An ADU generating rental income is a small landlord operation, and the IRS and Wisconsin DOR treat it that way. Rental income is taxable. Expenses are deductible. The net result can be significantly better than the gross rent number suggests — or worse if you’re not tracking expenses. This is not tax advice; consult a CPA for your specific situation. This is a practical orientation to what applies.
Rental Income Is Ordinary Income
Rent collected from your ADU is reported on Schedule E of your federal return. It’s ordinary income taxed at your marginal rate. If your ADU generates $18,000/year and you’re in the 22% bracket, that’s roughly $3,960 in federal tax on gross rent before deductions.
Deductible Expenses
Common deductions for ADU landlords on Schedule E include: mortgage interest on debt used to build the ADU (consult your CPA on tracing rules), property taxes attributable to the ADU, landlord insurance premiums, repairs and maintenance, utilities if included in rent, advertising and leasing costs, and professional fees. The IRS distinction between repairs (deductible when incurred) and improvements (capitalized and depreciated) matters — know which category your expenses fall into.
Depreciation: The Most Valuable Non-Cash Deduction
The ADU structure can be depreciated over 27.5 years. If your ADU cost $150,000 to build, the depreciable basis is roughly that amount minus any allocated land value. At $140,000 depreciable basis, that yields approximately $5,090 in annual depreciation — a non-cash deduction that doesn’t require spending money. On an ADU generating $18,000/year gross, depreciation alone can offset a substantial portion of the tax liability. Important caveat: depreciation recapture applies when you sell, taxed at 25%. Your CPA should model this in your long-term planning.
Wisconsin State Tax
Wisconsin taxes rental income as ordinary income at state marginal rates (currently 3.54%–7.65%). Wisconsin generally conforms to federal treatment on deductions. Track every expense associated with the ADU from day one. If you’re generating $15,000+ in annual rental income, a CPA who handles rental real estate is worth the cost — the depreciation strategy alone typically more than pays for it.
Short-Term Rentals (Airbnb) vs. Long-Term Rentals
Short-term rentals on platforms like Airbnb trigger different tax treatment. If you rent your ADU for 14 days or fewer per year, the income is completely tax-free under the Augusta Rule. Over 14 days, it becomes reportable income — but the expense allocation rules differ from long-term rentals. Days of personal use vs. rental use affect which deductions you can take. In Madison, short-term rentals also require a city license and are subject to local room tax. Model both scenarios before deciding on a rental strategy.
Record-Keeping From Day One
The IRS expects landlords to substantiate every deduction. Keep separate records for your ADU from the day construction begins: receipts, contractor invoices, loan documents, insurance policies, utility bills, and lease agreements. A dedicated bank account for ADU income and expenses simplifies Schedule E preparation and creates a clean audit trail. Mixing ADU finances with personal accounts creates avoidable complexity at tax time.
When to Bring In a CPA
A CPA experienced with rental real estate is worth the cost in most cases. They’ll properly calculate your depreciable basis, model depreciation recapture on a future sale, advise on the Section 121 exclusion implications if the ADU is on your primary residence, and handle Wisconsin-specific conformity issues. If your ADU generates over $15,000 per year in rent, the first year of professional tax preparation typically pays for itself through depreciation alone. Plan for a one-time basis setup meeting before your first tax year ends.
