Value added - Adding units to SFR

[Nick Macklin]

I was curious, from an appraisal standpoint, what people have seen when adding additional income producing units to single family homes.  Doing CMAs is pretty straight forward for a regular home but when you make a single family home 3 separate units the comps are tough to figure out.  Anyone have any experience with how much value can be expected by adding one or two units (i.e. converting an attached and detached garage into 1 BR apartments)?