Converting Single Fam to Duplex

[Adam Allen]

Has anyone converted a single family to a duplex in Austin? 

Our current residents is 1600 sqft 4/2, zoned SF-3 with 1850sqft lot. Poking around online we should be able to build onto (second story) the property and “easily” permit as a duplex. 

The goal is to add a 800 sqft unit (bed, bath, kitchen, and laundry) atop the attached garage with a side entry for rental income. Project cost estimate about $85-100K with our resources and rent for $1500-1600.

I’ve seen many new builds going up as a duplex but don’t know of many conversions happening... anyone familiar with this strategy? Still working through actual costs for design, engineering, permitting, and build but rough numbers look pretty good... 

Has anyone done this before?