This Weekend, New York Opens 300 Doors. Here Is What to Look for Behind Them.

Open House New York Weekend starts Friday. From October 16 to 18, more than 300 buildings across the five boroughs open their doors, most of them for free, many of them places you would never otherwise get into. Close to 50,000 people went last year.

The rooftops and Gilded Age mansions get most of the attention. Look past them, though, and this year's list says a lot about how New York builds now: old buildings taking on new jobs, new ones heated by the ground, and houses designed for the next flood. Here are six worth seeing.

77 Water Street

A 26-story office tower in the Financial District is halfway to becoming apartments, a project by Vanbarton Group and CetraRuddy. We wrote about office conversions back in March. Seeing one mid-construction is better, because that is when the hard choices are still visible.

Keeping the structure saves most of the carbon a new tower would burn through before anyone moved in. The catch is geometry. Office floors are deep, and bedrooms need windows. Look at how far the core sits from the glass, and where new kitchens and bathrooms find plumbing that was never planned for them. Tickets required.

The Peninsula

In Hunts Point, WXY Studio turned the former Spofford Juvenile Detention Center into 740 permanently affordable homes, plus about 50,000 square feet of light industrial space, community facilities, and gardens.

A detention center is built to keep people in and the street out. A neighborhood needs the opposite, and it is worth seeing how the design got there. Notice the industrial space too. Jobs on the same site as housing mean shorter commutes, which is sustainability even if nobody calls it that. Tickets required.

The Riverie

This 834-unit rental complex on the Greenpoint waterfront, designed by Marvel with INC Architecture & Design, has no boiler. It is all-electric, heated and cooled through roughly 320 geothermal boreholes under the site. Lendlease, the developer, calls it the largest geothermal residential building in New York State and expects about 53 percent less carbon from heating and cooling than a conventional building. The rooftop pool runs on it too.

The idea is simple. A few hundred feet down, the ground stays the same temperature all year. Heat pumps pull warmth out of it in winter and push heat back into it in summer.

What makes it an architecture story is timing. The boreholes go in before the building does, so you cannot add this to a finished tower later. Ask on the tour how it shaped the foundations. Tickets required.

Red Hook Treehouses

Sandy flooded Red Hook badly in 2012. These four townhouses by Gradient Architecture were built for the next one. They are heavy timber, arranged around a silver maple about 120 years old, and Ben Krone of Gradient leads the tours.

Keeping the tree cost floor area and made the foundations harder. Someone decided it was worth it, and a tree that size does more for shade and stormwater than a new one will for decades. Then look at the ground floor. In a flood zone, that is where the real decisions happen: what gets raised, what is allowed to get wet, where the equipment goes. Our September piece on flooding explains why. This is the small-scale version. Tickets required.

Wyckoff House and the Hunterfly Road Houses

The Wyckoff House in East Flatbush is a 17th-century Dutch farmhouse and the first building in New York City to be landmarked. This year's tours include the root cellar, which runs on the same idea as the Riverie's boreholes: the ground holds steady, so food stays cool in summer and does not freeze in winter. Same principle, three centuries earlier.

In Crown Heights, the Hunterfly Road Houses preserve three 19th-century homes from Weeksville, the second-largest free Black community in the country before the Civil War.

Both are still standing because people kept repairing them instead of replacing them. Every reuse project on this list is a newer version of that. No tickets needed at either.

Before You Go

Most ticketed sites sold out fast. Reservations opened October 2, mostly at $7, and last year 80 percent of about 10,000 tickets went in minutes. If you missed out, go drop-in. The OHNY website flags which sites get long lines.

Wherever you end up, ignore the finishes for a minute and ask a few things. Was this building here before? How is it heated? Where does the water go in a storm? What did the design choose to keep? The answers will tell you more about how the place will hold up than anything in the lobby.

What makes it an architecture story is timing