Separate Cost Seg for ADU?

Question

I converted a floor of my home to be exclusively an STR in 2025.

I want to do a cost segregation study and bonus depreciate the portion of my home that is related to the STR.

I wanted to know if it is okay to do a cost segregation study on the whole home and apply a portion of the cost segregation study based on square footage and bonus depreciate that? For example if they identify 500K of bonus depreciable assets and the square footage of the str is 800 sqft and the home square footage is 2000 sqft, can I bonus depreciate 500K * (800/2000)?

Or do I need to get a separate cost segregation study done on only the portion of the home that is an STR? Or are both methods ok? Why or why not?

Malik Javed · Answer

@Robert S.

This is a really common question, and your logic isn’t crazy. The issue is how you’re applying it.

Your square footage approach (800 / 2000) is fine for allocating the property itself between personal and rental use. That part is totally acceptable.

Where it gets shaky is taking a full-property cost seg result (like the 500K) and then just pro-rating that number. A cost seg study isn’t just a lump sum, it’s identifying specific assets in specific parts of the property. When you do it on the whole home, it includes personal areas and components that may not even exist in the STR portion.

So if you just multiply 500K by 40%, you’re kind of assuming all those accelerated assets exist proportionally in the rental space, which often isn’t true.

What works better (and is more defensible):

• Either do the cost seg specifically on the STR portion, or
• Allocate the property first (based on sqft), then apply cost seg to that portion only

That way, the accelerated depreciation is tied directly to assets actually used in the rental activity.

Bottom line:

• Square footage allocation is fine
• Pro-rating the results of a full cost seg is risky
• Allocating first, then doing cost seg (or scoping it to the STR) is the clean way to do it

It’s one of those situations where the math feels right, but the IRS cares more about how the assets are actually being used than just the ratio.

Jonathan Hersh · Answer

@Jason Malabute Good call on the passive loss question -- it needs to come first. No sense paying for a study if the losses are just going to sit there.

On the methodology side though, I'd be careful with the whole home prorate approach. It can work on paper,   but cost seg studies are identifying specific physical stuff: the kitchen cabinets, bathroom tile, lighting fixtures, etc. That stuff isn't spread evenly across a house. The STR floor might be fully renovated while the rest of the home still has original finishes from the 90s. So when you apply a flat 40% to the total,    you're assuming all the accelerated components are distributed proportionally, and in my experience that's usually not the case.

Scoping the study to just the rental portion keeps everything tied to actual assets in the actual space. Way cleaner if the IRS ever comes knocking, especially on a mixeduse property where they're already going to look more closely at that personal vs rental line.

Jason Malabute · Answer

Robert, your thinking is on the right track, and the square footage allocation approach is generally the correct method for splitting expenses between personal and business use. You can absolutely do a cost segregation study on the whole home and then allocate the results based on the rental square footage percentage (800/2000 = 40% in your example).

The key thing to understand is that only the rental-use portion of those accelerated depreciation components is eligible for bonus depreciation. So if the cost seg identifies $500K in shorter-lived assets, you would apply your 40% business-use ratio and bonus depreciate $200K. You don't need a separate cost seg study just for the STR portion — one study on the whole property with a proper allocation works fine.

A few things to keep in mind. Make sure you can substantiate the exclusive business use of that floor. The IRS looks closely at mixed-use properties, so having a clear separation (separate entrance, dedicated space, etc.) helps. Also, if this STR has an average guest stay of 7 days or less, it can qualify as a non-passive activity, which means those bonus depreciation losses could potentially offset your W-2 or other active income rather than being suspended as passive losses. That is a huge benefit that a lot of investors miss.

Of course, the exact treatment depends on your full tax picture, so work with a CPA who understands both cost segregation and the STR loophole to make sure everything is set up correctly.

Jonathan Hersh · Answer

@Robert S. Malik's right that you don't want to just prorate a whole home study. The cleaner approach is to scope the study to the STR portion from the start.

If the STR doesn't have its own kitchen and bathroom, you may have a bigger issue than the cost seg methodology. The IRS is going to want to see that the rental portion is genuinely separate and exclusively used for the STR. That affects your entire depreciation claim, not just the cost seg piece.

Assuming the unit is self contained: get the cost seg done on just the STR portion, bonus depreciate the reclassified components, and you'll be saving in no time!

Separate Cost Seg for ADU?

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