Mountainside, NJ Q3 2026 Home Sales Report: 27 Closings, a $1,065,000 Median, and Two Houses That Stopped at $999,999

Twenty-seven Mountainside closings carry a recorded date between July 1 and September 30, 2026. The median is $1,065,000, the mean $1,209,611, and the range runs from $635,000 to $2,775,000. We published thirteen of those closings first, and a $999,999 median, because the portal we started from dates only a fraction of what it lists. Two further sources date all twenty-seven and agree with each other on every overlapping date. The correction matters in a specific way: at thirteen closings the median sat one dollar below the $1,000,000 mansion-tax threshold, and at twenty-seven it sits $65,001 above it. Fourteen of the twenty-seven owed the tax. Thirteen did not. The $999,999 pattern is real and survives the recount — but it belongs to two individual houses, not to the town's midpoint.

Twenty-seven closings, July 1 through September 30, 2026. Median $1,065,000. Mean $1,209,611. Low $635,000, high $2,775,000. The mean sitting about $145,000 above the median is what a single large sale does to a small sample — 1613 Rising Way at $2,775,000 is $961,000 clear of the next closing, and in a twenty-seven-sale quarter one house like that moves the average and leaves the midpoint alone.

The first version of this report counted thirteen. That was not a sampling choice; it was the ceiling of what one source would date. The portal we began with lists twenty-five Mountainside properties and attaches a sale date to five of them. Everything else shows a price and a blank. We dated what we could, said so, and published a median built on thirteen houses.

Two other sources date all twenty-seven, and they agree with each other on every single overlapping date. Not approximately — exactly. That is the strongest evidence in this report, and it is what gave us the confidence to replace our own figure rather than hedge it.

September is the heaviest month and the most expensive, which is the reverse of the recording-lag pattern we found in Westfield, where September looked thin because deeds had not caught up. Here the September closings are dated by two sources independently, so the weight is real rather than an artefact of when we looked.

What changed, precisely. At thirteen closings the median was $999,999. At twenty-seven it is $1,065,000. The figure moved $65,001, and in moving it crossed the one threshold in New Jersey residential practice that costs money. A seller reading the first version would have concluded that the typical Mountainside house sells just under the mansion-tax line. The typical Mountainside house sells just over it.

248 Appletree Lane closed at $999,999 on July 22. 1080 Prospect Avenue closed at $999,999 on September 9. Two different houses, two months apart, at the same price to the dollar, and that price is one dollar below the point where New Jersey's seller-paid mansion tax begins.

New Jersey's mansion tax applies its rate to the entire consideration rather than to the portion above each threshold, which makes every threshold a cliff rather than a step. At $999,999 the seller owes nothing. At $1,000,001 the seller owes one percent of the whole price — about $10,000. To be better off crossing the line at all, a seller needs roughly $1,010,100, because anything between $1,000,000 and that figure nets less than stopping a dollar short.

Four of twenty-seven closings — fifteen percent of the quarter — landed within $20,000 below the threshold. Not one closed between $1,000,000 and $1,010,100, the band where crossing costs the seller money. The next closing above the line is 364 Dogwood Way at $1,065,000, comfortably past the break-even.

We cannot see inside these negotiations and do not claim any price was deliberately structured. We can say that $999,999 is not a figure two separate sets of parties reach by accident while negotiating in round numbers, and that the gap immediately above the line is empty in a quarter where the band immediately below it holds four houses.

1613 Rising Way at $2,775,000 is the quarter's outlier and the reason the mean and median diverge. It closed July 31 and sits $961,000 above 1023 Prospect Avenue, the next sale down. Nothing else in the quarter came within three-quarters of a million dollars of it.

Below that the top of the market is dense rather than thin. Six closings between $1,500,000 and $1,814,000, all within about $290,000 of each other, and four of those six closed in a three-week span in September. That is a different shape from Westfield next door, where the sales above $2,000,000 spread across the whole quarter.

Charles Street appears twice at opposite ends. 1003 Charles Street closed at $1,527,500 on September 15; 1075 Charles Street closed at $860,000 on July 1. Same street, $667,500 apart, eleven weeks apart. In a borough this small the street name tells you almost nothing about price, which is the practical reason an automated estimate built on street-level comparables struggles here.

Mountainside produces too few transactions for any one source to be checked against itself. A town with four hundred closings a quarter forgives a source that misses forty of them. A town with twenty-seven does not: miss fourteen and the median moves across a tax threshold, which is exactly what happened to our first version.

The four sources we used behave differently enough that the differences are worth naming. One lists twenty-five Mountainside properties and dates five. A second lists twenty-four and dates none. A third and fourth date every closing they carry, and agree with one another exactly. No single one of the four held all twenty-seven.

Two of the undated ten sit at exactly $999,000, which is a different number from $999,999 and worth not conflating. We mention them because if either closed in this quarter the cluster below the tax line grows from four closings to six, and because neither can be dated from any source we have.

The borough's own history explains part of why the sample is thin. Mountainside has held its zoning largely unchanged for decades, with almost no multi-family construction and no transit-oriented redevelopment of the kind Garwood and Cranford have absorbed. The housing stock turns over slowly because there is very little of it being added. Twenty-seven closings in a quarter is not a measurement failure; it is close to the whole market.

If your Mountainside house is likely to price between $950,000 and $1,050,000, this is the single most consequential decision in your sale. Fourteen of this quarter's twenty-seven closings owed the mansion tax and thirteen did not, and the boundary runs straight through the middle of the borough's ordinary range — which is not true of Westfield above it or Cranford below it.

The arithmetic is unforgiving in one direction only. Below $1,000,000 you owe nothing. At $1,000,001 you owe one percent of the entire price. Between $1,000,000 and about $1,010,100 you net less than you would have at $999,999, which is why four houses stopped below the line this quarter and none landed in that band. Above $1,010,100 the problem disappears and you should price to the market. The full bracket arithmetic, including the break-evens at $2,000,000 and $3,000,000, is worked through in our Westfield Q3 report.

For buyers, the practical consequence is comparable scarcity rather than price. Twenty-seven closings across 4.1 square miles means that for most houses there are perhaps three or four genuinely comparable recent sales, and an automated valuation reaching for more will cross into Westfield, Scotch Plains or Springfield to find them. One national search page we checked while compiling this report returned Westfield, Springfield and Scotch Plains properties under a Mountainside heading.

Dual-licensed in New York and New Jersey, with a Staten Island and New York City buyer base that reaches Union County directly. For a borough that produces twenty-seven sales a quarter, reach matters more than volume.

Get a valuation built on this quarter's twenty-seven dated closings and the threshold arithmetic that decides what you keep — not on whichever sales one portal happened to date.

Call 718-873-7345

What was the median home sale price in Mountainside, NJ in Q3 2026?

We computed $1,065,000 from 27 closings with Mountainside addresses and recorded sale dates between July 1 and September 30, 2026. The mean was $1,209,611 and the range ran from $635,000 to $2,775,000. July recorded 8 closings at a $995,500 median, August 7 at $901,000 and September 12 at $1,170,000.

Why does this report now show 27 closings instead of 13?

Our first version was built on the one source we started from, which lists twenty-five Mountainside properties but attaches a sale date to only five. We dated thirteen closings and published a $999,999 median. Two further sources date all twenty-seven and agree with each other on every overlapping date, so we replaced our figure rather than hedging it. The median moved $65,001, from one dollar below the mansion-tax threshold to $65,001 above it.

Did Mountainside sellers pay the mansion tax in Q3 2026?

Fourteen of the twenty-seven did and thirteen did not. New Jersey's mansion tax begins above $1,000,000 and the seller has paid it since the 2025 amendments, applied to the entire purchase price rather than the portion above the line. Mountainside is the town in this series where the threshold runs through the middle of the ordinary range rather than the top or the bottom of it.

Why did two Mountainside homes sell for exactly $999,999?

248 Appletree Lane closed at $999,999 on July 22 and 1080 Prospect Avenue closed at $999,999 on September 9. One dollar more and the seller would owe about $10,000, because the rate applies to the whole price. A seller needs roughly $1,010,100 to be better off crossing the line at all, and no closing this quarter landed between $1,000,000 and that figure. We cannot see inside these negotiations and do not claim any price was deliberately structured.

What was the highest home sale in Mountainside in Q3 2026?

1613 Rising Way at $2,775,000, dated July 31, 2026 — $961,000 above the next closing. Then 1023 Prospect Avenue at $1,814,000 on September 22, 244 Pembrook Road at $1,746,000 on August 17, 301 Garrett Road at $1,655,000 and 1233 Poplar Avenue at $1,650,000, both on September 18.

Can I trust an online valuation for a Mountainside home?

Treat it as a starting point. Twenty-seven closings across 4.1 square miles leaves most houses with three or four genuinely comparable recent sales, and an automated model reaching for more will cross into Westfield, Scotch Plains or Springfield. One national search page we checked while compiling this report returned Westfield, Springfield and Scotch Plains properties under a Mountainside heading.

Westfield Q3 2026 — $1,500,000 Median, 81 Closings
Cranford Q3 2026 — 57 Closings and Two Sales at $999,999
Clark Q3 2026 — A Sold List Contaminated by Seven Towns
Garwood Q3 2026 — Nobody Paid the Mansion Tax
Mountainside Market Report — The Town That Froze Itself in Place
Mountainside Housing Market — Spring 2026
Mountainside Neighborhoods & Homes Guide
Complete Guide to Buying and Selling in Mountainside
How to Sell Your Mountainside Home
Moving to Mountainside from New York
Mountainside & the Watchung Reservation
Scotch Plains Market Update — October 2026
Union County Towns by Price