How these figures are calculated

The records behind the numbers

The headline figure, about 1 in 10, comes from the communities’ own published residential sales studies: 12,175 home sales from 2024 and 2025 in 34 communities across Oakland, Kent, Ingham, Washtenaw and Genesee counties. A sale counts when the assessment the study prints for that home is more than 55 percent of the sale price, which puts the community’s value more than 10 percent above what the buyer paid. 1,202 of the 12,175 sales meet that test.

Only 2024 and 2025 sales are counted. Some studies print the current assessment rather than the one in place when the home sold, and against an older sale that later figure overstates the gap. The rate varies widely from one community to the next, from about 1 in 4 to about 1 in 49, so it describes the studies together rather than any single town. Oakland County supplies most of the sales; the figure will move as more communities are added.

Oakland County’s study sales were also checked against the county’s parcel records, to confirm each parcel exists, sits in the community the study names, and is residential.

The case studies

The three homes on the home page, in West Bloomfield, Farmington Hills and East Grand Rapids, come from the same documents: each community’s own published residential sales study. Each is a sale the study itself treats as a valid market sale.

Assessor’s value is twice the assessed value the study prints for the home. Michigan assesses property at half its market value, so doubling the assessment gives the value the assessor has placed on the home. Most studies print the assessment in place when the home sold; Farmington Hills’ two-year study prints the 2025 assessment, the first one set after its 2024 sale.

Tax on the gap is half the difference between the assessor’s value and the sale price, which is the part of the assessment the sale does not support, multiplied by the 2025 homestead millage rate for the home’s community and school district from the Michigan Department of Treasury.

Homes were left out where the gap has another explanation: condominiums, and houses that sold for less than the value of the land alone, which usually means a teardown.

What the figures are not

Every “tax on the gap” figure on this site is the tax on the difference between the assessor’s value and a recorded sale price. It is not a predicted refund and it is not a promised reduction.

Because of that provision, a gap between an assessment and a sale price is a reason to look rather than a result. An appeal succeeds on the assessor’s own valuation method applied correctly to the property, which is a separate piece of work from the screen these figures come from.

Known limitations

• Homestead is assumed. Case-study tax figures use the homestead (principal residence) rate. A rental or second home pays a higher rate, so the same gap would cost its owner more.
• The studies are samples. Communities publish the sales they used to set values, not every sale, and each lays its study out differently. Studies we could not read reliably, including scanned ones, are not counted yet.
• Millage rates are prior-year. Figures use 2025 Michigan Treasury rates. Rates move a little each year and a current bill may differ slightly.
• Sale prices are as recorded. A recorded price may include or exclude concessions, and we cannot see the terms of a transaction from the record alone.
• The screen is not the analysis. These figures identify properties worth examining. They do not establish that any particular assessment is wrong.

Public sources used

• Oakland County open parcel data, covering roughly 490,000 parcels across 62 communities, used to check the Oakland study sales.
• Michigan Department of Treasury millage rate tables, including the 2025 Total Rates report for every Michigan community and school district, and State Tax Commission forms and bulletins.
• Residential sales and economic condition factor studies posted by Oakland County communities, including Farmington Hills and Farmington, and by Oakland County Equalization for the communities it assesses, 2026 roll.
• 2026 residential sales and economic condition factor studies published by East Grand Rapids and Cascade Township (Kent County), Meridian Township (Ingham County), Ann Arbor (Washtenaw County) and Flushing Township (Genesee County).
• The Michigan General Property Tax Act, Public Act 206 of 1893, as amended.

If you want to check any figure on this site against its source, write to us and we will tell you exactly where it comes from.