Two co-ops go under contract the same week, both two-bedroom units, both a short walk from the Metro-North station. One closes at $965,000. The other closes at $1,015,000, a $50,000 gap on paper. Yet the buyer of the "cheaper" unit ends up writing a bigger check at closing than the buyer who paid more. That is not a typo and it is not bad math from either attorney. It is what happens when a tax written for eight-figure Manhattan penthouses lands, almost by accident, on the exact price point where half of Mamaroneck's housing stock sits.
New York's so-called mansion tax kicks in at $1 million. In Scarsdale, where the median sale price ran to roughly $2.465 million in 2025, that line is a formality buyers clear before they've finished their first walkthrough. In Mamaroneck, it sits inside the market itself, close enough to the median that whether a given deal falls above or below it can depend on which three months of data you're looking at and which building the unit is in.
A Flat One Percent, Drawn in 1989 and Never Moved
The mansion tax started in 1989 as a flat 1% surcharge on residential sales of $1 million or more, and that threshold has never been adjusted for inflation since. Inside New York City, the legislature layered on progressive brackets in 2019, so a $10 million Manhattan sale now faces a rate north of 3%. Outside the five boroughs, including every inch of Westchester County, the original version still applies: a flat 1% on the full purchase price once you cross $1 million, nothing above that. The New York State Department of Taxation and Finance still processes these filings through the same transfer tax return that's been in use for years, and the rate hasn't changed regardless of how much the surrounding market has.
That simplicity is exactly why it matters more here than it does thirty minutes north. A flat rate means the tax either applies in full or it doesn't apply at all. There's no partial exposure, no phase-in. The entire purchase price gets taxed the moment the total crosses seven figures, which makes the location of that line, not its size, the thing worth paying attention to.
The Line That's a Formality in One Town and a Coin Flip in the Next
Median prices from the past several months put Mamaroneck almost squarely on top of the threshold, depending on how you slice the geography:
| Market | Window | Median sale price |
|---|---|---|
| Mamaroneck, town-wide | 3 months ending May 2026 | $874,000 |
| Mamaroneck ZIP 10543 (village core) | 3 months ending May 2026 | $970,000 |
| Scarsdale | 2025 full year | approximately $2,465,000 |
Narrow the window to just the village core zip code and Mamaroneck's median closes to within about 3% of the tax line. Widen it to the whole town and the gap opens to roughly 14%, still close enough that a handful of higher-priced closings in a given month can flip the average from one side of $1 million to the other. That kind of volatility around a fixed threshold does not happen in a town where the typical sale is two or three times the trigger point. It happens here because Mamaroneck's price distribution, from starter condos to Sound-side houses, straddles $1 million rather than clearing it.
Why Co-ops Compute the Line Differently
Under state tax law, the mansion tax applies to any residential conveyance, which the statute defines to include one, two, or three-family houses, individual condominium units, and individual cooperative apartment units. Co-ops are covered the same as houses and condos. But the way the taxable amount gets calculated for a co-op is not the same as the contract price on the offer sheet.
For a co-op sale, the taxable basis includes the buyer's proportional share of the building's underlying mortgage, allocated to that specific unit, added on top of the price paid for the shares. That allocation lives in the co-op's financials, not in the listing. A shareholder selling a unit for $960,000 in a building with a modest underlying mortgage might close well under the line. The same $960,000 sale in a building carrying meaningful underlying debt could push the effective taxable basis past $1 million once that unit's share of the mortgage gets added in, triggering the tax even though the number on the contract never touched seven figures.
This is not an abstract wrinkle in Mamaroneck. The town's co-op stock includes buildings from very different eras and very different debt positions, among them The Alden House and Oakehaven, both dating to the early 1900s, the mid-century Mamaroneck Gardens, and Palmer Terrace in the Sound Shore community. Older buildings with refinanced or newer underlying mortgages are exactly where this math tends to surprise people, because the per-share debt allocation can be a meaningful fraction of the unit's own price.
The Pricing Behavior This Creates
Sellers and their agents know about the mansion tax line even when buyers don't, and pricing tends to reflect it. In one widely cited market snapshot of listings clustered near the threshold, hundreds of units sat priced between $999,000 and $999,999, while only a small handful sat between $1,000,000 and $1,001,000. The incentive is straightforward: pricing a unit at $1,010,000 instead of $999,000 doesn't just add $11,000 to the sticker, it adds a full 1% tax on the entire price, since the tax applies to the whole purchase amount once triggered, not just the portion above the line.
In most Westchester towns this pricing behavior is a rounding error nobody thinks about because most sales are nowhere near $1 million either way. In Mamaroneck, where a meaningful share of listings sit within striking distance of the threshold in both directions, that clustering incentive shapes real list prices, not hypothetical ones.
What This Means Before You Sign Anything
For a co-op seller, the practical move is getting the building's current underlying mortgage balance and per-share allocation from the managing agent before setting a list price, and sharing that figure with your broker rather than assuming the contract price alone tells the whole story. For a co-op buyer looking at anything priced between roughly $900,000 and $999,000, the same request belongs on your list before you get emotionally attached: ask for the per-share underlying debt figure so your closing attorney can calculate your actual mansion tax exposure, not just the exposure implied by the sticker price.
Not every property in town carries this risk in the same way:
- Starter condos like those at Chatsworth Gardens, with one-bedroom units historically starting around $330,000, sit far enough under the line that this math rarely applies.
- Newer midrise buildings such as Sweetwater, built in 2008, and larger luxury units at Philips Harbor, where three-bedroom layouts over 3,000 square feet can run up toward $2 million, tend to clear the threshold cleanly in either direction, avoiding the ambiguity altogether.
- It's the mid-range co-ops, roughly $850,000 to $1,050,000 on paper, where the underlying mortgage allocation actually changes the outcome.
The buyer typically pays the mansion tax by default, though this can be negotiated between the parties. The separate 0.4% New York State transfer tax runs on top of that and is customarily paid by the seller. The full fee schedule, including both taxes, is laid out on the Westchester County Clerk's land records page, which is worth a look before you finalize either side of a deal near this line.
A Few Direct Questions
Does the mansion tax apply to co-ops in Mamaroneck the same way it applies to condos and houses? Yes. The statute treats individual cooperative apartment units as residential property, subject to the same $1 million threshold and the same flat 1% rate that applies outside New York City.
Who actually pays it, buyer or seller? The buyer pays the mansion tax by default. The seller pays the separate 0.4% New York State transfer tax. Either can be negotiated as part of the deal, but that's the standard starting point.
Can a co-op's underlying mortgage really push a sub-$1 million listing over the line? Yes, because the taxable basis for a co-op includes the buyer's proportional share of that mortgage on top of the share price. It's a building-by-building question, which is exactly why the managing agent's numbers matter more than the listing price alone.
If you're weighing a Mamaroneck co-op, condo, or house anywhere near this threshold, the numbers on the listing sheet are only half the picture. Roseanna Tedone works this market from both sides of the closing table, with the licensed background to walk through a building's financials before you make an offer or set a list price. Let's Connect and get the real math in front of you before it's a surprise at the closing table.
