The most common Madison assessment errors are wrong square footage, basements misclassified as finished, incorrect bedroom or bathroom counts, and phantom features like demolished garages still on record. Even a 300 sq ft error can inflate your assessment by $10,000 to $20,000. Madison's mass appraisal system can be off by up to 20% on individual homes — here are the specific mistakes to look for on your property record.
This is probably the single most common assessment error and the easiest to verify. The City of Madison Assessor publishes a property record card for every parcel showing what they have on file for your home, including total finished square footage.
Pull yours up and compare it to:
Even a 100 square foot error can move your assessment by thousands of dollars. A 300 square foot error — which happens more often than you'd expect — can mean $10,000 to $20,000 of inflated assessment.
The mistakes usually go one direction: the city counts space that isn't actually finished living area. Common culprits include unfinished basement areas counted as finished, unheated three-season porches counted as living space, and bonus rooms over garages that don't meet the technical definition of finished space.
Wisconsin assessment models treat finished basements very differently from unfinished basements. A finished basement adds significant value. An unfinished basement adds storage value only.
If your property record says your basement is finished and it isn't — or partially finished and most of it is unfinished concrete and storage — that's a meaningful error. Take photos. Note the percentage of basement that's actually finished living space. This is one of the easier corrections to win.
Bedroom and bathroom count is a primary input to the city's valuation model. If the city has you listed at 4 bedrooms and 3 bathrooms but you actually have 3 bedrooms and 2 bathrooms, you're being assessed for value that doesn't exist.
Half-baths, three-quarter baths, and full baths all carry different values. A "full bath" requires a tub or shower, sink, and toilet. A "three-quarter bath" has a shower but no tub. A "half bath" has just a sink and toilet. Make sure the city's count matches reality.
Property record cards sometimes include features your home doesn't actually have:
These are pure errors and they're worth fixing. Walk through your property record line by line and flag anything that doesn't match.
This is the largest category of overassessments and the hardest to spot, because the error isn't in the data the city has — it's in the data the city doesn't have. Mass appraisal assumes "average condition" unless someone tells it otherwise. If your home is in significantly worse condition than average, your assessment is too high by default.
Major systems near end of life — a roof that needs replacement in the next two years, an HVAC system from the 1990s, original windows in a 1920s house, knob-and-tube wiring, galvanized plumbing — get priced in by buyers and knock real dollars off what someone would actually pay for the house. The assessor's model has no idea any of this is going on.
Foundation, basement, and moisture issues — cracks, settling, water in the basement after heavy rain, mold, efflorescence on basement walls — can knock $20,000 to $50,000 off real market value depending on severity. They're invisible to the assessor.
Outdated kitchens and bathrooms matter too. A 1985 kitchen with original cabinets and laminate counters in a market where buyers expect updated kitchens is a real value penalty. Same with original bathrooms. Buyers either knock the cost of a remodel off their offer or skip the house entirely.
Floor plan and functional issues — awkward layouts, rooms you have to walk through to access other rooms, no main-floor bathroom, low ceilings in finished basements, weird additions that don't match the original house — show up in real sale prices but not in mass appraisal models.
The "effective age" of a home — how old it functions, given any updates — affects assessment significantly. If your home was built in 1925 and has never been substantially updated, but the city has it listed as "renovated 1995," your effective age may be wrong. This is harder to demonstrate but worth noting if you can support it.
Drainage problems, encroachments, easements, oddly shaped lots, lots with steep slopes that limit usable yard space, lots adjacent to commercial property or busy streets — all of these affect market value. None of them necessarily show up in the city's data.
Here's the part most homeowners miss: assessment errors compound. If the city has you at 200 extra square feet and a finished basement that isn't actually finished and doesn't know about your foundation issue, you're not looking at one $5,000 error — you're looking at three errors that together might inflate your assessment by $30,000 or more.
In Madison, where the combined property tax rate runs around 2%, a $30,000 overassessment means about $600 a year in extra taxes. Over five years until the next thorough revaluation cycle, that's $3,000 you didn't owe.
Document them carefully. Take photos. Pull measurements. Get contractor quotes for major condition issues if you can.
Then file an objection by May 15, 2026 at 4:30pm, sent to openbook@cityofmadison.com. Under Wis. Stat. § 70.075, the Board of Assessors will conduct an individual appraisal of your home — this is your chance to get the errors corrected and your assessment reduced.
We built a free tool that walks you through every common assessment error — square footage, basement, features, condition issues — and pulls comparable sales from all 2,668 real 2025 records the City of Madison Assessor used. Use our free tool at mycitymadeeasy.com to check your assessment and build your objection in 2 minutes. No signup, no cost.