15 West 20th Street (Altair 20)

The building rises 11 stories on a 56-foot-wide mid-block lot, with ground-floor retail and sidewalk landscaping at the base. The residential mix is dominated by full- and half-floor two- and three-bedroom lofts: the B-line units run to 27-to-28-foot living/dining rooms with pass-through kitchens, and several lines carry dedicated home offices. The trophy is the duplex penthouse — 3,223 square feet with three bedrooms and nearly 2,000 square feet of terraces, a limestone-clad foyer, vaulted-ceiling primary suite, and a private loggia. Wood-burning fireplaces run through the residences, a scarce feature in Flatiron condominium stock of any vintage.

This is a boutique condominium, and it operates like one: a part-time doorman, a live-in superintendent, and a 17-unit owners' association rather than a full-service staff stack. The amenity set is compact but real — a fitness center, private storage, and a landscaped common roof deck with cabanas and grills. Buyers comparing against full-service Flatiron buildings should weigh the materially lower staffing overhead against package logistics and off-hours coverage. Exterior work, including anything visible at the roofline, runs through Landmarks review given the historic district. The offering plan and amendments are on file in The Roebling Research Library and available to clients during diligence.

Recent sales

Recent closings at this building, from The Roebling Research Library. Prices are the transfer amounts recorded with the NYC Department of Finance; apartment-level detail is checked against the building’s own file in The Roebling Research Library before publishing.

Market read. Most recent trades (2025) cleared a median $1,407/sf (floor-adjusted) across 1 sale. Floor-adjusted means each sale is restated to a constant mix of floors, so the figure can differ from the plain median of the sales listed above. Median listing discount 1.1% from the last ask — a recurring negotiation gap worth pricing into any offer or listing strategy.

Sales sourced from NYC Department of Finance recorded transfers (BBL 1-00822-7504; each condominium apartment is its own tax lot). Apartment-level facts (line, condition, asking-price context) curated and cross-verified in The Roebling Research Library. Not all transactions cross-verify with ACRIS records — sponsor and LLC purchases sometimes record at stipulated values rather than market price; square footage from recorded condo declarations and offering plans.

The structure is the headline. Condominium ownership in a pre-war loft envelope — no co-op board interview; purchases run through the condominium's application and right-of-first-refusal process. Confirm transfer, entity and financing rules against the declaration and by-laws with your attorney.

Seventeen units is a small base. In a building this small, capital projects divide across few apartments, in proportions set by the governing documents, and a single major facade or roof cycle moves common charges meaningfully. Review the budget, reserve position, and recent meeting minutes carefully; the offering plan on file gives you the baseline operating framework to compare against.

Underwrite the staffing model honestly. A part-time doorman and live-in super suit most loft buyers; they do not suit buyers expecting white-glove service. Price your own service expectations before contract.

Landmarks applies to the envelope. The Ladies' Mile Historic District protects the facade — a guarantee of the block's character and a process constraint on window, mechanical, and rooftop work. Review the building's alteration history during diligence.

Run the math at the asking tier. Nearly every unit in the building trades above the mansion-tax thresholds. Run the Mansion Tax Calculator and the Buyer Closing Cost Calculator before offering.

Sell the scarcity stack. Boutique condo, pre-war loft envelope, wood-burning fireplace, historic-district block, two-or-fewer-units-per-floor privacy — name the structural facts. The Flatiron buyer cross-shopping new glass product responds to exactly this list.

Same-line history is thin, so position against the corridor. With 17 units, your comparable set is the Flatiron boutique conversion market, not just the building. We anchor pricing to corridor comps and the unit's specific light, condition, and fireplace count.

Document the fireplace and finish provenance. Conversion-era finishes are now twenty years old; renovated units should market the upgrade history, and original-condition units should be priced to the renovation math. Run the Renovation Cost Calculator against your asking strategy.

If you're considering 15 West 20th Street, also evaluate:

• Altair 18 (32 West 18th Street) — the sister conversion by the same sponsor team; the most direct peer
• 45 East 22nd Street — the Flatiron's new-construction tower alternative at a higher price point
• 15 Union Square West — boutique condo conversion at the square; the glass-and-history alternative
• 141 Fifth Avenue — domed corner condominium conversion of the same Ladies' Mile vintage
• The Grand Madison (225 Fifth Avenue) — larger pre-war conversion on Madison Square Park
• One Madison (23 East 22nd Street) — the park-facing modern boutique tower
• 260 Park Avenue South — landmarked pre-war condo conversion at the Gramercy seam