Our methodology

The public data we use

Everything is built from open government data, refreshed on a schedule:

• MassGIS Level 3 standardized parcels — the statewide assessor layer for all 351 cities and towns: assessed value, use code, living area, and each parcel’s most recent recorded sale.
• MA DOR / Division of Local Services LA3 sales — the municipal sales reports, which carry the assessor’s own arm’s-length determination (the non-arm’s-length code), grantor/grantee type, and the assessment-to-sale ratio for each sale.
• MA DLS tax rates — the certified residential rate for each town, used for the estimated tax-impact figure.

How we choose comparable homes

A comparison is only as good as its comparables. We start from homes that are genuinely like yours: the same use class (single-family, condo, two- or three-family), a similar living area (within about ±25%), in the same municipality. Where a town publishes assessor neighborhood codes, sales in your own neighborhood carry most of the weight (about 70%) while the rest of the town still contributes — so a local block drives the result without being the only input.

Which sales we trust (the sale-validity filter)

Tax records are full of transfers that are not open-market sales — $1 transfers into a family trust, estate and life-estate deeds, and plain data-entry errors. Letting those into a comparison quietly corrupts it, so every sale must clear a filter before it can influence value:

• Nominal transfers excluded — any sale at or below $1,000.
• Sale-to-assessment ratio must fall between 0.5 and 2.0 (Massachusetts assessments target ~100% of fair cash value, so real sales cluster near 1.0).
• Outlier rejection on sale price per square foot using a robust median/MAD test (modified z-score above 3.5), which resists the very outliers it is removing.
• Recency — only sales within the last four years.
• A minimum of three valid sales, or we withhold the sales read-out rather than show a thin, unreliable number.
• The assessor’s own call wins. Where the LA3 record flags a sale as non-arm’s-length, we drop it even if the price looks like market — catching intra-family, trust, and estate transfers the statistics alone would miss — and we confirm the rest as arm’s-length.

The two arguments we build

Massachusetts law recognizes two distinct grounds for an abatement, and we test for each independently — they map to the two reason boxes on State Tax Form 128:

• Disproportionate assessment (uniformity). We compare your assessed value per square foot with the median of comparable homes. If similar homes are assessed at a lower rate than yours, your assessment is disproportionate — independent of market value.
• Overvaluation (market value). From the surviving valid sales we derive an indicated value range — median sale price per square foot times your living area, reported as a range. An assessment above that range is an overvaluation signal.

If you bought recently

An arm’s-length purchase is among the strongest evidence of market value. When the assessor’s sales report confirms your own purchase was arm’s-length, we compare your assessment-to-sale ratio against the town’s median qualified-sale ratio. Being assessed at a higher fraction of your sale price than your neighbors is a direct, non-circular disproportionate-assessment argument — and because the market’s movement since the sale is baked into both ratios equally, the comparison is apples-to-apples.

Assessment vintage

A Massachusetts fiscal-year assessment reflects value as of the prior January 1, built from the previous calendar year’s sales. We track which year’s values you are looking at, weight recent comparables toward the period the town actually used, and — in the winter appeal window — prompt you for a newly mailed value so the check reflects the assessment you are actually appealing.

What we set aside, and why

We keep a count of the sales removed at each step — nominal, out-of-ratio, statistical outlier, stale, and assessor-coded non-arm’s-length. Showing what was excluded, and why, is both a trust-builder and a more defensible methodology in front of a Board of Assessors than a black-box number.

What this is not

This is an automated screen built from public records, not an appraisal and not advice. It does not determine market value, provide an appraisal, give legal or tax advice, represent you, file anything, or guarantee an outcome. Verify every figure, deadline, and procedure with your city or town, and confirm any sale you rely on was arm’s-length. The guided packet helps you prepare a ready-to-sign Form 128 with a supporting Exhibit A; the decision rests with your assessors.