The deduction sitting in a building your client already owns

Every year, CPAs look for current-year deductions in the usual places — retirement plans, accruals, credits. One of the largest is frequently sitting untouched on the depreciation schedule: a building that has been depreciating straight-line over 39 years (or 27.5 for residential) since the day it was placed in service.

A cost segregation study performed today on a building placed in service years ago does not just change depreciation going forward. Through a Section 481(a) catch-up adjustment filed on Form 3115, it delivers every dollar of depreciation the taxpayer should have taken in prior years as a single deduction on the current-year return. No amended returns. No reopening closed years. And because the change is automatic, no user fee and no advance IRS approval.

Here is how the mechanics work, what the numbers look like, and the passive-activity trap — and election — that determines whether the deduction is actually usable when the client’s operating business and its real estate sit in separate entities.

The mechanics: an impermissible-to-permissible method change

Depreciating an entire building over 39 years when a portion of its cost is properly 5-, 7-, or 15-year property is, after two or more returns, an adopted method of accounting. Correcting it requires the Commissioner’s consent under Section 446(e) — which, for this change, is granted automatically. The change is filed on Form 3115 under the automatic procedures of Rev. Proc. 2015-13 (list of automatic changes in Rev. Proc. 2024-23 or its successor), using DCN 7 — a change from an impermissible to a permissible method of depreciation.

The engineering-based study allocates the building’s cost among:

• 5- and 7-year personal property — carpeting, decorative millwork, specialty electrical and plumbing serving equipment, cabinetry, process-related systems;
• 15-year land improvements — paving, curbing, site utilities, landscaping, fencing, exterior lighting;
• 39-year (or 27.5-year) real property — the shell, core, and general building systems.

The Section 481(a) adjustment is the difference between depreciation actually taken and what would have been allowable had the correct classifications applied from day one — including bonus depreciation at the rate in effect for the year the property was placed in service. For property placed in service between September 28, 2017 and the end of 2022, that rate is 100%. Because the taxpayer under-depreciated, the adjustment is negative — a deduction — and under Rev. Proc. 2015-13 a negative adjustment is taken entirely in the year of change. (Positive adjustments spread over four years; negative ones do not.)

Two more features worth telling clients about:

• Timing. The Form 3115 is filed with the timely filed (including extensions) return for the year of change, with a duplicate copy to the IRS in Ogden. A study commissioned even late in filing season can still land on the current return.
• Audit protection. Under the automatic procedures, the IRS generally may not raise the same depreciation issue for the prior years being corrected. The taxpayer is not exposed by fixing it — the exposure runs the other way.

What the numbers look like

Assume a client purchased its building in March 2019 — $4,000,000 of depreciable basis (land excluded) — and has depreciated the whole thing straight-line over 39 years. A 2026 study reclassifies 17% to 5-year property and 11% to 15-year land improvements:

Component	Basis	Allowable through 2025 (proper method)
5-year property (100% bonus in 2019)	$680,000	$680,000
15-year land improvements (100% bonus in 2019)	$440,000	$440,000
39-year building (≈17.4% cumulative)	$2,880,000	$501,600
Total allowable	$4,000,000	$1,621,600

Depreciation actually taken on the as-filed 39-year schedule through 2025 is roughly $696,700. The Section 481(a) adjustment is $1,621,600 − $696,700 = a $924,900 deduction on the 2026 return. At a 37% marginal rate, that is approximately $342,000 of current-year federal tax — from costs the client already incurred, on a return not yet filed.

For property acquired after January 19, 2025, the OBBBA made 100% bonus depreciation permanent, which makes the same analysis on newly acquired buildings a year-one planning item rather than a look-back exercise. For look-back studies, the bonus rate follows the placed-in-service year — 80% for 2023, 60% for 2024 — unless an election out was made.

The trap: the building is usually in a different entity than the business

Here is where a technically perfect study can fail to produce a usable deduction.

The standard structure — for liability and estate planning reasons, often at the CPA’s own recommendation — puts the real estate in an LLC that leases it to the commonly owned operating company. Under Section 469, that rental is a passive activity, and rental losses are passive per se regardless of how many hours the owner works. The catch-up deduction lands in the rental entity, produces a large rental loss, and — absent other passive income — is suspended under Section 469 rather than offsetting the owner’s active business income.

The self-rental rule makes this worse, not better. Under Reg. §1.469-2(f)(6), net rental income from property rented to a business in which the taxpayer materially participates is recharacterized as nonpassive — so it cannot absorb passive losses from other activities — while a net rental loss from the same arrangement stays passive. Heads the IRS wins, tails the taxpayer waits.

The fix: the Reg. §1.469-4 grouping election

Reg. §1.469-4 permits a taxpayer to treat multiple activities as a single activity if they form an appropriate economic unit. Rental activities face a higher bar for grouping with a trade or business — Reg. §1.469-4(d)(1) allows it only where the rental is insubstantial relative to the business (or vice versa), or where each owner of the business has the same proportionate ownership interest in the rental.

That second door is exactly the owner-occupied fact pattern: the same individuals own 100% of the operating company and 100% of the real estate LLC. Grouped, the rental and the business are one activity; the owner’s material participation in the business covers the combined activity; and the cost segregation catch-up deduction offsets active business income in the current year instead of waiting in a suspended-loss carryforward.

The procedural points that matter:

• Disclosure. Rev. Proc. 2010-13 requires a written statement with the original return for the year a new grouping is made, identifying the activities grouped and declaring they constitute an appropriate economic unit.
• Stickiness. Groupings are generally binding for all future years. Regrouping is permitted only on a material change in facts (or if the original grouping was clearly inappropriate). Group deliberately — this is a structural decision, not an annual toggle.
• Consistency with prior treatment. If the activities have simply been reported separately without an affirmative grouping, taking the position that a new grouping is being made now is common — but review the history before assuming it. Prior affirmative treatment can foreclose the election.
• Who it applies to. Section 469 applies to individuals, estates, trusts, personal service corporations, and closely held C corporations. For pass-through structures, the grouping analysis runs at the owner level.
• Alternatives. A taxpayer who qualifies as a real estate professional under Section 469(c)(7) — with material participation in the rental — reaches a similar result by a different road. For most operating-business owners, the hours tests make the grouping election the far more realistic path.

Sequencing the engagement

1. Screen the fixed asset schedule. Any building with $1M+ of depreciable basis, placed in service in the last 15 years and not previously studied, is a candidate. Purchased buildings qualify, not just new construction.
2. Confirm the entity map and Section 469 posture first. Where does the loss land, and can it be used? If the answer requires a grouping election, plan the election and the disclosure statement with the return the study will ride on.
3. Commission the engineering study. Reclassification percentages of 20–35% are typical for commercial buildings; the IRS’s own Cost Segregation Audit Techniques Guide describes the engineering-based approach as the most reliable.
4. File the Form 3115 with the current-year return (duplicate to Ogden), with the Section 481(a) computation attached, and the grouping statement where applicable.
5. Model the exit. Reclassified 5/7/15-year property generates Section 1245 recapture on sale, and grouped activities affect how suspended losses free up on disposition. The deduction is an acceleration, not free money — its value is time, rate arbitrage, and present dollars.

The takeaway

A look-back cost segregation study is one of the few planning tools that converts prior-year conservatism into a current-year deduction with no amended returns and audit protection included. But for the common structure — operating business in one entity, building in another — the depreciation analysis and the Section 469 analysis are one engagement, not two. The study creates the deduction; the grouping election is what lets the client actually use it.

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Corporate Tax Advisors performs engineering-based cost segregation studies and works alongside CPAs on the Form 3115 filings, Section 481(a) computations, and passive-activity analysis that turn a study into a usable current-year deduction. If you have a client fact pattern you would like to walk through, we are glad to look at it with you before anything is filed.

This article is general information, not tax advice, and does not create a client relationship. The passive activity rules in particular are fact-dependent — apply them to your specific situation with your tax advisor.