Methodology

Every number on this map is computed from public parcel data with a transparent formula. Here is exactly how, and what the limits are.

The data

Parcel records come from the Washtenaw County / MapWashtenaw assessment roll (the latest published roll, retrieved June 2026) — all of Washtenaw County, about 121,000 taxable parcels across its 28 cities and townships. We include only taxable parcels — residential, commercial, industrial, agricultural, and their vacant variants — with a positive assessed and taxable value. Exempt, municipal, and zero-value parcels are excluded because they carry no levy. Sale dates, sale prices, and year built come from Regrid (covering roughly 80% of parcels), joined to the county roll on parcel number.

Key terms

• SEV (State Equalized Value) — the assessor's value, by law about half of market value. We show estimated market value as SEV × 2 (an approximation; assessors do not hit exactly 50%).
• Taxable Value (TV) — the Proposal A–capped base your tax is actually charged on. Proposal A (1994) limits how fast TV can grow each year and resets it to market only when a property sells ("uncapping").
• Prop A discount — (SEV − TV) / SEV. How far the capped base sits below assessed value.

Modeled tax

The assessment roll has no per-parcel tax amount, so we model it: tax = TV × millage. Millage depends on where a parcel sits — every parcel pays a stack of overlapping levies: county, its city or township, its school district, the intermediate school district, community college, library, and any local authority. We use the total homestead and non-homestead rate for each (jurisdiction × school district) combination from the 2025 Washtenaw County Apportionment Report — 154 rates covering all 77 combinations in the county. In the City of Ann Arbor, for instance, that is 52.6657 mills for principal residences and 68.6263 mills for non-homestead property; rural townships run as low as ~27 mills and the City of Ypsilanti as high as ~81.

We validated against real 2025 tax bills from the county's BS&A system: a principal residence reconciles to within 1% (the small gap is parcel-level special assessments). We model the base millage only — special assessments are parcel-specific — so treat dollar figures as close estimates. Because the current assessment roll's own tax bills don't exist yet, per-parcel dollars are modeled at the most recent published (2025) rates; discount and fair-rate percentages are exact regardless of millage.

The revenue-neutral "fair rate"

We model a single reform: remove only the Proposal A cap, tax every parcel on its full SEV, and reset the rate so the total amount collected is unchanged. We do not touch the homestead exemption — that is a separate policy this project does not contest. We hold revenue neutral within each taxing district: for each (jurisdiction × school district × homestead class) we scale by k = ΣTV / ΣSEV and compute fair tax = SEV × millage × k. Because each district is reset against its own levy, removing the cap is not a tax increase in aggregate — it is a redistribution within each community.

A parcel's fair-rate change is fair tax − current tax. Positive means it is taxed below a fair rate today (a beneficiary of the cap); negative means it already pays more than a fair rate would charge — often a recently-sold property.

Limitations

• Single snapshot. The current assessment roll only. We can show where each parcel stands, not how its gap grew over time.
• Sale history is third-party. Sale dates, prices, and year built come from Regrid and cover roughly 80% of parcels; where missing, the size of a parcel's current discount stands in as a proxy for how long it has been capped.
• Modeled dollars at 2025 rates. Tax is computed from each district's base millage (2025, the latest published), not read from bills, and excludes parcel-specific special assessments and modified-rate abatements (IFT, NEZ); treat dollar figures as close estimates. Percentages are exact.
• Revenue-neutral within each district. The fair-rate reset balances each taxing district against itself; it is not a single county-wide rate, and it does not attempt to redistribute across district lines.

Support this work

I build and maintain Mind the Cap in my free time, and pay Regrid for parcel data and cover hosting costs out of my own pocket. If you find it useful and want to help keep it going, you can buy me a coffee to help support it — every bit helps. I'm also glad to hear constructive feedback or answer questions.