A home of $4M or more in Atherton or Hillsborough is financed in 2026 almost always with a jumbo mortgage, because the 2026 conforming loan limit for San Mateo County is $1,249,125 — the national high-cost ceiling, and a fraction of what an estate on these lots sells for. Buyers here rarely borrow the maximum: many pay cash or close to it, and those who finance tend to structure the loan around capital they would rather not sell. One structure built for exactly that position is The Family Office Mortgage, a privately offered program that pairs a jumbo mortgage with a structured financial product for households buying a home of $4M or more.
Why nearly every purchase here is a jumbo loan
Each year the Federal Housing Finance Agency sets the largest mortgage Fannie Mae and Freddie Mac may buy. For 2026 the limit is $832,750 for a one-unit home in most of the country and up to $1,249,125 in high-cost areas — 150 percent of the baseline, and the highest value the statute allows. The FHFA's published county list places San Mateo County at that ceiling.
The arithmetic leaves no middle ground. A buyer who borrows half the price of an $8,000,000 home takes a $4,000,000 loan — more than three times the county limit. Across the price points that define this market, from around $4,000,000 for a smaller or older home to eight figures for a finished estate, every financed purchase is a jumbo, and most are several multiples of the limit — the range lenders informally call super-jumbo.
The two towns are small and largely built out. Atherton runs from the village streets near the Caltrain station through West Atherton to the Lindenwood blocks west of El Camino Real. Hillsborough spreads across wooded hills between Highway 101 and Interstate 280, with the Carolands area and its quiet streets of homes on lots measured in acres. The technology wealth accumulated along the Peninsula's corridors north and south has long settled on these school districts and these lots, and buyers arriving from a company sale or liquidity event are a recurring feature here rather than an exception. Inventory is thin, many sales are arranged quietly rather than broadly marketed, and an older home is often bought with a rebuild in mind. Because a jumbo cannot be sold to the government-sponsored enterprises, each lender keeps the loan or places it privately and writes its own terms — down payment, reserves, rate structure. How an interest-only jumbo mortgage works covers that loan in detail.
How buyers here actually finance
Three patterns recur.
Cash or near-cash. A cash offer removes the lender from the negotiation, and in a thin market sellers value the certainty. The cost is liquidity: once several million dollars are in the house, they come back only through a sale or a new loan, and raising the cash by selling appreciated assets creates a capital gains tax bill that never appears on the closing statement. Paying cash versus financing works through that trade.
A jumbo sized to a large down payment. Buyers who could pay cash often borrow anyway, at 40% or 50% down, cutting the interest cost and keeping capital working. Lenders in this range underwrite the whole household: documented income, the assets that remain after closing, and sometimes pledged assets accepted in place of liquidated ones.
Borrowing against a portfolio. Rather than sell positions, a buyer draws on a securities-based line or pledges investment assets to the lender. The portfolio stays intact and the tax event is deferred; what is added is a variable-rate debt against marketable securities, on which a market decline can bring a maintenance call. Buy, borrow, die and where the home fits describes that route and its risks.
Within these, interest-only terms are common at this level: pay only the interest for a fixed period and decide deliberately what repays the principal, rather than letting the loan decide.
The local costs that change the math
Property tax. Under Proposition 13, the base property tax rate in California is 1 percent of assessed value, plus the rates needed to pay voter-approved bonds and certain direct assessments, as the State Board of Equalization's overview lays out. Two features matter here. First, a sale triggers reassessment: the assessed value resets to the price paid, so a home held for decades at a low value is taxed near its new price from the day it changes hands. On an $8,000,000 purchase the 1 percent base alone is $80,000 a year, before bond rates and direct assessments. Second, once established, the assessed value can generally rise no more than 2 percent a year, which rewards a long hold. The county also issues a supplemental assessment covering the balance of the first tax year after a purchase.
Transfer tax. San Mateo County's documentary transfer tax is $1.10 per $1,000 of value, according to the Assessor-County Clerk-Recorder — 55 cents per $500, computed on the price net of any liens the buyer assumes. Both towns add their own charge: Atherton's document transfer tax ordinance (Atherton Municipal Code §3.12.020, in force since 1967) and Hillsborough's real property transfer tax (Hillsborough Municipal Code §3.20.020, adopted the same year) each set $0.55 per $1,000, the half-rate city tax that Revenue and Taxation Code Section 11911 permits. That statute also credits the county tax by the town tax, so the combined tax in these two towns is $1.10 per $1,000 — $8,800 on an $8,000,000 sale — rather than double. By custom the seller pays, and the allocation is negotiable in the contract. Rates differ elsewhere in the county — the City of San Mateo levies a larger city conveyance tax — so escrow asks for the municipality before quoting a figure.
The deduction limit. Federal law caps the home mortgage interest deduction at interest on the first $750,000 of debt, or $375,000 if married filing separately, for loans taken out after December 15, 2017, per IRS Publication 936. On a loan of several million dollars the deduction reaches only a small share of the interest. Your CPA can state the after-tax cost of a given loan for your case.
Where the program fits
The program is candid about where it belongs in this market. It suits a household buying a home of $4M or more, holding $5M or more in assets beyond the home and earning $500,000 or more, that intends to keep the home and would rather commit capital up front than borrow the maximum. It is a poor fit for a buyer who needs full liquidity elsewhere or expects to sell within a few years.
On an $8,000,000 home at the program's standard terms, the household commits half the price at closing: $1,600,000 as the down payment and $2,400,000 as the contribution to the financial instrument. The mortgage — made by a third-party lender, because the program is not a lender, bank, insurance company or government program — is $6,400,000, interest only for the first fifteen years and principal and interest for the fifteen after. The rate is an assumption for illustration, not a rate offer: at an assumed 6.5 percent the interest-only payment is $34,667 a month, and in years 16 to 30 the scheduled payment is $55,751 a month. The financial instrument's projected payout is sized to that payment. The payout is a projection, not a guarantee; the mortgage still runs its full thirty years; designs are tested with no growth credited in their first two years, and the instrument's values are reviewed against the plan every year.
The trade is deliberate: $4,000,000 committed at closing, in exchange for fifteen years of the lower payment and a designed source for the later ones. The team behind the program has done hundreds of transactions. Whether the structure beats paying cash or taking a plain jumbo depends on the household, which is what the comparison and the calculator are for. The eligibility page states the criteria in full, and what a Family Office Mortgage is explains the structure for a household meeting it for the first time.
