How To Determine ADU Rent

[Ryan Russel]

I am in the process of investigating the prospect constructing 1 or 2 ADU's. I am not sure on the size I should build and thus the rent I could charge. Anything above 500sq ft costs considerably more in permitting and utility hookup fees, so if the rent increase doesn't make up for it I'll stick to a Junior ADU. Location: Marin County Zip 94947

Option 1: Detached any sq ft (476sq studio ft or 756sq ft are my comparisons; about $40k more for the larger one)

Option 2: Garage conversion on 326sq ft (rickety garage that would probably need to be torn down and rebuilt)

What is an effective methodology for comping rents when I can't directly compare to other ADU's? 75% of a commensurate apartment? If a nearby dated 1BR apartment with 750sqft is going for $2,000, what can I charge for my new construction 476sqft studio ADU?

Thank you!

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Bonus Question: It looks like the project estimate will be close to $200k all said and done which seems like a lot (Prefab, permits, foundation, utility installs, etc). If I use a HELOC for financing, 1% payoff will be $2000 and thus break-even. No wiggle room. Am I doomed or is there something else I'm not considering? This is all in an effort to provide my aging mother who doesn't have much to fall back on with cash flow so I may preserve her quality of life as she gets older. What's the absolute most cost effective way to go?