Every Peninsula homeowner has heard the national story by now: most people refinanced into cheap mortgages during the pandemic, almost nobody wants to give them up, and that is why so few homes are for sale. It is true. But the national headline treats the country as one undifferentiated market, and the Peninsula is anything but. Atherton is not Mountain View. A street of estates where families have held for thirty years behaves nothing like a corridor of recently sold condominiums.
So I built a way to measure the difference. The Peninsula Lock-In Index is a monthly estimate of how frozen each local market is, city by city, on a single 0 to 100 scale. It exists to answer a question I get constantly from sellers: is my market actually as stuck as the news says, and if so, what does that mean for me? This is the inaugural May 2026 reading. I will publish a fresh one each month, so over time you can watch the freeze tighten or thaw in your specific city.
What Is the Peninsula Lock-In Index?
The Peninsula Lock-In Index is a 0 to 100 score estimating how strongly the owners in a given city are financially discouraged from selling. A higher score means more households have good reason to stay put, which means scarcer inventory and more leverage for the rare seller who does list. It is a measure of market friction, not home value.
It is built from three real, well-documented forces, combined into one number. I am deliberately transparent about the method, because an index is only useful if you can see how it was made.
How the Index is built
Each city's score blends three weighted inputs into a 0 to 100 scale:
- Rate lock (50%). The estimated share of mortgaged owners holding a rate below 5 percent, anchored to the statewide figure that 77 percent of California homeowners were below 5 percent as of September 2025, then adjusted by each city's purchase and refinance vintage.
- Proposition 13 reset penalty (30%). The gap between assessed and market value, which rises with owner tenure. The wider the gap, the more annual property tax a seller would add by rebuying, and the stronger the disincentive to move.
- Turnover drag (20%). Recent sales velocity. Slower-moving markets with longer tenure score higher; higher-turnover markets score lower.
These are modeled estimates built from public county, census, and mortgage-distribution data, not a census of individual loans. No source publishes the exact share of locked-in owners for a single city, so the Index is a transparent directional model, refined each month as new data arrives. Treat the scores as a well-grounded ranking, not a decimal-precise fact.
Which Peninsula Cities Are Most Frozen Right Now?
In the May 2026 reading, the estate towns are the most locked: Atherton tops the Index at roughly 83, with Hillsborough and Los Altos Hills close behind. The least frozen major market is Mountain View at about 64, where more recent buyers and a larger condominium base keep turnover comparatively higher. Here is the full inaugural ranking.
| Rank | City | Lock-In Index | Read |
|---|---|---|---|
| 1 | Atherton | 83 | Deeply frozen |
| 2 | Hillsborough | 81 | Deeply frozen |
| 3 | Los Altos Hills | 80 | Deeply frozen |
| 4 | Los Altos | 78 | Very tight |
| 5 | Palo Alto | 76 | Very tight |
| 6 | Menlo Park | 75 | Very tight |
| 7 | Burlingame | 73 | Tight |
| 8 | San Carlos | 72 | Tight |
| 9 | Belmont | 71 | Tight |
| 10 | San Mateo | 68 | Firm |
| 11 | Redwood City | 66 | Firm |
| 12 | Sunnyvale | 65 | Firm |
| 13 | Mountain View | 64 | Firm |
Notice how narrow the band is. Even the least frozen city on this list scores a 64, which in absolute terms is still a profoundly supply-starved market. For context, San Mateo County is running near 1.9 months of inventory this spring and Santa Clara County near 1.8 months, where five to six months marks a balanced market. The whole Peninsula is frozen. The Index simply tells you how frozen, and where seller leverage is most concentrated.
Why Are Atherton and the Estate Towns the Most Locked?
The estate towns score highest because two powerful lock-in forces compound there more than anywhere else. The rate lock is real, but in these communities the Proposition 13 reset penalty is often the larger anchor.
Consider a family that bought in Atherton or Hillsborough two decades ago. Their assessed value, capped by Proposition 13 at roughly 2 percent annual growth, may sit at a fraction of today's market value. Selling and rebuying anywhere nearby would reset that basis to current value and could add tens of thousands of dollars per year in property tax, permanently. Layer a sub-3 percent pandemic-era mortgage on top, and the financial logic of staying put becomes overwhelming. These are not owners waiting for a slightly better month to list. Many have no intention of moving at all.
In the estate towns, the lock is structural, not seasonal. Proposition 13 and a cheap mortgage together turn the cost of moving into a number large enough that most owners simply opt out for good. That is why a single well-prepared listing in Atherton can command the attention of every qualified buyer in the price band.
The lower-scoring cities are not less desirable. They simply have a different ownership mix: more recent purchases at higher rates, more condominiums and townhomes, younger households earlier in their tenure, and therefore more natural turnover. The freeze is still severe, but a few more owners have a reason to move in any given year.
What Does a High Lock-In Score Mean if I Want to Sell?
A high score is good news for a seller, not bad. It means most of your would-be competitors are staying off the market, so when you list, you are one of very few options for every motivated buyer in your price band and school district. Scarcity transfers pricing power to you.
This is the part homeowners most often get backwards. They read that the market is frozen, assume frozen means weak, and decide to wait. But the data says the opposite. Santa Clara County single-family listings ran near 105 percent of list price this spring with a median of roughly 9 days on market. San Mateo County single-family homes are appreciating in the 4 to 5 percent range year over year. Those are not the numbers of a soft market. They are the numbers of a market where deep, well-capitalized demand is competing for a tiny pool of homes.
There is also a timing signal buried in the Index. The instinct to wait for mortgage rates to fall is the same instinct keeping your competition off the market. When rates ease, the lock loosens, more owners list, and Index scores fall. The very moment more sellers feel comfortable coming to market is the moment your scarcity advantage starts to shrink. A high score today is a window, not a permanent condition.
Wondering whether a move actually pencils out for your household before you weigh the lock-in math? Take our free 60-second affordability quiz to see where your budget and priorities land across Peninsula communities, then we can talk about whether listing into this scarcity makes sense for you.
What Does the Index Mean for Buyers?
For buyers, a high Lock-In score is a warning to come prepared. Direct answer: the more frozen a city, the fewer homes will reach the market and the more competition each one will draw, so success depends on being ready to move decisively rather than waiting for selection to improve.
In the highest-score cities, inventory will not loosen meaningfully until rates fall enough to unlock sellers, and even then the Proposition 13 anchor will keep long-tenure owners in place. Buyers who insist on waiting for a deep menu of choices in Atherton or Los Altos Hills are waiting for a market that the structure of California property tax will not deliver. The more realistic strategy is to define your criteria precisely, secure financing and proof of funds in advance, and be ready to act when the rare right home appears. The buyers winning on the Peninsula in 2026 are not the ones who waited. They are the ones who were prepared.
Reading the Index Over Time
The single most useful thing about an index is the trend, not the snapshot. One month tells you where leverage sits today. Twelve months tell you where the market is heading. As I publish each new reading, three movements are worth watching:
- Falling scores signal a thaw. If mortgage rates ease through 2026 as some forecasts suggest, the rate-lock component will soften first, scores will tick down, and more inventory will follow. That is the early signal that the seller's scarcity window is narrowing.
- The estate-town gap is sticky. Because the Proposition 13 penalty does not move with interest rates, expect Atherton, Hillsborough, and Los Altos Hills to stay near the top of the Index even if rates fall. Their lock is structural.
- Turnover cities move first. Watch the lower half of the table. Mountain View, Sunnyvale, and Redwood City will show loosening before the estate towns do, making them the canaries for any broader Peninsula thaw.
That is the value of a recurring read. The lock-in effect is not a static fact you absorb once. It is a force that tightens and loosens, unevenly, city by city, and the seller or buyer who is watching it move has a real advantage over the one reacting to a national headline three months late.
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Frequently Asked Questions
Q: What is the Peninsula Lock-In Index?
A: The Peninsula Lock-In Index is a monthly estimate of how frozen each Peninsula city's for-sale market is, on a scale of 0 to 100. A higher score means more owner households have a strong financial reason not to sell, which means scarcer inventory and more leverage for the few sellers who list. It combines three modeled inputs: the estimated share of mortgaged owners below a 5 percent rate, the Proposition 13 tax-reset penalty weighted by tenure, and recent sales turnover. The scores are transparent estimates, not a census of individual loans.
Q: Which Peninsula city has the most frozen housing market in 2026?
A: In the May 2026 reading, Atherton holds the highest Lock-In Index score at roughly 83, followed by Hillsborough near 81 and Los Altos Hills near 80. These long-tenure estate communities combine deep mortgage rate lock-in with the largest Proposition 13 tax-reset penalties. Mountain View, at roughly 64, is the least frozen major Peninsula city because it has more recent buyers, more condominiums, and higher turnover.
Q: Why does Proposition 13 make Peninsula homeowners less likely to sell?
A: Proposition 13 caps a California home's assessed value at its purchase price plus a small annual increase, so a longtime owner often pays property tax on a fraction of today's market value. Selling and rebuying resets that basis to current value, which can add tens of thousands of dollars in annual property tax to the cost of moving. On the Peninsula, where homes are expensive and tenure is long, this penalty stacks on top of the mortgage rate lock-in and keeps inventory unusually tight.
Q: Does a high Lock-In score mean it is a bad time to sell?
A: No, it usually means the opposite. A high score signals that most owners are staying put, so the seller who does list faces very little competition for a deep, well-capitalized buyer pool. Scarcity transfers pricing power to the seller. In high-score Peninsula cities, a well-prepared listing routinely draws multiple offers and sells above asking because qualified buyers have so few homes to choose from.
Q: How often is the Peninsula Lock-In Index updated?
A: The Index is published monthly. Each reading reflects current mortgage rates, the latest California rate-distribution data, and recent county sales velocity, so scores shift as rates move and inventory loosens or tightens. As rates ease and more owners regain a reason to list, scores will gradually fall, which is itself a signal that seller scarcity is weakening.
Know anyone weighing a Peninsula move? Send them a free home valuation.