A home of $4M or more in Boston's Back Bay or Beacon Hill is financed in 2026 almost always with a jumbo mortgage, because Suffolk County's 2026 conforming loan limit is $962,550 — roughly a fifth of this page's $4,500,000 representative price. Buyers here bring far more than a standard down payment: many pay cash or close to it, and those who borrow 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 the highest-cost areas. The FHFA's published county list prices the Boston-area counties — Essex, Middlesex, Norfolk, Plymouth and Suffolk — at $962,550 for a one-unit home; that is up from $914,250 in Suffolk County in 2025, so the limit moved rather than holding flat. Dukes and Nantucket carry the $1,249,125 ceiling; the rest of Massachusetts sits at the baseline.
The arithmetic leaves no middle ground. A buyer who borrows half the price of a $4,500,000 home takes a $2,250,000 loan — more than double the county limit. The homes that define this market, from the Back Bay rowhouse blocks of Commonwealth Avenue and Marlborough Street to the Beacon Hill streets around Louisburg Square, trade at prices several times the limit, so essentially every financed purchase is a jumbo, and most are several multiples of it — the range lenders informally call super-jumbo.
Much of the stock changes hands quietly: whole townhouses and full-floor residences in these neighborhoods are often arranged before they are marketed, and a cash-heavy buyer pool gives sellers little reason to wait on financing contingencies. 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. Raising the cash can itself be taxable — Massachusetts taxes long-term capital gains at 5 percent, and the state's published rates add a 4 percent surtax on income above $1,107,750 for 2026 — a 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. Homes here are frequently held in trusts or limited liability companies, which jumbo lenders will underwrite with added documentation.
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 same arithmetic governs other dense, high-priced urban markets; Manhattan covers New York's version of these questions.
The local costs that change the math
Property tax. Boston's most recently certified residential rate, for fiscal 2026, is $12.40 per $1,000 of assessed value, per the city's assessing department. The tax follows assessed value, and assessments track market value, so a buyer should expect the bill to follow the price rather than the seller's old bill. On a $4,500,000 home the annual tax is $55,800 before exemptions, billed quarterly. Boston sets its rates in a classification vote each fall and the new rate appears on the third-quarter bill, typically issued in late December — so $12.40 is a snapshot, not a fixture. A primary-residence owner who qualifies for the city's residential exemption saved up to $4,353.74 in fiscal 2026, bringing such a bill to about $51,446 — meaningful relief on a modest home, a small offset at this price level.
Deeds excise. Massachusetts charges an excise on every conveyance of real estate at $2.28 per $500 of consideration — $4.56 per $1,000 — as the Department of Revenue's published rates state, computed on the price less any mortgage the buyer assumes. On a $4,500,000 sale the excise is $20,520. It is the seller's cost, and the stamped deed is recorded with the Suffolk County Registry of Deeds.
Historic districts. Back Bay and Beacon Hill are designated local historic districts. Exterior work visible from a public way requires review and approval by the neighborhood's architectural commission before it begins, and the commissions enforce design standards that reach materials, windows, rooflines and additions. The review adds time and limits to renovation plans. A buyer pricing a gut renovation of a 19th-century rowhouse should treat the design-review process as part of the project, not a formality after it.
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 a $4,500,000 home at the program's standard terms, the household commits half the price at closing: $900,000 as the down payment and $1,350,000 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 $3,600,000, 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 $19,500 a month, and in years 16 to 30 the scheduled payment is $31,360 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; and the instrument's values are reviewed against the plan every year.
The trade is deliberate: $2,250,000 committed at closing, in exchange for fifteen years of the lower payment and a designed source for the later ones. 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.
