One ZIP, two tax bills: why the county line matters to the underwrite
ZCTA 30005 sits in two Georgia counties. On a $400,000 rental, a Johns Creek parcel's 2025 rate costs about $854 a year more than an unincorporated Forsyth parcel. Same ZIP, different tax line.
A ZIP code feels like a place. It is printed on the listing, it fills the search box, and most rent and tax lookups start with it. But a ZIP code is a mail route. The post office draws it to deliver mail, and it does not stop at county lines. Property taxes do. They are set by the county, the school system, and sometimes a city, and they follow the parcel, not the ZIP.
So two houses with the same ZIP can carry very different tax bills. If the underwrite uses the wrong county’s rate, the tax line is off from the first month.
How to see a ZIP that crosses a county line
The U.S. Census Bureau publishes ZIP Code Tabulation Areas (ZCTAs). They are approximations of ZIP codes built from census blocks, and they are close enough to show where a ZIP spreads across more than one county. The Census relationship file lists every county each ZCTA touches.[1]
ZCTA 30005, around Alpharetta and Johns Creek in metro Atlanta, is one of them. Census shows it with land in both Fulton County and Forsyth County, Georgia.[1] Same five digits, two counties, two school systems.
ProfitGuard uses the full street address, not just the ZIP, to match a property to its county and that county's tax rate, so a split ZIP like this one lands on the right bill. That data comes from public sources that can lag or contain errors, so always confirm the rate on the county tax assessor's site before you lock your numbers.
What each side charges
To keep this concrete, the comparison is a parcel inside the city of Johns Creek on the Fulton side against a parcel in unincorporated Forsyth County, meaning outside any city. Those are two real options inside ZCTA 30005, but they are not the only ones. Other parts of the ZCTA sit in other cities, with other rates.
Georgia taxes real property on 40% of fair market value, and the tax rate is quoted in mills. One mill is $1 of tax per $1,000 of assessed value.[2] Both rates below are the full levy for that location in tax year 2025, as each county publishes it.
Unincorporated Forsyth County: 24.522 mills. Forsyth lists five levies: county operations 4.791, fire 2.805, and county bond 0.300 (7.896 for county government), plus school operations 15.208 and school bond 1.418 (16.626 for schools). There is no city levy outside city limits.[2]
Johns Creek, Fulton County: 29.861 mills. The Fulton County Tax Commissioner lists this as the total Johns Creek millage, covering county, school, and city. The same sheet lists 26.119 mills as the Fulton County and school rate inside cities, which leaves 3.742 mills for the city of Johns Creek.[3]
The total gap is 5.339 mills, and most of it is not the county line itself. About 3.742 mills is the Johns Creek city levy. The other 1.597 mills is the difference between Fulton’s and Forsyth’s county and school rates. A house in a different Fulton city, or in a city on the Forsyth side, would land somewhere else. That is the reason to look at the parcel, not the ZIP.
Worked example: one house, two sides of the line
Take a fictional rental worth $400,000, held as an investment, so no homestead exemption applies.[4] Run the same house through both rates.
Line | Unincorporated Forsyth County | Johns Creek, Fulton County |
|---|---|---|
Fair market value (assumed) | $400,000 | $400,000 |
Assessed value (40%) | $160,000 | $160,000 |
Total millage, tax year 2025 | 24.522 | 29.861 |
Yearly tax | $3,923.52 | $4,777.76 |
Monthly tax | $326.96 | $398.15 |
The Johns Creek parcel costs about $854 more a year, or about $71 more a month, for the same value of house in the same ZCTA. Of that, about $599 a year is the city levy and about $256 a year is the county and school difference.
What it does to cash flow
Say the underwrite was built with the Forsyth rate and showed $200 a month of cash flow. That is an example assumption. The house turns out to sit inside Johns Creek on the Fulton side.
Line (monthly) | Amount |
|---|---|
Planned cash flow, using the Forsyth rate | $200.00 |
Tax difference, Johns Creek parcel | −$71.19 |
Cash flow on the Johns Creek parcel | $128.81 |
Change over a year | −$854.24 |
Rent did not change. The loan did not change. The tax line did, and it takes about a third of the planned cash flow with it. The same amount comes out of net operating income (NOI), so the debt service coverage ratio (DSCR) drops too. The loan is part of the rental underwrite walks through how lenders read that ratio.
If the house is escrowed, the higher bill also shows up in the payment once the servicer catches up, which is the pattern described in the year-two escrow post.
How to get the right rate on day one
✓ Start from the parcel, not the ZIP. Find the parcel on the county assessor or tax commissioner site. If the address does not show up in the county you expected, check the neighboring one.
✓ Read the current bill. It lists each taxing authority: county, school, city, and any special district. Add them up yourself.
✓ Use the rate for the right year. Rates are adopted each year. Note the tax year next to the number in your sheet.
✓ Drop the seller’s exemptions. A homestead exemption belongs to an owner who lives there. A rental usually will not carry it.
✓ Price the value the county will use. Assessed value follows the county’s own appraisal, which can move after a sale. The full cash-flow stack keeps a buffer on the tax line for this.
Close the loop
A ZIP code tells the mail carrier where to go. The county line tells the tax collector what to charge. When a ZIP crosses that line, the same house can cost hundreds of dollars more a year depending on which side it sits on. Look up the parcel, add up every taxing authority on the bill, and write the tax year next to the rate. Then the tax line in the underwrite is the one the county will actually send.
Next up: Analyze the full cash-flow stack before you offer — every line, from rent to reserves, before the bid.