Where Growth Came From

What this is: real FY2009→FY2025 tax growth (not 2026 - see below) minus 75.28%, the real 20-year growth an average existing, unchanged property should have seen per scripts/existing_vs_new_property_tax.py - a full reconstruction of actual Prop 2.5 mechanics (2.5%/yr + real overrides + real debt exclusions), not a stylized formula. Positive = grew faster than an ordinary unchanged property would have; the further above zero, the stronger the signal something besides ordinary appreciation happened. Tax-only (the baseline is a levy concept, not an assessed- value one) - the Metric toggle above is ignored in this mode. Uses FY2025 as the endpoint, one year earlier than the other three views, because the real baseline only has audited data through FY2025; stretching it to FY2026 would mean estimating, not measuring.

Reading this map: every parcel sits on one continuous color scale — FY2009→FY2026 growth, in whichever of the three normalizations above you pick — rather than construction/renovation getting a separate flat color the way the FY17-24 version does. The outlines are layered on top (toggle them off with the button above the outline legend if they get visually busy) so you can see for yourself whether the reddest parcels line up with flagged construction/ renovation, instead of the map assuming they do. They only partly do (see the box above) — the classifier's FY18-25 window and known low detection rates (residential renovation ~20-25%, commercial near 0%, see METHODOLOGY.md §5) mean most real value-adding activity, especially FY2009-2017 and anything commercial, isn't flagged at all even where it's real. Reclassification (cyan) is deliberately separate from new construction (red): it's every case where the only signal was a commercial-value code change, not a countable unit or square-footage change - MA CAMA data has no commercial-sqft field at all, so these 8 events townwide can't be verified from this data alone and are treated as unverified until checked against real records (permits, sales, news).

$/sqft vs. % growth give different answers on purpose: $/sqft favors dense, small-lot parcels almost by construction (any given dollar of growth spreads over less land) and reflects large-lot, lower-density areas (like South Brookline) as flatter even where relative growth was real. % growth normalizes that away and instead favors parcels with a low starting value — a cheap parcel that doubles looks the same as an expensive one that doubles, even if the dollar amounts are very different. Neither is "correct" - they answer different questions.