Accessory dwelling units are having a moment. A wave of state and local zoning reform has made it easier than it’s been in decades to add a backyard cottage, convert a garage, or finish a basement into a legal rental unit, and for a lot of owners, that’s the cheapest square footage they’ll ever add to a property. What doesn’t follow automatically is that the tax treatment is simple, or that an ADU just tags along with however the main house was already handled.
An ADU isn’t a smaller version of the main house. For depreciation, it’s its own asset.
Whether it’s a detached cottage, a converted garage, or a finished basement, an ADU’s construction or conversion cost is tracked as its own addition to basis, with its own placed-in-service date, separate from the house it sits next to. That separateness is exactly why an ADU is worth a second look under cost segregation. A newly built ADU comes with a full kitchen, its own bathroom fixtures, its own HVAC (often a ductless mini-split), sometimes its own electrical sub-panel and meter, its own flooring, and its own driveway pad, walkway, and landscaping. That’s a dense concentration of short-life components relative to the square footage, more, proportionally, than you’ll usually find in an equivalent slice of an older main house.
Does building an ADU actually clear the 100% bonus depreciation bar?
100% bonus depreciation is back for property acquired after January 19, 2025, and “acquired” works differently depending on how you got the ADU. If you bought a property that already had an ADU on it, the binding-contract date on your purchase controls, same as any acquisition. But if you built or converted the ADU yourself, self-constructed property uses a different test: the acquisition date is generally the date construction began, not when you signed a contract, and not whenever you bought the underlying lot. In practice, that means an ADU you broke ground on after January 19, 2025 can qualify for the full 100% rate even if you’ve owned the main house since 2015 and it’s already fully depreciated on the old schedule.
What a study actually finds inside an ADU.
The categories look familiar from any residential study, but the concentration is higher. Kitchen cabinetry, countertops, and appliance hookups; bathroom fixtures and specialty plumbing; a dedicated HVAC system or mini-split; a new electrical sub-panel and the wiring that feeds it; decorative flooring and window treatments, these typically land in 5- or 7-year property. Outside the structure, the ADU’s own driveway pad or parking area, a separate walkway, fencing that encloses its yard, exterior lighting, and the landscaping added around it usually qualify as 15-year land improvements. None of that is exotic. It’s the same set of asset categories a cost segregation study always looks for, there’s just more of them packed into a smaller building.
One advantage for owners of older properties.
Because an ADU addition is its own asset with its own placed-in-service date, it doesn’t matter how long you’ve owned the main house. A property acquired in 2012, studied or not, doesn’t drag its old acquisition date onto a cottage you build in 2027. The new construction stands on its own placed-in-service date and its own bonus depreciation eligibility. We wrote a longer piece on how that logic works for ground-up builds in cost segregation on new construction, and the reasoning carries over here almost exactly.
The math on a $210K backyard ADU.
Take a detached 750-square-foot ADU added to an existing rental, cost $210,000, construction began after January 19, 2025 and the unit was placed in service the same year. A cost segregation study on that addition alone might reclassify around 30% of the cost, roughly $63,000, into 5-, 7-, and 15-year property: kitchen and bath fixtures, the mini-split system, the dedicated electrical sub-panel, decorative flooring, and the unit’s own driveway pad and fencing. Under 100% bonus depreciation, the entire $63,000 is deductible in the year the ADU is placed in service. At a combined 35% federal-and-state marginal rate, that’s roughly $22,050 in first-year tax savings, on an addition that’s already generating rent, separate from whatever the main house itself is worth if it hasn’t been studied yet.
Living in the main house and renting out the ADU?
This is where an ADU is actually simpler than house hacking a duplex. When you live in one unit of a multi-unit building, the personal-use portion sits inside the same structure, and you have to allocate the building’s basis between personal and rental use, unit by unit or square foot by square foot. An ADU is a physically separate structure. If you live in the main house and rent the ADU to someone else without personally using it, the ADU is a distinct rental asset, fully depreciable on its own, without a fractional allocation to work out. You can read more about how our engagements are scoped on our process page.
Should the ADU be its own study, or folded into one for the whole property?
If the main house hasn’t been studied yet, do them together: one engagement, one report, covering the full purchase price plus the ADU addition. If the main house was already studied in an earlier year, a supplemental study on just the ADU’s cost captures the new addition without touching the original report or reopening an earlier tax year.