How we source and compute every number on this site

Every county-specific figure on countyhomecosts.com is traceable to a named public source. Where the calculator needs numbers that aren't published per county (closing cost line items, mortgage rate, insurance), it uses national ranges and assumptions that are labeled as estimates and documented below. This page covers where each data field comes from, how often it's refreshed, and how derived values are computed. For how we review content and handle corrections, see our editorial policy.

Property tax rates and median home values

Source: US Census Bureau, American Community Survey (ACS) 5-year estimates. Accessed via the Census API at api.census.gov.
Vintage: 2024 (covering survey years 2020–2024). Released December 2025 by the Census Bureau.
Tables used: B25103_001E (median real estate taxes paid) and B25077_001E (median home value), joined on the county GEOID (5-digit FIPS).
Median tax-to-value ratio: Computed as median real estate taxes paid divided by median home value. Both inputs are separate county-wide medians, so the result is a ratio of two medians — not the median of individual households' tax rates, and not a nominal millage rate. It describes the county in aggregate. No individual property is charged this rate, and we do not present it as one.
Validation: All counties must have a 5-digit FIPS, effective rate under 5%, and median home value over $50,000. Territories (Puerto Rico, USVI, Guam, American Samoa, Northern Mariana) are excluded. 3,134 counties across the 50 states plus DC passed validation.
Update cadence: Annually each December when the Census Bureau releases the next 5-year ACS vintage.

State real estate transfer taxes

Source: Primary source per state: the state Department of Revenue, Tax Commission, or authorizing statute. Every entry in data/transfer_taxes.json links to a live .gov URL.
Verified: April 2026. All 51 jurisdictions (50 states plus DC) were individually researched against their authoritative source.
What we capture: Buyer rate, seller rate, whether counties can add their own transfer taxes, any notable county-level additions (e.g., NYC RPTT, Chicago CTA portion, Miami-Dade surtax), and the source URL for each. Rates are decimal fractions, not percentages.
Custom vs. statute: Where statute says "either party" and custom assigns payment to the seller or buyer, we record custombecause that's what you'll actually owe at closing.
Update cadence: Annually, or when major legislation changes the rate (e.g., Rhode Island's 63% increase effective January 2026 was captured in the current data).

Flood zone risk and SFHA percentage

Source: Federal Emergency Management Agency (FEMA), National Risk Index (NRI), county-level feature layer. Queried via the published ArcGIS FeatureServer.
Vintage: NRI version 1.20 (December 2025).
SFHA percentage: Computed as (Coastal Flood Exposure Area + Inland Flood Exposure Area) ÷ Total County Area, using FEMA's CFLD_EXP_AREA, IFLD_EXP_AREA, and AREA fields. Result is the share of county land within a Special Flood Hazard Area.
Risk buckets: LOW (<5%), MODERATE (5–15%), HIGH (15–30%), VERY_HIGH (>30%).
Coverage: 3,144 US counties. This is the authoritative FEMA aggregate, not a parcel-level flood zone lookup. For specific properties, users are directed to msc.fema.gov.
Update cadence: Annually when FEMA releases a new NRI version.

Closing cost components

Sources: Consumer Financial Protection Bureau (CFPB), American Land Title Association (ALTA), Bankrate 2025–2026 closing cost survey, Angi/HomeAdvisor cost databases for inspection and appraisal ranges, Fannie Mae and Rocket Mortgage for loan origination norms.
Components covered: Title insurance, loan origination, appraisal, home inspection, attorney fees (attorney-required states only), recording fees, prepaid interest, insurance escrow, and property tax escrow.
Caveat: These are national ranges, not county-specific. Title insurance rates are regulated in FL, TX, and NM; attorney fees only apply in attorney-required closing states. The calculator applies the correct subset based on the county's state.
Update cadence: Re-verified annually against the latest CFPB and Bankrate surveys.

Calculator assumptions

The calculator combines the county data above with a small set of fixed assumptions. Every result is labeled as an estimate; these are the inputs behind it.

Annual property tax: Purchase price × the county's median tax-to-value ratio. That ratio is a county-wide statistic (median taxes paid divided by median home value) and is neither a millage rate nor a rate levied on any particular parcel. Your actual bill depends on your assessed value, the millage set by your overlapping taxing districts, and any exemptions you qualify for. Note the direction of the likely error: because the ratio reflects taxes actually paid by current owners, many of whom hold homestead exemptions and assessment caps, a buyer paying today's market price often pays more than the ratio implies.

Where a state resets the assessment on a sale and sets the buyer's rate by law, that rate replaces the ratio. Today that is California: Proposition 13 resets the assessed value to the purchase price, and a buyer pays the 1% base rate plus voter-approved debt levies, which average about 0.1% statewide, so the calculator uses 1.10%. The county ratio is still shown, as context.

In Arkansas, the District of Columbia, Florida, Georgia, Maryland, Michigan, New Mexico, Oklahoma, South Carolina and Texas, an assessment cap also lifts on a sale, but the rate varies locally, so the calculator keeps the county ratio and labels the figure as likely understated for a buyer. In Arizona, Nevada and Oregon the cap stays with the property, so a buyer inherits it and the ratio is not biased for this reason. Each state's rule and source is in data/property_tax_rules.json.
Special assessments: Optional, entered per year: Mello-Roos in California, CDD assessments in Florida, and other district levies billed with property tax. They are added to the monthly total, the year-one total and the property tax escrow at closing. We have no data for them, so they default to $0. Check the seller's latest tax bill.
Transfer tax: The buyer's share of the state transfer tax rate, charged on the basis the state declares — the purchase price in every state we currently publish, or the loan amount where a state levies it that way. Known county and city add-ons are described in each county page's FAQ but are not added to the calculated total.
Mortgage payment: Principal and interest on a fixed-rate loan for the term you choose. Each term uses its own published national average from the Freddie Mac Primary Mortgage Market Survey (PMMS), currently 7.24% for 30 years and 6.60% for 15 years, week ending October 29, 2026. PMMS publishes weekly; we refresh the figure each Friday and redeploy when it changes. A national average is not a quote. Your lender sets the actual rate. The same rate is used for prepaid interest at closing.
Homeowners insurance: A range of $1,000–$2,000 per year for the monthly estimate, and $1,500 per year for the year-one total and insurance escrow. Flood insurance is not included; see the county's flood exposure instead.
Private mortgage insurance (PMI): Applied when the loan is more than 80% of the purchase price, at 0.85% of the loan amount per year.
Closing costs: The national ranges described above, with attorney fees applied only in attorney-closing states.
Year-one total: Total cash out of pocket across your first year: the down payment, transfer tax and the midpoint of the closing-cost range, plus twelve months of mortgage principal and interest, property tax, insurance, PMI where it applies, and HOA dues. It is a cash-flow figure, not a cost of ownership — most of the mortgage portion is principal, which buys you equity rather than disappearing.

State property tax exemptions

Source: Primary source per state: Department of Revenue, Tax Commission, or statute. All 51 jurisdictions individually researched. Every exemption in data/state_exemptions.json links to a live .gov URL.
Categories captured: Homestead, senior (65+), disabled veteran, general disability, assessment caps (e.g., California Prop 13, Michigan Proposal A, Texas 10% cap), first-time buyer programs, and agricultural/current-use.
What we do NOT capture: County-level local exemptions that supplement state programs. For example, Texas school districts each set their own local homestead amount above the state minimum. Users are directed to their county assessor for final verification.
Verified: April 2026.

What we deliberately don't do

No affiliate links. No mortgage lender, title company, or agent referrals. The site earns revenue only from the companion mobile app, not from steering buyers.
No ads. No display ads, no sponsored content. Pages load fast because they carry no ad JavaScript.
No invented data.Every figure traces to a public source. Where we don't have data for a county, the page says so. We don't fill gaps with estimates.
No per-property flood zone lookup. Our flood data is a county-level FEMA aggregate. For your specific property, use msc.fema.gov.

Reproducibility

The data pipeline is open and scriptable. Every dataset on the site can be regenerated from its public source:

scripts/build-counties-csv.mjs pulls the Census ACS data and produces counties.csv
scripts/build-flood-zones.py queries FEMA NRI and produces flood_zones.csv
scripts/build-county-narratives.mjs generates per-county narrative paragraphs deterministically from the data files
scripts/build-llms-full.mjs generates the llms-full.txt index for LLM crawlers

Transfer taxes and state exemptions are maintained as hand-verified JSON files because they require live .gov source verification; running a script against the state DOR pages would not be reliable.

Found an error? See how to report it and how we handle corrections in our editorial policy.