ADU Cost Segregation: How Accessory Dwelling Units Qualify for Bonus Depreciation
ADUs are the most cost-seg-friendly property type I encounter. Here is why, and how to maximize your deductions.
Matthew Gigantelli
Lead Cost Seg Engineer · ASCSP M009-25
ADUs are the fastest-growing segment of residential construction in the United States. California alone permitted over 20,000 ADUs in 2023. But most ADU investors treat the tax side as an afterthought, letting their CPA depreciate the entire structure over 27.5 years, generating roughly $6,909 per year on a $200,000 build. With cost segregation and 100% bonus depreciation — permanently restored by the One Big Beautiful Bill Act for property placed in service after January 19, 2025 — that same ADU can generate $50,000-$70,000 in first-year deductions. At a 37% marginal rate, that is $18,500-$25,900 in actual cash tax savings. The study typically costs $1,500-$3,500, producing a 5x-17x return.
Why ADUs Are a Cost Segregation Sweet Spot
Four structural factors make ADUs produce higher reclassification rates (25-35%) than standard residential acquisitions (20-28%).
Clean construction records. Unlike acquisition studies where engineers estimate component costs using RS Means, ADU builders have exact invoices for every line item. The cost segregation engineer does not need to estimate what the appliances cost or what percentage went to landscaping — it is on the invoices.
Nearly 100% depreciable construction costs. When you buy an existing property, you allocate 15-25% to non-depreciable land. With an ADU, the land already exists as part of your primary residence. ADU construction cost is almost entirely depreciable, with only 5-10% allocated to incremental land value.
Heavy weighting toward short-lived property. A typical detached ADU includes appliances, cabinetry, flooring, specialty lighting, window treatments, landscaping, fencing, driveway extensions, and outdoor lighting — all 5-year or 15-year property. The structural shell represents a smaller percentage of total cost than in a full-size home because ADUs are compact (400-1,200 SF).
Disproportionate site work. ADU construction requires significant site work: utility trenching, sewer/water connections, electrical service upgrades, grading, drainage, and landscaping restoration. Site improvements are 15-year property eligible for 100% bonus depreciation. On a typical detached ADU, site work represents 12-18% of total construction cost.
ADU Cost Segregation by Type
| ADU Type | Typical Cost | Depreciable Basis | Est. Accelerated (25-35%) |
|---|---|---|---|
| Detached new construction | $150K-$350K+ | 90-95% of cost | $38K-$116K |
| Garage conversion | $80K-$180K | 92-97% of cost | $20K-$60K |
| Attached addition | $120K-$280K | 90-95% of cost | $30K-$93K |
| Basement conversion | $60K-$150K | 95-100% of cost | $15K-$52K |
State Conformity Warning
California, New York, and New Jersey do NOT conform to federal bonus depreciation. If you live in one of these states, your federal and state depreciation schedules will diverge. Your CPA needs to track both. The federal benefit of cost segregation remains substantial, but do not assume your state tax return will reflect the same first-year deductions.
Worked Example: $220K Detached ADU in California
Project Details
ADU type: 650 SF detached new construction in Los Angeles
Total construction cost: $220,000
Land allocation: $13,200 (6% — minimal because land already exists as part of primary residence)
Depreciable basis: $206,800
Cost segregation component breakdown:
- Appliances (refrigerator, range, dishwasher, washer/dryer): $8,200 — 5-year
- Cabinetry and countertops: $14,500 — 5-year
- LVP flooring throughout: $7,800 — 5-year
- Light fixtures, ceiling fans, bathroom fixtures: $5,400 — 5-year
- Window treatments: $1,800 — 5-year
- Utility trenching and sewer connection: $12,000 — 15-year
- Landscaping restoration and fencing: $9,500 — 15-year
- Driveway extension and grading: $6,800 — 15-year
Total reclassified: $66,000 (32% of depreciable basis)
Year 1 bonus depreciation: $66,000
Year 1 straight-line on remaining $140,800: $5,120
Total Year 1 deductions: $71,120
Federal tax savings at 37%: $26,314
California note: CA does not allow bonus depreciation. State depreciation: $206,800 ÷ 27.5 = $7,520/year. The $26,314 federal savings still applies — you just will not see accelerated deductions on your CA return.
Study cost: $2,000-$3,500 for a new-construction ADU with organized invoices. ROI: 7.5x-13x.
ADU Cost Segregation Checklist
During construction: Keep every invoice organized by trade (electrical, plumbing, flooring, appliances, site work). This is the single biggest factor in maximizing reclassification.
Order the study early: Engage the cost segregation engineer during construction so they can flag components in real time. This produces higher-quality results than a post-completion study.
Document placed-in-service date: The date the ADU receives its certificate of occupancy (or is otherwise available for rent) determines which tax year you claim deductions.
Check state conformity: If you are in CA, NY, or NJ, your CPA must track separate federal and state depreciation schedules.
Photograph everything: Before drywall closes, photograph plumbing runs, electrical rough-in, and HVAC ductwork. These photos support the engineering classification.
Frequently Asked Questions
Do ADUs qualify for cost segregation?
Yes. ADUs used as rental property qualify for cost segregation and, if placed in service after January 19, 2025, for 100% bonus depreciation on reclassified components. ADUs typically reclassify 25-35% of construction cost to accelerated categories — higher than standard residential acquisitions.
How much can I save with cost segregation on an ADU?
A typical $200,000 detached ADU can generate $50,000-$70,000 in first-year deductions through cost segregation with 100% bonus depreciation. At a 37% marginal rate, that translates to $18,500-$25,900 in actual tax savings in the year the unit is placed in service.
When should I order a cost segregation study for my ADU?
During construction, not after completion. When the engineer receives organized cost records from an active project, they can flag components in real time and ensure nothing gets categorized incorrectly. This produces the highest-quality study with maximum reclassification.
Can I use the STR loophole with an ADU?
Yes. If you list the ADU as a short-term rental with an average guest stay of 7 days or less and materially participate, the losses (amplified by cost segregation) can offset your W-2 income directly. ADUs in tourist or urban areas are particularly well-suited for this strategy. See our W-2 earner cost segregation guide for details.
For ADU-specific cost segregation tax strategies, see Overline's ADU cost segregation tax strategy guide.