I say do whatever you can to make it a good home. Soundproofing is worth it. We didn’t do all we could for our suite and I regret it daily.

Earplugs are absent from your list.

1- The person who owns the property and who pays for the ADU makes the decisions.

2- In many markets, rental units next to the freeway rent for significantly less than those a few blocks away. Being able to advertise that a unit has extra soundproofing may or may not help with getting a good renter for a reasonable price.

All I know is that rock wool is the gold standard, also fire resistant. My daughter used it in the walls between her industrial condo and the neighbors. Other people’s wall mounted TVs were no longer a problem.

Buy the Master Handbook of Acoustics. 7th Edition is about to be printed.

Rock Wool is not the gold standard. That's total BS.

There is an entire industry supporting soundproofing science and materials. It's not a trivial matter.

Given the rents in your area, I would expect that great soundproofing would add at least $100 per month to the rate. Even if it didn't add to the rent amount, it would help get and retain tenants to be able to walk in, shut the door and not hear the outside world. Keeping tenants long term will make your venture more profitable so compare cost of adding soundproofing to the future benefit. At today's interest rates, $100 higher monthly payment will cover around $25,000 on the mortgage. How much extra money are you expecting to pay for the soundproofing?

What are you planning for windows?

IMO triple paned windows would be the first choice for me. As for the soundproofing what sound is it you are trying to minimize, the road noise or sound from your unit. IMO a poor spot for anyone to live is next to a freeway and I can't imagine paying $2500 for a space like that . I would do as much as possible to now make the space quieter.

The AVS Soundproofing master thread has more info on soundproofing than we can possibly put here.

www.avsforum.com/forum/19-dedicated-theater-design-construction/1509173-soundproofing-master-thread.html%3famp=1

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There is an entire industry supporting soundproofing science and materials. It's not a trivial matter.

There is also an entire industry supporting business marketing. In that industry you learn product positioning, where incredibly small improvements can be marketed as specialty products for a premium.

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@Jae T - Read the information on the AVS forums as a good place to start. Soundproofing (really sound reduction) is a lot more about knowledge than products. Regular drywall will outperform acoustic drywall if designed and installed intentionally to reduce noise.

There are no products that offer universal solutions. Different sound frequencies need different approaches, for example, the power tools in my shop have two different types of motors, universal and induction, each motor requires different materials to reduce noise as they produce different frequencies. My theater uses an entirely different type of soundproofing.

However, when in doubt add mass.

Triple pane is not inherently better than dual pane for sound blocking despite all the online hype from some window companies and other sources.

But choice of window and glazing options can make a huge difference in sound performance.

The author of the Master Handbook of Acoustics suggests that you invest in the most airtight windows you can afford, and then make every effort to block/seal any other areas where air could get in.