Calculating value added for "granny flat" additions or ADU's
Is there a formula used by RE Professionals that calculates the viability of adding room additions, granny flats, garage apartments, aka Accessory Dwelling Units (ADU's), etc. to a property that meets the zoning, building codes, neighborhood
i.e. HOA, POA, CC&R's. I'm just wondering if there are any existing formulas that anyone knows about that can be used to determine at what point that type of improvement out values or overbuilds the property for that area.