When building an ADU, you should consider three things. Money, who's gonna do it, the contractor, and the timing.
When you're gonna enjoy your ADU with money, you should know how much you want to spend. ADU use can go from a hundred thousand dollars to $500,000. So you gotta know what kind of materials you want to use either on your bathroom, the kind of tile, the kind of valves vanities on your kitchen countertops, cabinets. Even the appliances will reflect on the money you want to spend.
On the contractor, you should know who's gonna be able to help you through the process from the beginning. They're gonna be able to help you with the design. Are they gonna help you getting out your stress by helping you collecting all those finished materials? Are they're certified? Do they have the main power to tackle this, this big project?
And Timing, you should know when you wanna start your project 'cause the weather cannot affect the construction. You should know when either your mom or your son is gonna move in.