A cost segregation study is done on a $400k property with annual rental income of around $3500 per month, $42k per year.  

Make sure the study methodology is defensible.

Do case studies.

The STR loophole utilizing the 100-hour test that Julio mentioned is the exact strategy I’m leveraging for my own cost seg study this year. 

I also had a recent cost segregation study performed and they had the roof components prior to the replacement at all 39 years. 

While house hacking, only the rented portion qualifies, so the study benefits are proportionally smaller and the math rarely justifies the cost.

When I evaluate providers, I usually look for groups that have done a lot of multifamily or apartment studies and have been around for a while.

When a primary residence is converted to a short-term rental, it is considered placed in service when it becomes available for rent, and a cost segregation study may identify components eligible for bonus depreciation.

I agree that 5-year depreciable assets can qualify for bonus depreciation etc - but that stands true independent of needing a cost segregation study to make that allowable. 

Since it's a new detached structure placed in service in 2025, you treat it like a newly acquired rental asset — the 2022 purchase price and overall property basis are irrelevant for this depreciation calculation.On the cost seg question: at $120K construction cost, the math on a full engineering-based study might be tight depending on what's inside.