Your backyard could be paying you $2,000 a month — and quietly building a tax bill you won't see until the day you sell. Accessory dwelling units (ADUs), the garage conversions and backyard cottages multiplying across the country, are one of the few home improvements that generate income from day one. In California they now account for roughly one in five newly built homes, and national construction costs in 2026 average around $180,000 per unit. But the difference between an ADU that funds your retirement and one that funds your accountant's boat is the paperwork: how you track costs, split expenses, claim depreciation, and plan for the eventual sale. Here is the full profit-and-loss picture, from framing to final tax return.
What an ADU Costs to Build in 2026
Start with honest numbers, because the sticker price determines every tax figure downstream: your depreciable basis, your break-even rent, and your eventual gain on sale.
National data puts the typical ADU project between $40,000 and $360,000, averaging close to $180,000, or roughly $150 to $300 per square foot. The spread is enormous because "ADU" covers very different projects:
- Garage or basement conversion: $120,000–$250,000. The shell already exists, so you are paying for kitchens, baths, insulation, egress, and bringing old wiring and plumbing up to code. Surprises hide in slab foundations and undersized electrical panels.
- Attached addition: $180,000–$350,000. New foundation and roof tie-ins add cost, but you share at least one wall and often the main utility connections.
- Detached backyard cottage: $250,000–$500,000+. A fully independent structure with its own foundation, roof, and utility runs. In high-cost coastal metros, premium custom units run past $500,000.
On top of hard construction costs, budget 15–25% for soft costs: architectural plans, structural engineering, permit and impact fees, utility connection or upsizing charges, and financing costs during the build. Two line items catch first-timers off guard. First, separate utility meters: sharing the main house's water and power is cheaper upfront but makes the Schedule E expense split (below) a permanent estimation exercise. Second, permit timelines: even in ADU-friendly states, plan review plus construction commonly runs 9–18 months, during which your capital is locked up and earning nothing.
Financing matters to the P&L too. Most owners fund ADUs with a HELOC, home-equity loan, cash-out refinance, or renovation loan. Interest on money used to build a rental unit is generally deductible as a rental expense — but only the portion allocable to the rental, and only once the unit is placed in service. Interest paid during construction gets capitalized into the building's basis instead. Keep the loan draws and construction invoices in one dedicated folder (or better, a dedicated bank account) from the first check you write.
What It Can Earn: Running the Rental Math
Rental income is where ADUs shine, because you already own the land — the single most expensive input in rental real estate. Small one-bedroom ADUs in mid-cost metros commonly rent for $1,500–$2,800 per month; in supply-starved coastal cities, $1,800–$3,000 is realistic for a well-finished unit.
Run your own break-even before you commit:
- Gross annual rent: monthly rent × 12, minus a vacancy allowance (5–8% for long-term tenants, more for furnished mid-term rentals).
- Operating costs: the ADU's share of insurance, property tax increase, utilities, maintenance, and any HOA or property-management fee.
- Debt service: the annual cost of whatever you borrowed to build.
- Net cash flow: gross rent minus operating costs minus debt service.
A $280,000 detached ADU renting for $2,200 a month grosses $26,400 a year. After roughly $4,000 in allocated operating costs and no debt (cash build), that is about an 8% gross yield on construction cost — before depreciation and taxes. With financing, cash flow shrinks but leverage boosts cash-on-cash returns, at the cost of risk if rents soften or the unit sits vacant.
One strategic choice shapes both income and taxes: long-term tenant versus short-term rental. A year-round lease means stable income taxed as ordinary rental income on Schedule E. Nightly or weekly furnished stays can gross more per month but may trigger local transient-occupancy taxes, stricter permit rules, and — if you provide substantial services like daily cleaning and meals — different tax treatment altogether. Check your city's short-term-rental ordinance before assuming Airbnb income in your pro forma; many ADU permits explicitly forbid rentals shorter than 30 days.
Running It on Schedule E: Splitting Income and Expenses
Once the unit is rented, the IRS treats you as the owner of residential rental property, reported on Schedule E. The rent checks are the easy part. The skill is in dividing every shared cost between your personal residence and the rental — because only the rental slice is deductible.
The exclusive-use rule comes first
To claim rental deductions at all, the tenant's space must be used exclusively by the tenant. A renter with their own kitchen and bathroom in a converted basement or detached cottage qualifies. Someone renting a bedroom while sharing your kitchen is a roommate or guest arrangement, not a rental unit, and the expense-splitting rules below don't apply the same way. For a self-contained ADU this test is usually easy to meet — just don't store your belongings in "their" garage bay or work from "their" spare room and expect the IRS to look the other way.
Three buckets of expenses
Think of every dollar you spend on the property as falling into one of three buckets:
- Direct rental expenses — 100% deductible. Repairs inside the ADU, tenant screening fees, advertising the vacancy, the ADU's own insurance rider, and appliances bought for the unit.
- Direct personal expenses — 0% deductible. Repairs to your own kitchen, your personal landscaping choices, and anything that benefits only your household.
- Shared expenses — split by a reasonable method. Mortgage interest, property taxes, homeowners insurance, utilities on a shared meter, roof replacement, pest control, and internet serving both units.
For the split, IRS Publication 527 blesses two common methods — square footage and room count — and permits any other reasonable method. Square footage is the natural fit for ADUs: a 600-square-foot cottage behind a 1,800-square-foot house supports allocating 25% of shared costs to the rental. Some costs reasonably split differently — water by headcount, for example, if the ADU houses two people and the main house one. Whatever method you choose, document it in writing, apply it consistently year after year, and keep the worksheet with your tax file. The method you pick now also governs how sale proceeds get divided later, so consistency isn't just tidy — it's required.
Depreciating the rental portion
Depreciation is the ADU owner's best annual deduction and least understood. You recover the rental portion of the building's cost over 27.5 years, straight-line, starting when the unit is placed in service (available for rent, not when the first tenant signs).
The mechanics: take your total cost basis in the property improvements — original purchase price allocated to structures plus the ADU construction cost — subtract land value (land never depreciates), and allocate the remainder between personal and rental use, typically by square footage. A common shortcut for new detached ADUs is cleaner: the entire construction cost of the cottage (plus related soft costs) becomes the depreciable basis of the rental, since 100% of that structure is rental use. Either way, only the building counts — get a land-versus-improvement allocation from your closing statement, assessor's records, or an appraisal, and keep it.
Two warnings. First, construction costs are capitalized, not expensed: you cannot deduct the $280,000 build in year one. Second, depreciation is "allowed or allowable," meaning the IRS treats you as having taken it whether you claimed it or not. Skipping depreciation to "keep things simple" doesn't avoid the recapture tax at sale (see below) — it just forfeits years of deductions for nothing.
The passive-loss speed limit
Rental real estate is generally a passive activity, so if the ADU shows a tax loss — common in early years when depreciation exceeds net rent — you can usually use only up to $25,000 of it against wages and other ordinary income, and that allowance phases out between $100,000 and $150,000 of modified adjusted gross income. Unused losses aren't lost; they carry forward to offset future rental income or the gain when you sell. Track suspended losses every year so the benefit survives until sale day.
The Sale-Day Surprise: Depreciation Recapture and Gain Allocation
Here is the part most ADU guides bury: every dollar of depreciation you claimed (or could have claimed) lowers your basis and raises your taxable gain when you sell — and that slice of gain gets special, unfavorable treatment.
When you sell a property that is part residence and part rental, you must allocate the sale price, selling expenses, and basis between the two portions using the same method you used for depreciation. The residence portion may qualify for the Section 121 exclusion — up to $250,000 of gain ($500,000 married filing jointly) if you owned and lived there two of the last five years. The rental portion does not.
Worse, the depreciation slice is carved out as "unrecaptured Section 1250 gain" and taxed at a maximum 25% rate — higher than the 0%/15%/20% long-term capital gains rates most sellers expect. The Section 121 exclusion cannot shelter depreciation taken after May 6, 1997, so even if your residence gain is fully excluded, the ADU's accumulated depreciation still produces a tax bill. The sale is reported on Form 4797, with the unrecaptured gain flowing to Schedule D.
A concrete example: you built a $280,000 ADU, rented it for ten years, and claimed roughly $100,000 of depreciation. When you sell, that $100,000 of prior deductions comes back as gain taxed at up to 25% — a $25,000 tax cost to set against the thousands you saved each year. Depreciation is still worth taking (a dollar of tax saved today beats a discounted dollar paid a decade from now, and many owners are in lower brackets at sale), but model the recapture before you build, not after you accept an offer. And if you're considering converting the ADU to personal use before selling to "erase" the rental history — it doesn't work. The depreciation history follows the property.
How an ADU Changes Your Property Tax Bill
Property tax is the local-government counterpart to the federal story, and it cuts in your favor more often than owners expect.
In most states, building an ADU is assessable new construction: the assessor adds the value of the addition to your tax roll. The key protection — in California under Proposition 13 and under similar acquisition-value systems elsewhere — is that only the new construction is assessed at current market value. Your existing home keeps its old base-year value; the ADU does not trigger a reassessment of the whole parcel. Expect your bill to rise by roughly the local tax rate (about 1% plus local bonds and fees in much of California) times the assessed value of the addition — perhaps $2,000–$4,000 a year on a $280,000 cottage.
California went further. SB 1164 created a temporary new-construction exclusion for ADUs completed between January 1, 2025 and January 1, 2030: qualifying units are not added to the assessment at all until the exclusion expires (ten years after completion) or the property changes ownership. Claiming it requires notifying the county assessor before or within 30 days of completion and filing an affidavit that you'll make a good-faith effort to keep the unit in residential use. Miss the notice window and you pay tax you could have avoided for a decade — put the assessor letter on the same checklist as the final inspection.
Outside California, rules vary widely: some jurisdictions offer ADU-specific exemptions or freezes, others assess the full addition immediately, and a few still treat ADUs inconsistently from one assessor to the next. Call your county assessor's office before finalizing your pro forma, ask specifically how ADU new construction is enrolled, and get the answer in writing. While you're at it, tell your homeowners insurer about the new unit — an undisclosed rental structure is a classic claim-denial trap, and the premium increase is partly deductible as a rental expense anyway.
Bookkeeping That Keeps the ADU Audit-Ready
None of the tax benefits above survive without records. The IRS expects rental owners to keep leases, rent receipts, expense invoices, a log of days rented versus days of personal use, and documentation of every improvement with its cost and placed-in-service date. For an ADU, add three habits that pay for themselves:
- Separate the money. Run all ADU income and expenses through a dedicated checking account (and ideally a dedicated credit card). When every transaction is already segregated, the Schedule E practically writes itself, and an auditor sees a business, not a hobby.
- Track basis like an asset ledger. Log the original construction cost by category, every later improvement, the land allocation, depreciation taken each year, and suspended passive losses. This ledger is what makes the sale-year allocation and recapture computation possible a decade from now, when the contractor's invoices are long gone.
- Photograph and date everything. A timestamped photo of the completed unit establishes placed-in-service timing; receipts for each repair distinguish immediately-deductible fixes from improvements that must be capitalized and depreciated.
Common mistakes to avoid: expensing the build instead of capitalizing it, depreciating the land along with the building, splitting shared costs by a method you can't explain, "forgetting" rental income from a family member paying below-market rent (bargain rent to relatives can reclassify the unit as personal use), and converting to short-term stays without checking permit and occupancy-tax rules. Each one is cheap to prevent and expensive to fix under audit.
Keep Your Rental Books Organized from Day One
As you plan, build, and lease your backyard cottage, maintaining clear financial records is what turns construction chaos into clean tax returns — separate accounts for the build, a documented expense-split method, and a running basis ledger you can hand to your tax preparer. Beancount.io provides plain-text accounting that gives you complete transparency and control over your financial data — no black boxes, no vendor lock-in. Get started for free and see why developers and finance professionals are switching to plain-text accounting.
