Many municipalities have amended their zoning rules and regulations to allow for ADUs ( converted garages, converted sections of the main dwelling to have separate entrances with a kitchen and bathroom, Etc.) Per the subject's zoning, this may be allowable....you need to verify.
If the above is the case, you can find a couple sales with non permitted garage conversions like yours or a non permitted addition.... "something" to show market acceptance and what the market reaction is for said non permitted additions.
You need to find legal, permitted Adu builds to extract a percentage to find out the difference between legal and non-premitted ADU's are. You need to do the same thing for properties with and without garages as subject no longer has a garage.
Before doing this however, contact your client and let them know that's how you going to approach this assignment. Or, if they want a hypothetical as if the garage conversion never transpired.