What you want to do is pay an architect, and probably a very experienced real estate broker, for a "feasibility study" to determine the highest and best use.

How do you choose a reputable architect? 

My architect said the fees would be $3,000; we'll see.

A friend bought a house designed by some weirdo architect where all of the angles were either 120 or 60 degrees.

He should be taking care of everything for you, handling materials/subs/architects and everything else for his 20-25% markup.

Potentially a self-certified architect. 

Material cost wise, you can find $15-25  per linear foot, (plus labor cost) on the lower end if you go through an architect that has connections with local reps. 

You need : building plans with a drafter or architect Fees with cityAnd building costs (material and labor) We have relationships with all of them in our teams Takes between 3 to 6 months for permits and 3 to 5 months for construction (depending on scope of work).

@Kolton Weichman, I was the Architect for a 20-Unit new-construction building for Student Rentals which was finishing construction earlier this year.

Wife is an architect and I think a lot of people don't have the kind of funds to put into an ADU/MIL plus if the lot can even fit it, etc... conversions are common but personally I'd just look to buy one that already has the ADU/MIL on it versus dealing with GCs and such right now, market is nuts with labor and materials.