the house for more than 27.5 years?

Or,

is it the fair market value of the house when we converted the room to a “rental unit”?

 

Any insights are appreciated!