Does a cost seg apply to only 1 portion of a multi family?

Question

Here’s our situation: My partner and I bought a rundown two-family and put about $150K into it (electrical, HVAC, and a full cosmetic refresh on one of the two units). Both units are now rented, but the property is still operating at a loss.

We’re considering finishing the basement and converting it into a two-bedroom short-term rental. We’re close to Harvard Square, so the STR demand is potentially strong. The basement renovation will cost around $300K.

My understanding is that it doesn’t make sense to do a cost segregation study now because we don’t have rental income to offset yet (we’re operating at a loss). But if we finish the basement, launch it as an STR, and materially participate, then we could use a cost segregation study to offset our high W-2 incomes.

Does that logic make sense? And should we do it?

Someone mentioned that you can’t cost-seg the whole property if only one-third of it is being used as an STR, but I’m not sure that’s accurate. My understanding is that the rule isn’t about STR vs. LTR—it’s about whether you have non-passive income to offset and whether you materially participate.

Reyjie Sobebe · Answer

Hello Alicia,

Your logic is on the right track. Since the long-term units are currently producing a loss, a cost seg today wouldn’t actually help you because the losses would stay passive. But if you build out the basement STR, treat it as its own activity, and materially participate, then the bonus depreciation from a cost seg on the STR portion can offset your W-2 income. You don’t need the entire property to be an STR what matters is that the STR activity itself qualifies as non-passive. Many people get tripped up thinking you need to cost-seg the whole building, but you can absolutely allocate and run a study on the STR portion only. The key is having clean hour logs, especially once cleaners, handymen, or contractors are involved. I’ve found that using a simple, automated tracking system for rentals has made it way easier to document hours in a way that would hold up in an audit, so getting that in place before reno/launch is huge.

Ashish Acharya · Answer

@Alicia Palleschi, Your logic makes sense. Cost segregation studies are most beneficial when you have non-passive income to offset or you already have other passive income. The depreciation on top of losses is also good. It will not go to waste. If you don't do cost seg now, you will have to file 3115 later which makes things complicated.

Once you finish the basement and start generating short-term rental (STR) income, you could definitely use material participation and a cost segregation study to offset your high W-2 income. Even with the LTR, you can do the same with REPS.

You're also correct that the key factor for cost segregation isn’t whether it’s an STR or LTR, but whether you have non-passive income to offset and whether you meet the material participation requirements (or you have passive income).

I just sent you some resources that will be helpful for you. Feel free to check them out!

James Jones · Answer

Alicia, your logic is on the right track, but here’s the clearer framework so you don’t accidentally miss or misinterpret the rules.

1. Yes, you can cost-seg only the STR portion, but you’re not limited to only that.

You’re allowed to run a cost segregation study on the entire property even if only part of it is used as a short-term rental. The key is allocation.

A cost seg breaks out the components of the whole building. You then allocate depreciation between:

• The STR portion (non-passive if you materially participate)
• The LTR portion (passive unless you qualify as REPS)

The STR portion (non-passive if you materially participate)

The LTR portion (passive unless you qualify as REPS)

So you’re not restricted from studying the whole asset. You’re just restricted in what losses you can use depending on participation.

2. The STR rules matter more than LTR vs STR mix.

For the STR portion, if:

• The average stay is under 7 days and
• You materially participate

The average stay is under 7 days and

You materially participate

Then that STR activity is non-passive, even if you're nowhere near REPS.

That means: ➡️ STR-side bonus depreciation can offset W-2 income. ➡️ LTR-side depreciation stays passive and can’t offset W-2, but still accumulates.

This is why many investors carve out a unit for STR specifically.

3. Doing the cost seg before vs. after finishing the basement

You’re correct: It doesn’t make much sense to do a cost seg now while you have no offsettable income and no STR activity.

If you finish the basement and begin operating it as an STR:

• Now you have non-passive income (potentially)
• And a large additional depreciable basis to run a study on

Now you have non-passive income (potentially)

And a large additional depreciable basis to run a study on

That’s when the study becomes meaningful.

4. Yes, the entire $300K basement build-out becomes eligible basis

The finished STR space adds:

• 5-year property
• 7-year property
• 15-year land improvements

5-year property

7-year property

15-year land improvements

All of which are bonus-depreciable in a cost seg.

For many STR operators, the basement alone can generate six-figure paper losses.

Bottom Line

✔️ Your logic makes sense
✔️ You can cost-seg the whole property
✔️ What matters is material participation
✔️ The STR portion is what unlocks W-2 offsets
✔️ Waiting until the basement is done is the smart move

If the numbers work and you’re confident you can hit material participation hours, the strategy is very viable.

Sophanavy Prak · Answer

I would recommend speaking to a CPA that specializes in Cost Seg. My understanding to offset your W-2 is dependent on real estate professional status. Sorry I can’t be of any more help. Best of Luck!

Jeffrey Blais · Answer

Hello Alicia:

Do you need to finance the $300K, or do you and your partner have those funds currently available? Knowing that info would be helpful, please let me know and I'll let you know my honest opinion.

Thank you in advance,

Jeffrey

Does a cost seg apply to only 1 portion of a multi family?

Does a cost seg apply to only 1 portion of a multi family?

Most Popular Reply

5 Replies

1. Yes, you can cost-seg only the STR portion, but you’re not limited to only that.

2. The STR rules matter more than LTR vs STR mix.

3. Doing the cost seg before vs. after finishing the basement

4. Yes, the entire $300K basement build-out becomes eligible basis

Bottom Line