Cost vs. Value Methodology and Sources

Every number in the report is either computed by us from data we are allowed to use, or credited to the published study it comes from. Where neither exists, the cell stays blank.

Back to the report

Not an appraisal, a broker price opinion or a prediction of a sale price. Costs are budget estimates, not a bid, appraisal, or insurance claim valuation. Pilot.

Fixed project scopes

Each project is one fixed, written scope (size, materials and finish level), priced the same way in every city, so cities can be compared. The scopes are our own and are listed on each project page.

Cost: the Caissonix estimator

Every job is broken into line items: materials, labor hours by trade, permits, disposal, contractor overhead and profit, and a contingency.
Labor: each trade's billed hourly rate in the county, built from Bureau of Labor Statistics wages (OEWS), employer payroll costs, California workers' comp rates and contractor overhead.
Materials: current prices from home improvement stores, lumber yards, suppliers and manufacturer list prices, each with its link and date, plus waste and the local sales tax rate (CDTFA).
Uncertainty: three-point inputs and 4,000 simulated outcomes per job. Likely is the median; the range is the 10th to 90th percentile.
Cities: one representative ZIP per city. Costs in this edition were computed 2026-10-31 (engine 0.10.0, price book 2026-10-31.1).

Value: three kinds of evidence

Caissonix analysis of public records. San Francisco publishes its Assessor-Recorder secured roll under a public domain dedication. Under Proposition 13, a home's assessed value resets to its purchase price when it sells, so for single-family homes that sold in the year before a roll's lien date, the roll value is close to the sale price. We modeled the log of that value on living area, bathrooms, bedrooms, lot size and age, with neighborhood and year controls (2,362 homes sold in 2022 and 2023, R² 0.73). One more full bathroom: about $78,000 at the median home (95% range $47,000–$110,000). One more square foot of living area: about $643 (95% range $585–$701). The median just-sold home was $1,785,000 and 1,700 sq ft.
Peer-reviewed studies measured in California, applied only where they were measured. Example: the Los Angeles ADU study is turned into a dollar figure for Los Angeles only.
Studies shown as context. National, older or small-sample studies (heat pumps, solar, seismic retrofit) are summarized and credited but not turned into city figures.

Limits: roll values can include transfers that were not open-market sales, and the model cannot see finish quality or condition. A bathroom or square-foot value measures what buyers paid for homes that already had it, which is not the same as the payoff from building it.

County values: local and regional estimates

The county table covers all 58 counties in two clearly separated ways.

Local: counties with open, licensed sales or assessor data. We fit the same model in each (log sale price, or Prop 13 reset roll value, on living area, bathrooms, bedrooms, age and, where recorded, lot size, pool and second units, with neighborhood or ZIP and year controls; robust standard errors):
- Los Angeles County: 43,190 single-family sales, R² 0.805. County of Los Angeles, Assessor Parcel Data (Rolls 2006-present), eGIS Hub; values reset at sale under Prop 13, 2024 roll (sales recorded July 2022 to June 2023).
- San Francisco County: 2,362 single-family sales, R² 0.73. SF Assessor-Recorder secured roll, roll years 2023-2024 (DataSF wv5m-vpq2, PDDL 1.0).
- Sonoma County: 9,921 single-family sales, R² 0.687. County of Sonoma, Clerk-Recorder-Assessor, Parcels Public (CC BY-SA 3.0 US); recorded full-value sale prices, 2023-2026.
Regional estimate (low confidence): for every other county we borrow from the local results with a partial-pooling (random-effects) model. Each local county is weighted by its precision and by how similar it is to the target county: same region, price tier (Census ACS median home value) and housing stock (share of homes built before 1960). The range adds the measured spread between counties, so it is wider than a local range. The percentage effect is then applied to the county's own ACS median home value. Pooled measures: bathroom from 3 counties (between-county spread 0.01 log points); living area from 3 counties (between-county spread 0.08 log points). A measure is pooled only when at least two counties measured it locally, and ADU values are never pooled. A regional estimate is never shown as local data.

We searched all 58 counties for open assessor, sales and permit data and checked each license; the county data inventory lists what we found. Counties move from regional to local as usable open data appears.

What we do not use

Other publishers' remodeling value surveys or their figures.
Real estate portal data whose terms do not clearly allow use in a commercial product.
Insurance claim pricing data.
Guesses. A blank cell means we do not have verified data yet.

Confidence labels

High: two or more independent California sources agree (none yet).
Medium: one solid analysis measured in that city with a reasonably tight range.
Low: a small sample, a wide range, or a study applied close to but not exactly where it was measured.
Context: credited and summarized, not turned into a city figure.
Regional estimate (low): a county figure borrowed from similar counties with local data, not measured in that county.

Coming in 2027

Permit-linked sales: homes that sold after a permitted kitchen, bath, roof or other project, compared with similar homes that did not, starting with counties that publish both permits and sales or assessor data openly.
A Caissonix survey of California real estate agents and appraisers, by region, using our own questions and project scopes.
More counties with local values, starting with those that publish home details today and those whose assessors provide data on request.

Sources

Caissonix analysis of the San Francisco Assessor-Recorder secured roll, roll years 2023 and 2024. Open Data Commons PDDL 1.0 (public domain dedication). https://data.sfgov.org/d/wv5m-vpq2
Brueckner and Thomaz, "ADUs in Los Angeles: Where are they located and by how much do they raise property value?", Real Estate Economics (2024). Published finding, cited (facts are not copyrightable). https://doi.org/10.1111/1540-6229.12471
Shen, Liu, Qiu, Patwardhan and Vaishnav, "Estimation of change in house sales prices in the United States after heat pump adoption", Nature Energy (2021). Published finding, cited. https://www.nature.com/articles/s41560-020-00706-4
Hoen et al., "Selling Into the Sun: Price Premium Analysis of a Multi-State Dataset of Solar Homes", Lawrence Berkeley National Laboratory (2015). U.S. government-funded report, cited. https://eta-publications.lbl.gov/publications/selling-sun-price-premium-analysis
Porter, Alhumaidi and Reed, "Seismic Retrofit Added 17% to the Resale Value of Older California Houses", Natural Hazards Review (2022). Published finding, cited. https://doi.org/10.1061/(ASCE)NH.1527-6996.0000579
U.S. Census Bureau, American Community Survey 2025 1-year, table B25077 (median value, owner-occupied homes). U.S. government work, public domain. https://www2.census.gov/programs-surveys/acs/summary_file/2025/table-based-SF/
County of Los Angeles, Assessor Parcel Data (Rolls 2006 to present), eGIS. Courtesy of the County of Los Angeles; used under its terms, which allow commercial use with attribution. No endorsement implied.
County of Sonoma, Parcels Public. Licensed under CC BY-SA 3.0 US; figures derived from it are shared under CC BY-SA 4.0. No endorsement implied.
U.S. Census Bureau, American Community Survey tables B25077 (median value) and B25034 (year built), 2025 1-year and 2020–2024 5-year. Public domain.
Bureau of Labor Statistics OEWS wages, CDTFA sales tax rates and FHFA house price indexes (U.S. and California public data), as used in the estimator.

Not an appraisal

Not an appraisal, a broker price opinion or a prediction of a sale price. Costs are budget estimates, not a bid, appraisal, or insurance claim valuation. The report is informational. It does not give legal, tax, lending or insurance advice. For a valuation of a specific home, ask a licensed appraiser or a local real estate professional.