Detached Structures: Barn, ADU, Detached Garage in Cost Seg
How a detached barn, garage, or ADU shows up as a 15-year land improvement bump, how the detached_sqft scaling works, and a worked example.
Detached Structures: Barn, ADU, Detached Garage in Cost Seg
This tutorial covers a single mechanic: how the calculator scales the 15-year land-improvement bucket based on the size of detached structures on the property — a barn, a workshop, a detached garage, an accessory dwelling unit. The mechanic lives in the detached_structures and detached_sqft fields and produces a percentage bump capped at 3% of basis.
Why a detached structure is not part of the building shell
The 27.5-year (residential rental) and 39-year (nonresidential real property) MACRS schedules apply to the building shell: walls, roof, foundation, structural framing of the dwelling unit being rented. A separately-constructed structure on the same parcel — a barn, a detached two-car garage, an ADU on a slab in the backyard — is a separate asset. Pub 946 Asset Class 00.3 (land improvements) and 01.1 through 01.4 (farm-related structures) handle most of these as 15-year or 20-year MACRS property.
The cost-seg implication: the cost basis attributable to a detached structure should not ride the 27.5-year schedule by default. Engineering deliverables call this out explicitly — a detached 1,200-sqft barn on a rural Hill Country STR represents real basis dollars that an engineer will reclassify to a faster recovery period.
What the calculator's detached_sqft field does
The questionnaire collects two inputs:
detached_structures— a list of structure types (garage,carport,barn,workshop,adu, etc.).detached_sqft— the total enclosed square footage across those structures.
The engine maps detached_sqft to a 15-year-bucket bump in _compute_adjustments (cost_seg/engine.py):
``python if inp.detached_structures and inp.detached_sqft > 0: pct = min(inp.detached_sqft / 100 * 0.001, 0.03) ``
That formula reads: 0.1% per 100 square feet, capped at 3%. A 600 sqft detached garage adds 0.6% to the 15-year class. A 1,500 sqft barn adds 1.5%. A 3,500 sqft set of structures hits the 3% cap. The cap exists because beyond a certain size the structure starts approaching the dollar-impact of the dwelling itself, and the formula can no longer pretend a single percentage of total basis represents the structure honestly.
This is a screening simplification. A real engineering study would assign a cost-per-square-foot to the detached structure (RS Means or vendor invoice) and reclassify that dollar amount precisely; the calculator's approach is good enough to surface whether a study is worth commissioning.
Worked example — STR with detached barn and ADU
A property owner buys a 4-bedroom rural STR in March 2026 for $850,000 with $200,000 allocated to land, leaving $650,000 of depreciable basis. The property has a 1,400 sqft detached barn (used for tenant storage and event hosting) and a 600 sqft detached ADU rented separately on the same listing — total detached_sqft = 2,000.
The detached bump: 2,000 / 100 × 0.001 = 0.020 = 2.0% added to the 15-year class.
Base STR allocation (13% / 1% / 8%) plus the 2.0% detached bump on the 15-year class:
- 5-year: $650,000 × 13% = $84,500
- 7-year: $650,000 × 1% = $6,500
- 15-year: $650,000 × (8% + 2%) = $650,000 × 10% = $65,000
- 27.5-year building: $650,000 × 76% = $494,000
At 100% bonus depreciation, the year-1 deduction on the accelerable bucket ($156,000) is $156,000 plus partial-year building S/L (March placement, 9.5 months) of $494,000 / 27.5 × 9.5/12 ≈ $15,200. Total year-1 deduction ≈ $171,200. At a 32% bracket, that's roughly $54,800 of year-1 federal tax effect.
The detached structures are contributing roughly $13,000 of accelerable basis ($650,000 × 2%), which translates to about $4,200 of year-1 tax effect on a 100% bonus year — the difference between a 50% bracket and a 60% bracket on a CPA conversation.
Try an STR with detached structures →
When a detached structure is its own asset entry
Engineering deliverables sometimes treat a substantial detached structure (large barn, full-sized ADU, separate workshop) as a separately-listed asset on the depreciation schedule, with its own placed-in-service date and basis. The cost is allocated from the purchase price (or from construction invoices if built later), and the structure depreciates on its own MACRS line. This produces a more defensible audit position than rolling the structure into a percentage of total basis.
For a screening calculator the bump is sufficient — but the property owner should know the engineering version goes further. An ADU large enough to operate as a separate rental unit, in particular, is best handled as its own depreciable asset.
Compare a property without detached structures →
Citations
- IRC §168(e)(3)(E)(iv) — 15-year MACRS for qualified land improvements.
- IRS Publication 946, Asset Class 00.3 — land improvements definition.
- IRS Publication 946, Asset Classes 01.1-01.4 — farm-related structures.
- IRC §168(k)(6) — 100% bonus depreciation, OBBBA permanent.
Disclaimer. This tutorial describes general federal tax concepts. TaxProtestTx (Nought Labs LLC) is a feasibility-screening tool, not tax advice or a cost segregation study. Calculator output cannot be relied on under Treasury Circular 230. Consult a qualified CPA, EA, or attorney before filing. Results are not guaranteed.