Middlesex County, Massachusetts property tax

ACS 5-year median real estate taxes paid (table B25103) divided by ACS 5-year median value of owner-occupied housing units (table B25077), county-wide. This is a ratio of two independently estimated medians — not the median of the ratio, and not a rate any parcel is billed. It reflects the actual mix of overlapping districts, exemptions and assessment caps as experienced by owner-occupants, and it lags: a 5-year sample centres about two and a half years before its end date.

Layer 1 of the county property-tax corpus: a MEASURED baseline from ACS 2020-2024 5-year, not a published rate. No statutory rate, assessment ratio or exemption has been collected for these counties yet, so nothing here should be read as what a particular property would be billed.

Three things here must NOT be modelled as reducing a tax bill. (1) Clause 41A is a DEFERRAL, not an exemption: taxes accrue at 8% interest and are recoverable on sale or transfer, so it postpones a bill and creates a lien rather than shrinking it. (2) The senior circuit breaker is a refundable credit against the STATE INCOME TAX under c.62 §6(k), never touching the municipal bill — it is available to renters, who have no property tax bill at all. (3) The "county tax" of c.35 §31 is assessed on member municipalities, not on parcels, and is already inside the municipal levy. Two traps for anyone consuming the classification shift. The 150%/175% limit constrains a class's share of the LEVY relative to its share of value, NOT the ratio between rates: Burlington runs a lawful commercial-to-residential rate ratio of 2.967. And the maximum shift a municipality may adopt is frequently neither 1.50 nor 1.75 but an intermediate figure fixed by its own levy-share history (Cambridge 1.515098, Canton 1.618167), which cannot be derived from current-year data and must be read per municipality from DOR. Finally, split-rate status must be read from the CIP shift factor, not by comparing published rates: Tisbury shifts 1.08507 yet prints identical residential and commercial rates, because its residential exemption offsets the shift exactly.