The same house pays about $3,690 more a year in Fairfax than in Loudoun. Here is the math.

The number on our home page says a home at the two counties' average sale price pays about $3,690 more in county real estate tax each year in Fairfax than in Loudoun. We would rather show our work than ask you to trust us, so here is the whole calculation.

Step 1: the rates

Each county sets a real estate tax rate per $100 of assessed value. For tax year 2026, Loudoun's rate is $0.805 and Fairfax's is $1.12. The difference is $0.315 per $100, or 0.315% of assessed value.

Step 2: the house

To compare like with like, we need the same house in both counties. We used the average sale price of a detached home in July 2026: about $1.13 million in Loudoun, calculated from the Dulles Area Association of Realtors' monthly totals, and $1,208,722 in Fairfax, from MarketStats. The mean of the two is about $1.17 million. That is the house.

Step 3: the bills

That is it. Multiply $1,170,000 by 0.00805 and by 0.0112, and subtract.

Why the gap exists

Loudoun's rate was not always the lowest in the region. In 2016 it was $1.145, higher than Fairfax's is today. It fell, year after year, because the county kept adding commercial tax base. Commercial and industrial property is now 36.6% of Loudoun's real estate base, and data centers alone supply about $1.3 billion a year, roughly 40% of the county's General Fund. In Fairfax, commercial and industrial property is 14.4% of the base, excluding apartments, and commercial values there have fallen for three straight years.

Here is what that history is worth to one homeowner. Take the same $1.17 million home and apply the rate Loudoun charged in 2016.

If Loudoun still charged its 2016 rate, that home would owe about $13,400 a year, nearly $4,000 more than it pays now, and a few hundred dollars more than the same home pays in Fairfax today. The difference is the commercial base that Loudoun said yes to and that has not been built elsewhere. A decade of approvals shows up on the bill as a rate cut of 34 cents.

When businesses carry more of the base, homeowners pay a lower rate. When the commercial base shrinks and nothing replaces it, homeowners pay the difference. The $3,690 is what that looks like on one house. The $4,000 is what it would have looked like if Loudoun had said no.

One caution, which we make everywhere we use this number: a base that depends heavily on a single industry has its own risk. Loudoun's own budget guidance says data center revenue "will plateau in the next 5 to 10 years." The lesson is a broad base, spread across homes, shops, offices, industry and technology, not a bet on any one of them.

What the number leaves out

• Assessment versus price. We applied the rate to a sale price, which is a stand-in for assessed value. Both counties assess at or near market value, and the Virginia Department of Taxation's 2025 sales ratio study puts the effective rate on market value at $0.75 in Loudoun and $1.02 in Fairfax. On the same $1.17 million home, that measure gives bills of about $8,780 and $11,930, a gap of about $3,150. The gap is real on either measure.
• The 2016 comparison. Applying a 2016 rate to a 2026 home price is a what-if, not a historical bill. Home values were lower in 2016, so the actual bill that year was smaller. The point of the comparison is the rate, which is the part the county controls, held against the same house.
• Other levies. The figure is the county real estate rate only. Towns levy their own rates on top, and both counties have service districts and fees that vary by address.
• Each county's own average. If you price the house at each county's own average rather than the mean of the two, the gap is larger, about $4,400, because Fairfax's average home costs more.