Methodology
How this works
We build an independent estimate of what each Philadelphia home is worth, using the same public records the city uses. Then we compare our estimate with the city’s assessment.
What a property assessment is
Every year, Philadelphia’s Office of Property Assessment (OPA) estimates what your home is worth. Your property tax is about 1.4% of that number. If the number is too high, you pay too much tax. If your neighbor’s is too low, they pay too little, and everyone else covers the difference.
Where our numbers come from
- More than 200,000 real home sales from city deed records (2016 to today).
- City property records: size, age, style, and condition on file.
- City licenses and inspections: permits, complaints, violations, and vacancy.
- Public maps: parcel shapes, transit, and parks.
A computer model learns from those sales. It asks what homes like this one actually sold for, then estimates today’s value for every home in the city. What we never use: race, income, or anything about the people who live in a home. People-data is never part of the price. We use it only afterward, to check the model for neighborhood bias.
What does “90% sure” mean?
Each dot is a home like yours that sold. Pick a confidence level. The range grows or shrinks so that many dots land inside it.
72 of 80 sold inside the range. Open dots are sales that landed outside the range.
We only flag an assessment when the city’s value falls outside the 90% range, and two different statistical methods have to agree first.
How our estimates hold up
Checked against 19,484 real sales the model never saw (out-of-time test, 2026).
| Our model | City values | |
|---|---|---|
| Typical miss vs. actual sale price | 26% | 34% |
| Sales landing inside our stated range | 91 of 100 | no range given |
| Cheaper homes treated same as pricier ones | Close: passes the official test | Uneven: fails it |
See the full head-to-head proof. It runs the official test, scored line by line.
What we cannot see
Public records don’t show a renovated kitchen, a leaking roof, or anything else inside your walls. If the inside of your home differs a lot from its records, our estimate can be wrong in either direction. That is why every report shows a range, and why the facts table matters more than any single number.
Who made this
This is an independent, open project by Nick Hand. It is not a city service. The full code and data pipeline and the technical model documentation are public, including the measurements that didn’t work. If you find a mistake, we want to know.
Data updated 2026-07-04 · Model code and validation are public · Independent, not a City of Philadelphia site