ADU Cost Segregation: What Counts as Basis When You Built It Yourself (2026)

Accessory dwelling units reclassify well, and owner-builders get the basis wrong more often than any other group we work with. An ADU packs a full kitchen, full bath, complete MEP and finished interior into a compact footprint, so a larger share of the money sits in short-lived components — cabinets, appliances, flooring, fixtures, walkways, landscaping — than in structural shell. Typical reclassification runs 7–14% of depreciable basis, with around 9% representative. With 100% bonus depreciation permanent under the One Big Beautiful Bill Act, every reclassified dollar is deductible in Year 1. For a guide to what cost segregation is and how it works, start there.

This guide leads with basis rather than with benefit, because on a self-built ADU that is where the money is won or lost. If you hired a general contractor and have one final invoice, skip ahead to detached vs. attached.

What Counts as Your ADU’s Cost Basis

Depreciable basis is what the ADU cost you. Not what it is worth, not what it would cost to hire out, not what you would have charged a client. Cost.

For a self-built ADU, that means everything you actually paid for:

• Materials — lumber, concrete, roofing, windows, siding, drywall, flooring, tile, fixtures, cabinets, finishes, and the sales tax on all of it
• Subcontractors — every trade you wrote a check to: concrete, framing help, electrical, plumbing, HVAC, roofing, stucco, drywall
• Permits and fees — building permits, plan check, school and park impact fees, utility connection and meter fees
• Design and engineering — architect, draftsman, Title 24 or energy calcs, structural engineer, survey, soils report
• Equipment and site costs — tool and equipment rental, scaffolding, dumpsters and hauling, portable toilet, temporary power
• Utility work — trenching, sewer lateral, water and gas line extensions, panel upgrade
• Construction-period interest and certain carrying costs, if you financed the build

What does not count: your own labor

If you framed it yourself, your hours are not basis. Not at your shop rate, not at a market rate, not at any rate.

The reason is structural rather than technical. Basis is cost, and your own time did not cost you cash — you never took its value into income, so there is nothing there to recover through depreciation. It is the same reason you cannot deduct the value of volunteer services you donate to a charity. If sweat equity created basis, you would be deducting income you never reported.

This is the single most common surprise for owner-builders, and it usually surfaces in the worst way: someone totals what the ADU “cost to build” including their own labor, we build the study off that number, and the basis is overstated by exactly the part that isn’t deductible. A study built from your actual ledger is not the conservative option here. It is the only correct one — a study modeled from square footage would hand you a bigger number than the law allows, and it is your return, not our model, that has to survive.

There is one legitimate path, and it is usually a wash. If you operate a contracting business, that business can bill the rental activity at arm’s length. Then there is a real cost, and it is basis. But the business takes the fee into income and you pay tax on it now — likely ordinary rates plus self-employment tax — to buy depreciation you would mostly accelerate into Year 1 anyway. It is also a related-party arrangement that has to be papered properly. Sometimes the timing works; usually it doesn’t. This is a conversation for your CPA before the work is billed, not after.

What about the land?

If you built the ADU on land you already owned, there is no land allocation against your build cost. Your construction spend is 100% improvements. The land already has its own basis, it isn’t depreciable, and it doesn’t change because you built on it.

This matters more than it sounds. Owner-builders often assume the standard 20–25% land haircut applies to their build number the way it does on a purchase. It doesn’t, and applying it throws away a fifth of the deduction.

The land allocation question only appears when you bought a property that already had an ADU on it. Then the purchase price does have to be split between land and improvements, and again between the main house and the accessory unit.

Getting Your Invoices Organized

Most owner-builders have a shoebox, a bank feed, and a memory. What the study needs is a list.

Download the cost schedule template (or the CSV) and put one line per item: what it was, what you paid, and when it was finished. You do not assign tax categories — we do that, because a study where the taxpayer picked the recovery periods is not a defensible study.

Two things make a large difference to what gets accelerated:

Describe the work, not the payee. “Epic Electric” tells us nothing. “Rough electrical” tells us it’s structural; “recessed can lights and fixtures” tells us part of it isn’t. Lines that carry only a contractor’s name, a check number, or a supplier invoice number can’t be classified from the description, and they default to the long structural life.

Be specific about finishes. “Bathroom” is structural. “Tile — bathroom floor and surround” is flooring. Cabinets, countertops, vanities, flooring, light fixtures, appliances, blinds, walkways, fencing, landscaping and irrigation are all worth calling out by name rather than folding into a line that says “interior finishes.”

If the invoices are easier to send than the spreadsheet, send the invoices. We can read them. What we can’t do is recover a scope of work that was never written down anywhere.

Detached vs. Attached ADUs: Different Tax Treatment

This distinction matters more than most ADU owners realize.

Detached ADUs — backyard cottages, casitas, converted garages that are physically separate from the main house — are treated as separate residential rental property. If you rent the unit, it depreciates over 27.5 years under the standard MACRS schedule for residential rental property. This is the straightforward case. The ADU is its own building, its own tax asset, and its own depreciation schedule.

A purpose-built backyard cottage is also new construction, so the placed-in-service date is the one on your certificate of occupancy — or when the unit was ready and available to rent, whichever the facts support. See cost segregation for new construction.

Attached ADUs — garage conversions, basement apartments, additions that share walls or a roofline with the main house — are part of the main building rather than their own, so the building’s classification governs the structural residual. That raises the 80% residential rental test, and it is worth being precise about, because it is widely repeated in a form that isn’t right.

Under IRC Section 168(e)(2)(A), a building is “residential rental property” (27.5-year life) when 80% or more of its gross rental income comes from dwelling units. The common telling is that living in the main house makes the building fail, because your own half produces no rent. That is not how the section works. Section 168(e)(2)(A)(ii)(I) provides that if you occupy part of the building, the rental value of the portion you occupy is included in gross rental income — and because your half is itself a dwelling unit, it counts toward the 80% as well as into the total. Two dwelling units, one of them yours: 100% of gross rental income is from dwelling units, and the building is residential rental at 27.5 years.

What the test actually screens for is mixed-use: apartments over a shop, a storefront with units above. If the commercial rent exceeds 20% of the building’s total, the whole structure becomes nonresidential and depreciates over 39 years. An owner-occupied two-family is not that case.

So for most attached ADUs the answer is 27.5 years. Raise it with your CPA if there is genuine commercial space in the building; otherwise it is a distraction.

Either way, cost segregation accelerates a portion of your ADU’s depreciable basis into 5-year and 15-year classes. The base depreciation schedule affects the residual, not the reclassified amount — so the 27.5-vs-39 question changes the tail, never the part you deduct in Year 1.

One term worth not borrowing: qualified improvement property (QIP) does not apply to an ADU. QIP under IRC Section 168(e)(6) is an interior improvement to nonresidential real property. A residential rental — attached or detached — is outside it entirely.

Why ADUs Reclassify Well

In a typical 2,000-square-foot house, the kitchen and bathrooms might be 15–20% of construction cost. In a 500-square-foot ADU, those same systems — cabinets, countertops, appliances, plumbing fixtures, tile, vanities — can be 30–40%. Most of those components are 5-year personal property, so the reclassification percentage runs higher than the house they sit behind.

Other factors that help:

• Full MEP in a small footprint — HVAC, electrical panel, plumbing rough-in and water heater are required regardless of square footage, so they’re a larger share of the total
• Dedicated site improvements — walkways, landscaping, irrigation, fencing, utility runs and outdoor lighting installed for the ADU are 15-year land improvements
• Newer construction with real documentation — you know what went in and what it cost, which is exactly what a documented-cost study runs on
• Simpler structural profiles — slab foundation, standard framing, single roofline means less budget in shell and more in finishes

What this does not mean is an unusually high headline percentage. Measured across delivered ADU studies, reclassification runs 7–14% of depreciable basis, with about 9% representative. Anyone quoting you 20%+ on an ADU is quoting a number we don’t see.

The Numbers: What to Expect

Example 1 — $150,000 detached ADU, owner-built, Los Angeles

Documented build cost (materials, subs, permits, design)	$150,000
Land allocation	none — built on land you already own
Depreciable basis	$150,000
5-year personal property	$9,750
15-year land improvements	$3,750
27.5-year structural residual	$136,500
Accelerated total (9%)	$13,500

At 100% bonus, the full $13,500 is deductible in Year 1. At a 37% marginal rate that’s about $5,000 of federal tax against a $1,295 study fee — roughly 3.9:1 in Year 1, before the ongoing schedule.

Note what did not happen: no land was subtracted, because none was bought. And if you framed and finished it yourself, the $150,000 is what you paid out — your labor isn’t in there.

Illustrative. Actual percentages vary with finish level, site work, and what the documentation supports.

Example 2 — $250,000 attached ADU (garage conversion), Portland

A conversion often lands at the upper end of the range, because the shell already exists and nearly all of the new spend goes into finishes, MEP and fixtures rather than foundation and framing.

Documented conversion cost	$250,000
Depreciable basis	$250,000
Accelerated at 13%	$32,500
Year-1 federal tax at 37%	~$12,025
Study fee	$1,495

Oregon conforms to federal bonus depreciation, so the full benefit flows to the state return with no add-back.

Want numbers for your own ADU? The cost segregation calculator takes 30 seconds and needs no signup.

Your Hours Aren’t Basis — But They May Still Matter

Here’s the part that surprises owner-builders in the other direction.

Your construction hours do nothing for basis. But under IRC Section 469(c)(7), a “real property trade or business” expressly includes construction and reconstruction. So hours you spend building can count toward real estate professional status — the 750-hour test and the “more than half your personal services” test.

That matters because REPS is what decides whether a cost segregation loss on a long-term rental ADU can offset your other income. Without it, the loss is passive and sits suspended until you have passive income or sell. If you work in the trades, you may already be closer to the threshold than you think.

Three caveats worth being straight about. REPS is tested year by year, not once. The hours must be in real property trades or businesses in which you materially participate. And clearing REPS still leaves a separate material-participation question on the rental activity itself, which your CPA may address with a grouping election under Reg. Section 1.469-9(g).

So keep a log — a real one, written as you go rather than reconstructed in April. Our time log template has a dashboard that runs the hour tests for you, and a tab for contractor hours, which is what the 100-hour test turns on.

ADU as Short-Term Rental: The STR Loophole

If you rent the unit on Airbnb or VRBO, there’s a second route to using the loss, and it doesn’t require REPS.

Under the normal passive activity rules, rental losses only offset passive income. The exception: short-term rentals with an average guest stay of 7 days or fewer are not treated as rental activity under those rules. If you also materially participate — commonly 100+ hours with nobody else doing more, or 500+ hours — the activity is non-passive, and accelerated depreciation can offset W-2 salary, 1099 income and other active income.

For a W-2 earner with a backyard ADU on Airbnb, this is usually the strongest configuration available. See our short-term rental page and the guide to using STR depreciation to offset W-2 income.

Already Built Your ADU? Form 3115

If you built your ADU in a prior year and have been depreciating it straight-line, you haven’t missed anything. The IRS allows a catch-up study through Form 3115 (Application for Change in Accounting Method).

Your CPA orders the study as though the ADU were placed in service today, then calculates the cumulative difference between the straight-line depreciation you claimed and the accelerated depreciation you should have claimed. That difference flows through as a single Section 481(a) adjustment on your current-year return. No amended returns. Form 3115 for cost segregation falls under the IRS’s automatic consent procedures (Rev. Proc. 2015-13, as modified), and the catch-up hits immediately.

If the ADU has been in service three or more years, the accumulated adjustment can be a large share of the whole benefit.

Which States Benefit Most?

State conformity decides how much of the federal benefit reaches your state return.

State	Bonus Depreciation	Impact
California	Does NOT conform	Federal benefit only; CA requires straight-line on the state return. Large ADU market, state add-back required.
Oregon	Conforms	Full bonus flows through to the state return.
Washington	No state income tax	Federal benefit only, no state return to manage.
Texas	No state income tax	Federal benefit only. Growing ADU market in Austin, Houston, Dallas.
Other states	Varies	Full state-by-state guide.

Is a Study Worth It for Your ADU?

Good candidates

• Rental ADU, long-term or short-term
• Documented build or conversion cost above roughly $150K
• You have taxable income to offset this year
• You plan to hold for 3+ years
• You have invoices, a builder’s ledger, or a bank feed you can reconstruct from

Weak candidates

• Build cost under about $125K, where the fee starts eating the benefit
• 22% marginal bracket or lower
• Selling within 1–2 years
• Used as a personal guest suite or home office rather than rented
• No documentation at all, and the build was mostly your own labor — there may be less basis than you think

Be honest about that last one. An owner-builder who spent $60,000 on materials and did everything else himself has $60,000 of basis, not the $180,000 the ADU would have cost to hire out. At 9% reclass that’s about $5,400 accelerated, and the study fee doesn’t clear.

For a broader look at pricing, see how much a cost segregation study costs, and how long the study takes — most reports deliver in 3–5 business days.

Frequently asked

Can I include the value of my own labor in my ADU's cost basis?

No. Depreciable basis is what the property cost you, and your own uncompensated time didn't cost you cash. You never took the value of your labor into income, so there is nothing to depreciate — the same principle that denies a charitable deduction for donated services. Materials, subcontractors, permits, plans, equipment rental, and utility connections all count. Your hours don't, at any hourly rate. This catches a lot of owner-builders by surprise, so it's worth knowing before you total up your costs.

Do ADUs qualify for cost segregation?

Yes. Any ADU used as rental or income-producing property qualifies for cost segregation. The ADU must be placed in service as a rental — whether long-term tenant, Airbnb, or other short-term rental arrangement. ADUs used solely as a personal guest suite or home office do not qualify. Detached ADUs (backyard cottages, casitas) are treated as separate residential rental property and depreciate over 27.5 years. Attached ADUs may depreciate over 39 years depending on the 80% residential rental test.

What if I rent my ADU on Airbnb?

ADUs rented as short-term rentals may qualify for the STR material participation loophole. If your average guest stay is 7 days or fewer and you materially participate (100+ hours per year of active management), the depreciation losses generated by cost segregation can offset your W-2 and other active income — not just passive rental income. This makes Airbnb ADUs one of the most tax-efficient configurations for W-2 earners.

Does the 80% residential rental test affect my ADU?

Less than most people are told. The test asks whether 80% or more of a building's gross rental income comes from dwelling units — it exists to separate residential buildings from mixed ones, like apartments over a shop. Owner-occupancy does not trigger it: under IRC Section 168(e)(2)(A)(ii)(I), if you occupy part of the building the rental value of your own portion is INCLUDED in gross rental income, and because your unit is itself a dwelling unit it counts toward the 80% as well. So a two-family where you live in one half and rent the other is residential rental property at 27.5 years, not 39. Where the test does bite is a building with genuine commercial space — ground-floor retail under the units — if that rent exceeds 20% of the total.

How much does an ADU cost segregation study cost?

An ADU you built or converted yourself is priced on its documented build cost: $995 up to $100K, $1,295 from $100K to $250K, and $1,495 from $250K to $500K. An ADU you acquired as part of a house, where basis is allocated by square footage, prices on the residential ladder starting at $495. Studies are delivered as a CPA-ready PDF with full component-level depreciation schedules.

Can I do cost segregation on an ADU I built years ago?

Yes. The IRS allows a lookback or catch-up study via Form 3115 (Application for Change in Accounting Method). Your CPA orders a cost segregation study on the ADU as if it were placed in service today, calculates the cumulative difference between the straight-line depreciation you claimed and the accelerated depreciation you should have claimed, and that entire difference flows through as a single Section 481(a) adjustment on your current-year tax return. No amended returns required.