The cottage is finished. Your dad has moved in, or your first tenant has signed the lease. Then, one spring, an envelope from the City Assessor arrives with a bigger number on it.
That number belongs in your budget before you build. A new ADU adds value to your property, and Wisconsin taxes property based on value. Knowing the calendar gives you time to plan. Checking your assessment gives you a chance to catch mistakes.
Renting out your ADU? Our rental tax post covers income taxes on rent. Here, we're looking at your property tax bill: when your ADU gets assessed, how it's valued, and what to do if the number looks wrong.
The January 1 Snapshot
Wisconsin law tells assessors to value all taxable property "as of the close of January 1 of each year." That single date drives everything.
Whatever stands on your lot at the close of January 1 is what gets assessed for that year. Your ADU shows up in the assessment for the first January 1 it exists, finished or not.
Mid-construction counts too. If your ADU is half-built on January 1, the assessor values it as it stood that day. The City's own Open Book guidance lists "a building permit that was not completed on January 1" as a detail worth checking on your record.
Timing matters for your budget. An ADU finished in December and one finished the following February can land a full year apart on the assessment calendar. Plan for the increase in the assessment year that begins with the first January 1 your ADU stands on your lot.
How the Assessor Puts a Value on It
State law requires full value: what the property "could ordinarily be obtained" for at a private sale. The assessor doesn't value the ADU in isolation. They value your whole parcel, house and ADU together.
The best evidence is a recent arm's-length sale of your property. Next comes recent sales of comparable properties. When neither is available, the assessor weighs all the factors that bear on value.
In Madison, the City starts with mass appraisal, valuing properties in groups. If you object, the City says its Board of Assessors will appraise your property individually.
Reading the Number on Your Notice
Madison mails assessment notices each spring. In 2026, notices went out May 1.
Your tax bill later shows two values. The assessed value is the City's figure for your parcel. The estimated fair market value is the assessed value divided by the City's average assessment ratio. State law requires the second number so you can judge whether your assessment is reasonable. The City notes that some variance between the two is expected.
When the notice comes, check the facts first. Is the ADU listed? Is it described correctly? Was it finished on January 1, or still under construction?
Two Ways to Fix It: Open Book and Objections
Madison gives you two separate tracks. Pick the one that fits your problem.
Open Book: the facts are wrong. For about a week after notices go out, you can review your record with City staff. If a detail is wrong, such as room count or a building permit that wasn't complete on January 1, they correct it and adjust your assessment. In 2026 this ran May 4–8. Contact openbook@cityofmadison.com or 608-266-4531.
Objection: the facts are right, the value isn't. If the details are correct but you think the market value is different, you file an objection with the Board of Assessors. In 2026 the deadline was 4:30 p.m. on May 15. The Board meets weekly through the summer and early fall.
Board of Review: the final City step. If you disagree with the Board of Assessors, the Board of Review makes the final municipal determination. It's made up of appointed City residents. Miss the objection deadline, and you'll need to request a waiver from the Board of Review.
These dates change every year. Watch the Assessor's Office page each spring for the current schedule.
Outside Madison
The January 1 rule and the full-value standard apply statewide. The appeal path is different. In most Dane County towns and villages, there's no Board of Assessors. You go straight to the local Board of Review, which meets during a 45-day window that starts the fourth Monday of April.
One rule catches people: you generally must give the board's clerk written or oral notice of your intent to object at least 48 hours before the board's first meeting. Waivers exist, but they're limited. Check with your municipal clerk for that year's dates.
Plan for It From Day One
Your ADU may give a parent a place nearby. It may bring in rent. The property taxes belong in that plan too, alongside your mortgage and insurance.
Ask the Assessor's Office how ADUs are being valued in your area. Understand how January 1 affects your assessment. Keep your permit records handy when your notice arrives.
