Listed by Compass New England, LLC · Mitch Varley · 978-248-8081
Single Family Residence · Portsmouth, NH
518 New Castle Avenue, Portsmouth, NH
$35,000,000
Est. $176,979/mo · 20% down, 30-yr, 6.5% (Owl’s Nest Real Estate estimate, not MLS data)
- Beds
- 6
- Baths
- 9
- Sq Ft
- 12,940
- Built
- 2001
- MLS®
- #5114173
Property Description
Welcome to 518 New Castle Avenue, a single-family home in Portsmouth, New Hampshire. The home offers 6 bedrooms, 9 bathrooms and 12,940 square feet of living space, listed at $35,000,000. Single-family ownership here means your own land, your own walls and room to make the property yours. The numbers worth knowing: a 7.7-acre lot, garage parking for 6 and 2001 construction. See the listing's full details below, and reach out for a private showing of 518 New Castle Avenue.
There are properties that simply cannot be replicated. Privately sited on 7.7 waterfront acres on Portsmouth's coveted Shapleigh Island, 518 New Castle Avenue is a once-in-a-generation New England estate, an extraordinary convergence of scale, privacy, architecture, and waterfront living just moments from the heart of downtown Portsmouth. With 13,450± square feet, this magnificent Shingle Style residence was conceived on a grand scale, pairing timeless coastal architecture with exceptional craftsmanship and sophisticated modern living. Georgian-inspired millwork, stately Tuscan columns, intricate wainscoting, and beautifully proportioned interiors create a residence of remarkable presence, while walls of windows capture sweeping water views and an ever-changing maritime landscape. Across 17 rooms, the estate offers 6 bedrooms, 7 full and 2 half baths, and an impressive collection of formal and informal living spaces equally suited for everyday living and grand-scale entertaining. A newly added balcony overlooking the pool creates an elevated vantage point of the estate's sweeping water views. Beyond the home, manicured grounds descend toward the Piscataqua River, where a private deep-water dock provides direct access to Portsmouth Harbor and the open Atlantic beyond. An attached 3-car garage is complemented by a separate 3-car garage with a finished 1-bedroom, 1-bath guest accommodation above, creating a private and flexible retreat for visitors. Nearly 6,000 square feet of unfinished lower-level space offers exceptional flexibility and storage, while a new roof completed in 2025 represents a significant recent improvement. Yet perhaps the estate's greatest luxury is one that cannot be recreated: complete seclusion across 7.7 waterfront acres while remaining connected to downtown Portsmouth and its cultural, culinary, and coastal lifestyle. 518 New Castle Ave is a landmark NH estate and a rare opportunity to steward one of the region's defining legacy properties.
Location Highlights
518 New Castle Avenue is a single family residence in Portsmouth, New Hampshire, listed at $35,000,000. That works out to about $2,705 per square foot, for a home built in 2001. It has been on the market for 19 days.
At this latitude, Portsmouth averages roughly 8.4 hours of sun a day in June and 3.6 in December.
Source: zoned school names are PrimeMLS listing data; distances, sun exposure and school facts are Owl’s Nest Real Estate additions (school facts from the NCES, U.S. Department of Education) — not PrimeMLS data.
Property Details
Essentials & finishes
- Bedrooms
- 6
- Total Baths
- 9
- Full Baths
- 7
- SqFt
- 12,940
- Price / SqFt
- $2704.79
- Property Type
- Single Family Residence
- Year Built
- 2001
- County
- NH-Rockingham
- Town
- Portsmouth, NH
- Status
- Active
- Days on Market
- 13
Primary Features
- County
- NH-Rockingham
- Property Type
- Residential
- Year Built
- 2001
- Zoning
- SRA
Interior Features
- Basement
- Yes
- Basement Access Type
- Interior
- Basement Description
- Partially Finished, Interior Stairs
- 1/4 Baths
- 0
- 3/4 Baths
- 0
- Cooling
- Central Air, Zoned
- Electric
- Circuit Breakers, Generator
- Heating
- Propane, Hot Air, Zoned, Radiant
- Interior Features
- Cathedral Ceiling(s), Gas Fireplace, Wood Fireplace, 3+ Fireplaces, Hot Tub, Kitchen Island, Sauna
- Rooms - Total
- 17
- SqFt-Approximate Finished Above Grade
- 12,940
- SqFt-Approximate Finished Below Grade
- 511
- SqFt Approximate Total Finished
- 13,451
- SqFt Approximate Total
- 18,939
Financial Details
- Tax - Gross Amount
- $162,922
- Tax Year
- 2025
- Taxes TBD
- No
Additional
- Construction Status
- Existing
- Construction
- Wood Frame, Shingle Siding, Stone
- Covenants
- Unknown
- Flood Zone
- Yes
- Pre-Construction
- No
- Property Sub Type
- Single Family Residence
- Seasonal
- No
- Total Stories
- Two
- Surveyed
- Yes
- Timeshare/Fractional Ownership
- No
- Water Access Details
- Deep Water Access, Dock Access
External Features
- Driveway
- Circular, Paved
- Exterior Features
- Boat Slip, Balcony, Hot Tub, Outbuilding, Patio, In-Ground Pool
- Foundation
- Concrete
- Garage
- Yes
- Garage Capacity
- 6
- Lot Features
- Landscaped, Level, Island, Near Country Club, Near Golf Course, Near Shopping, Neighborhood, Near Hospital, Near School(s)
- Lot - Acres
- 7.7
- Lot - SqFt
- 335,412
- Parking
- Auto Open, Direct Access, Heated Garage, Driveway, Parking Spaces 1 - 10, Paved, Attached, Detached
- Road Frontage Type
- Public
- Road Frontage
- Yes
- Road Frontage Length
- 700
- Roof
- Architectural Shingle
- Sewer
- Public
- Utilities
- Underground Utilities
- Water Source
- Private
- Water Body Type
- River
MLS®: PrimeMLS #5114173; Listed by Compass New England, LLC · Mitch Varley · 978-248-8081
Property History
Estimated annual taxes for this property
- 2024
- $595,000
- 2023
- $580,125
- 2022
- $565,250
Source: Owl’s Nest Real Estate estimate from the list price at New Hampshire’s average effective tax rate — not PrimeMLS or town assessor data. Verify with the town assessor.
Market Data
Live snapshot computed by Owl’s Nest Real Estate from PrimeMLS IDX active listings in this town
- Active listings in Portsmouth
- 123
- Average list price
- $1,159,033
- Price range
- $285,000 – $2,775,000
Mortgage Calculator
Estimate your monthly payment for this home
Estimated Monthly Payment
$233,854/ mo
- Principal & Interest
- $176,979
- Property Tax
- $49,583
- Home Insurance
- $7,292
Source: Owl’s Nest Real Estate estimate, not PrimeMLS data. Estimates use typical New Hampshire costs — a 1.7% effective property-tax rate, insurance at 0.25% of home value/yr, and PMI below 20% down. Actual town tax rates vary; verify with your lender and the town assessor.
Around This Property
Schools, everyday places and nearby destinations near 518 New Castle Avenue, Portsmouth, NH
Source: Google Maps Platform (Places) and Owl’s Nest Real Estate — not PrimeMLS data.
Questions About This Listing
How much does 518 New Castle Avenue cost?
518 New Castle Avenue in Portsmouth, NH is listed for $35,000,000 on PrimeMLS.
That is about $2,705 per square foot of finished living space.
It came on the market on October 14, 2026.
How many bedrooms and bathrooms does 518 New Castle Avenue have?
518 New Castle Avenue in Portsmouth, NH has 6 bedrooms and 9 bathrooms (7 full and 2 half) in 12,940 square feet of finished living space.
Counting 511 finished square feet below grade, the MLS lists 13,451 finished square feet in total.
What parking does 518 New Castle Avenue have?
518 New Castle Avenue in Portsmouth, NH has a 6-car garage and a circular and paved driveway.
Parking details: auto open, direct access, heated garage, driveway, parking spaces 1 - 10 and paved.
Is 518 New Castle Avenue a waterfront property?
Yes. The MLS lists 518 New Castle Avenue in Portsmouth, NH as a waterfront property on a river.
Water access: deep water access and dock access.
Confirm frontage, shoreland rules and any dock or beach rights during due diligence.
Is 518 New Castle Avenue still for sale?
Yes. 518 New Castle Avenue in Portsmouth, NH is listed as Active on PrimeMLS (MLS #5114173).
Status can change quickly; contact us to confirm availability before you visit.
Which brokerage has 518 New Castle Avenue listed?
518 New Castle Avenue in Portsmouth, NH is listed by Mitch Varley of Compass New England, LLC (978-248-8081).
Owl's Nest Real Estate shows it through the PrimeMLS IDX feed; we are not the listing brokerage.
What type of home is 518 New Castle Avenue?
518 New Castle Avenue in Portsmouth, NH is a single-family home built in 2001.
The MLS lists its style as Colonial, with two stories.
What town is 518 New Castle Avenue in?
518 New Castle Avenue is in Portsmouth, New Hampshire 03801, in Rockingham County.
Location notes from the MLS: near country club, near golf course, near shopping, near hospital and near school(s).
What is the lot size of 518 New Castle Avenue?
518 New Castle Avenue in Portsmouth, NH sits on 7.7 acres, per the MLS.
The lot is described as landscaped, level and island.
It has 700 feet of road frontage (public).
Zoning: SRA.
The MLS says the lot has been surveyed.
The MLS lists the property in a flood zone; check the FEMA flood map and insurance costs.
What are the annual property taxes for 518 New Castle Avenue?
Annual property taxes on 518 New Castle Avenue in Portsmouth, NH are $162,922 for the 2025 tax year, per the MLS.
New Hampshire towns set their own tax rates and revalue periodically, so confirm the current bill with the Portsmouth assessing office.
What heating and cooling does 518 New Castle Avenue have?
518 New Castle Avenue in Portsmouth, NH is heated by propane, hot air, zoned and radiant, per the MLS.
Cooling: central air and zoned.
What are the water and sewer systems at 518 New Castle Avenue?
518 New Castle Avenue in Portsmouth, NH has public sewer, per the MLS.
MLS details — water: private; sewer: public.
Electric: circuit breakers and generator.
Other utilities: underground utilities.
Does 518 New Castle Avenue have a basement?
Yes. 518 New Castle Avenue in Portsmouth, NH has a basement with interior access, including 511 finished square feet.
The MLS describes it as partially finished and interior stairs.
What are the key features of 518 New Castle Avenue?
Inside, the MLS lists these features at 518 New Castle Avenue in Portsmouth, NH: cathedral ceiling(s), gas fireplace, wood fireplace, 3+ fireplaces, hot tub, kitchen island and sauna.
Outside: boat slip, balcony, hot tub, outbuilding, patio and in-ground pool.
How does the price of 518 New Castle Avenue compare with other Portsmouth listings?
At $35,000,000, 518 New Castle Avenue in Portsmouth, NH is priced above the $1,495,000 median asking price of the 43 single-family homes for sale in Portsmouth on PrimeMLS as of November 3, 2026 (this listing included).
That figure is Owl's Nest Real Estate's analysis of active listings, and asking prices are not sale prices — ask us for recent comparable sales before you make an offer.
How can I see 518 New Castle Avenue in person?
To see 518 New Castle Avenue in Portsmouth, NH, use Request a Showing on this page or call Owl's Nest Real Estate at 401-648-5308.
We can show any MLS-listed home and represent you as your buyer's agent; the listing brokerage represents the seller.
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NOTE: This representation is based in whole or in part on data generated by PrimeMLS, Inc., which assumes no responsibility for its accuracy. The data relating to real estate displayed on this site comes in part from the IDX Program of PrimeMLS.
This listing data is intended for consumers’ personal, non-commercial use and may not be used for any purpose other than to identify prospective properties consumers may be interested in purchasing. Data last updated 11/02/26.