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Single Family ResidenceResidential
$916,283$585 / ft²
TopHap EstimateOur estimate of what this home would sell for today, based on nearby sales, the home's size and features, and local price trends. It is a starting point, not an appraisal.· confidence 95How well the pricing models agreed with each other on this home. Higher means the estimate rests on stronger, more consistent data.· as of Aug 30, 2026
Last sold for $660,000 on Jul 5, 2019

21 WILLIAM ST

ROCKVILLE CENTRE, NY 11570
2.5
Baths
1,566 ft²
Living AreaTotal finished area of the structures, from county records. Some counties include shared space like hallways or a building gym, so it may not match a listing.
4,000 ft²
Lot SizeThe size of the land the home sits on, from county records.
1925
Year BuiltThe year the home was originally built, according to county records.
2
StoriesHow many floors the home has above ground.
6
RoomsThe total room count in county records. Counties differ on which rooms they count, so compare it loosely.
1
ParkingThe number of parking spaces on the property, including the garage.

Valuation

$916K
$874K low$958K high
Last SaleThe most recent sale price and date from the recorded deed. Transfers between family members or companies may not reflect market value.$660,000 · Jul 5, 2019

Tax & Assessment

Property TaxThe property tax billed for this tax year. It usually pays for schools, roads, and local services.$13,627 (2025)
Assessed ValueThe value the county assigns to this home to calculate property tax. It is often lower than what the home would sell for.$578 (2026)
Assessed LandThe share of the county's assessed value that comes from the land itself, not the buildings.$326
Assessed ImprovementsThe share of the county's assessed value that comes from the buildings and other structures on the land.$252
Previous AssessedThe county's assessed value from the year before, useful for seeing how the assessment changed.$564
Assessor Market ValueThe county assessor's own opinion of market value. It is separate from our estimate and updates on the county's schedule.$578,000

Climate Risk

Projected Flood DepthHow deep a serious flood is expected to get here at some point in the next 30 years. Shown in inches.9 in
30-Year Flood ChanceThe chance of at least one serious flood here in the next 30 years, not in any single year.30%
FEMA Flood ZoneFEMA's flood zone for this address. Zones starting with A or V are high risk, and lenders usually require flood insurance there.minimal

Scores 0–100; higher = greater projected risk.

Property Facts

Property TypeThe broad category the county puts this property in, such as residential or commercial.Single Family Residence
CountyNASSAU
Lot (acres)The size of the land the home sits on, from county records.0.09
ConstructionThe main material the home's frame and walls are built from.Frame (Unspecified)
RoofThe main material covering the roof, such as asphalt shingle or tile.Asbestos
HeatingThe type of heating system in the home.Central
CoolingThe type of cooling or air conditioning in the home.Central
FireplaceWhether the home has a fireplace, and what kind.Yes
GarageWhether the property has a garage, and details like whether it is attached to the home.Garage, Detached
PoolWhether the property has a pool, and what kind, such as in-ground.No
FeaturesExtra features found in county records. Records are not complete everywhere, so a missing feature does not always mean the home lacks it.Patio · 120 ft²

Transaction History(2)

Sales and refinances filed with the county for this home. Not every transfer is a normal sale, so check the type before comparing prices.

Assigned SchoolsThe public schools this address is zoned for. Boundaries change, so confirm with the district before you count on it.

H
Southside High SchoolA
High · Grades 9–12 · 0.6 mi
E
Jennie E. Hewitt Elementary SchoolB
Elementary · Grades KG–5 · 0.6 mi
M
Southside Middle SchoolB+
Middle · Grades 6–8 · 0.6 mi

District: Rockville Centre Union Free School District

Neighborhood · ZIP 11570These are comparison scores, not percentages. 100 equals the national average, so 150 means about 50% above average.

CrimeLocal crime compared with the rest of the country, where 100 is average. It reflects things like population density, income, and how crime gets reported.82
Air pollutionOverall air pollution compared with the rest of the country, where 100 is average. Lower is cleaner.112
EarthquakeEarthquake risk and likely building damage compared with the rest of the country, where 100 is average. Scores can run far above 100.91

Index scale: 100 = US average (tick). Lower is better.

Homes occupiedThe share of homes here that someone lives in. The rest sit empty, including vacation and for-sale homes.97%
Bachelor's or higherThe share of adults 25 and older whose highest degree is a bachelor's degree.32%
Poverty rateThe share of people whose household income falls below the federal poverty line, which varies by family size.4.6%
UnemploymentThe share of men 16 and older who are out of work and actively looking. Only men are counted in this figure.1.0%
PopulationHow many people live in this area.28,370
DensityPeople per square mile. Higher numbers mean a more crowded, urban area.7,589 / mi²
Median incomeThe middle household income here — half of households earn more, half earn less.$144,451
Average incomeThe average household income. It runs higher than the median when a few high earners pull it up.$184,571
Average tempThe average temperature here across a full year.54°F
Annual precipHow much rain and melted snow falls here in a typical year.49 in
SunshineThe share of daylight hours that get direct sun in a typical year, counting cloud cover.56%

Owner Tools

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Data by ATTOM. Valuations are automated estimates, not appraisals. Photos from Google Street View and may not depict the current property state.

How this home compares

21 William St measured against every home in its neighborhood, its ZIP code and its county, using county assessor records refreshed nightly. A home can stand out in one of those areas and look ordinary in another, because each one holds a different mix of homes.

  • Costs more per square foot than 94% of homes in Rockville Centre 11570
  • Smaller than 82% of homes in Rockville Centre 11570
  • Among the smallest lots in Rockville Centre 11570
  • Older than 90% of homes in Nassau County

What homes are worth in Rockville Centre 11570

Each bar is how many homes fall in that price range. The marker is this home.

This home
$610K$1.7M

This home is worth more than 39% of homes in Rockville Centre 11570.

How it measures up in Rockville Centre 11570

The dot is this home. The mark in the middle is the typical home in Rockville Centre 11570.

Estimated value$916,283
CheaperTypical $974,937Pricier

Pricier than 39% of homes nearby

Price per ft²$585
CheaperTypical $487Pricier

Pricier than 94% of homes nearby

Living area1,566 ft²
SmallerTypical 2,057 ft²Bigger

Bigger than 18% of homes nearby

Lot size4,000 ft²
SmallerTypical 0.16 acBigger

Smaller than almost all homes nearby

Year built1925
OlderTypical 1935Newer

Newer than 24% of homes nearby

How 21 William St ranks against every home in each area.
MeasureThis homeRockville Centre 11570Nassau County
Estimated valuepricier than$916,28339% of homes62% of homes
Price per ft²pricier than$58594% of homes72% of homes
Living areabigger than1,566 ft²18% of homes39% of homes
Lot sizebigger than4,000 ft²almost no homes4% of homes
Year builtnewer than192524% of homes10% of homes

About 21 William St

How much is 21 William St, Rockville Centre, NY worth?

The TopHap Estimate for 21 William St, Rockville Centre, NY is $916,283 as of Aug 30, 2026. The likely range is $874,281 to $958,285. The estimate is modelled from recorded sales, assessments and the property's own characteristics — it is not an appraisal.

When did 21 William St, Rockville Centre, NY last sell?

21 William St, Rockville Centre, NY last sold on Jul 5, 2019 for $660,000, according to the county's recorded deed. TopHap holds 2 recorded sales for this parcel.

How big is 21 William St, Rockville Centre, NY?

21 William St, Rockville Centre, NY has 2.5 bathrooms, 1,566 square feet of living space and a 0.09-acre lot.

When was 21 William St, Rockville Centre, NY built?

21 William St, Rockville Centre, NY was built in 1925, making it 101 years old. The building has 2 stories.

What schools serve 21 William St, Rockville Centre, NY?

21 William St, Rockville Centre, NY is assigned to Southside High School (high), rated A; Jennie E. Hewitt Elementary School (elementary), rated B; Southside Middle School (middle), rated B+. All sit in Rockville Centre Union Free School District. Attendance boundaries change — confirm with the district before relying on an assignment.

How does 21 William St, Rockville Centre, NY compare to other homes in Rockville Centre 11570?

Among the 6,982 homes in Rockville Centre 11570, it is worth more than 39% of them ($916,283 against a typical $974,937), its price per square foot is higher than 94% of them ($585 against a typical $487), it is bigger than 18% of them (1,566 ft² against a typical 2,057 ft²), its lot is bigger than almost none of them (4,000 ft² against a typical 0.16 ac) and it is newer than 24% of them (1925 against a typical 1935). Measured against all of Nassau County, it is worth more than 62% of homes there. These shares are recounted nightly from the full county assessor record for each area, not from active listings.

What are the property taxes on 21 William St, Rockville Centre, NY?

The 2025 property tax bill for 21 William St, Rockville Centre, NY was $13,627. The county assessed it at $578.

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