HUD Fair Market Rent (the rent a modest unit should cost) for an area, by bedroom count. entityid is a HUD entity id: a 10-digit county FIPS (e.g. 3600599999 for Bronx County = 36005 + 99999) or a metro CBSA code. Use zip_crosswalk then list_counties to turn a ZIP into a county entityid.

HUD income limits for an area: the extremely-low (30% AMI), very-low (50%, the Section 8 voucher line), and low (80%) income thresholds, by household size. Pass household_size (1-8) to get the single threshold that applies to a family that size. entityid is a 10-digit county FIPS or a metro CBSA code (same as fmr_lookup).

Affordability verdicts computed server-side from the same HUD tables as fmr_lookup and income_limits: whether a proposed rent is above or below the Fair Market Rent for a bedroom size (dollar and percent gap), and which HUD income bands a household qualifies under (extremely low 30%, very low 50% = the Section 8 voucher line, low 80% of area median). Pass rent + bedrooms, income + household_size, or all four; returns the verdicts with the underlying numbers and table year for citation. entityid is a 10-digit county FIPS or a metro CBSA code (same as fmr_lookup).

Map a 5-digit ZIP to the county, tract, CBSA (metro), or congressional district it falls in, using the HUD-USPS crosswalk. Returns each matching geography with its residential-address share (res_ratio); the highest-share county is the one to resolve into an entityid via list_counties.

List the counties in a state with their 10-digit FIPS entity ids, so you can look up FMR or income limits by county name. Pass a 2-letter state code.

List HUD metropolitan areas (CBSAs) with their codes, for metro-level FMR and income-limit lookups.