In real estate, as in life, taking the wrong turn sometimes points you in the right direction. That’s what Mark Bearak and Michael Moore discovered while they were hunting for a house in Hudson, N.Y., as a weekend getaway from their apartment in Manhattan.

During a visit to Hudson in 2016, Mr. Moore decided to take a break from touring houses to drive to a local gym. “I made a wrong turn and stumbled upon this street just outside of town,” he said, where he admired a string of handsome homes bordering the Hudson River.

“And at the end of the road, I saw this vacant lot for sale and thought, ‘Well, this would be pretty cool,’” said Mr. Moore, 37, who works in finance.

Although they had been searching for a house to renovate, it wasn’t much of a stretch to consider building from the ground up instead. Mr. Bearak, 41, is an architect and the founder of the New York-based firm dtls.

They liked the location, but on closer inspection, the 25-acre lot didn’t look like a winner. It was an odd, V-shaped patch of sloping land that zigzagged around other, flatter lots. “The previous owner had been using it as a hunting ground, so it was really overgrown and had large piles of tree limbs that he could hide in,” Mr. Bearak said. “It took a few trips for us to understand if it was even a buildable piece of property.”

Eventually, they decided that with some clearing and grading they could create a building site — or a few — and closed on the property that October, for $130,000.

Back at his office, Mr. Bearak let his imagination run wild, envisioning not just one house, but a series of structures they could build over time, including a main house, a pool house, a guesthouse, a gym, a sauna, a teahouse and an office.

“Our first intention was to build the main house, which is actually on the best site on the property,” he said. But as they were in the early stages of adopting children, he said, “we didn’t know what our needs would be.”

So they shifted their focus to the pool house, a 1,300-square-foot structure with two bedrooms and two bathrooms. “We realized the pool house would be much more fun and be a lot more accessible for us to design and get built quickly,” Mr. Bearak said.

In the spring of 2017, the couple began preparing the site by clearing the land, putting in a driveway, bringing in electricity, digging a well and installing a septic system — all of which cost more than twice the $70,000 they had budgeted.

At the same time, they developed plans for the pool house — a glass box inspired by the cliff-topping, steel-framed Stahl House in Los Angeles, built by Pierre Koenig in 1960 as part of the Case Study House program. After a visit to study the original, they began working with Greencore Builders to translate concepts from the sunny Stahl House to a structure that would have to endure freezing winters.

To improve the thermal performance of the house, Mr. Bearak said, they used glued, laminated timbers for the structure, rather than steel. He designed the roof overhangs to allow plenty of sunshine during the winter to warm the concrete floor, but to shade it in the summer.

Embracing the spirit of the Case Study House program, which aimed to generate practical, affordable designs for houses with ideas that could be easily duplicated, Mr. Bearak tried to build the structure entirely from off-the-shelf parts. “All of the components that we used for this house were locally sourced, indigenous or something you could go to Home Depot and order,” he said. “We weren’t using anything custom.”

To create the glass walls, for instance, he used Andersen patio sliders in standard sizes, which “ended up creating the structural module for the house,” he said.

With few places for light switches on the walls, the house leans heavily on a Savant smart-home system that controls lighting, motorized shades, heating and cooling, and home entertainment.

Outside, a gabion wall — a steel cage filled with rocks — snakes around to create a flat platform. It is punctured by the pool, which extends from the house toward a distant view of the Catskill Mountains.

Construction of this pool house with benefits was completed at the end of 2018. The basic structure cost about $350,000, Mr. Bearak said, which is exactly what the couple budgeted. The pool, however, ended up costing more than $200,000 — far more than they anticipated. Along with the cost of the site work, a hot tub, the smart-home system and finishing touches, he said, they spent a total of roughly $1 million.

When the pandemic struck New York in March, Mr. Bearak and Mr. Moore stayed put in Manhattan with their children — they now have three daughters: Madison, 4, Lily, 2, and Margaux, 1 — in part because they had rented their house out on Airbnb.

“There was this really nice couple, and they were just supposed to stay there for a weekend,” Mr. Bearak said. The guests were expecting a baby, so he and Mr. Moore were loath to end the rental when they asked for an extension. “They ended up staying for two and a half months.” (The couple are currently working with Mr. Bearak’s firm on a home of their own.)

As for Mr. Bearak and Mr. Moore, with a growing family, they already need more space. This spring, they plan to begin construction of the guesthouse, so they can host overnight visitors, and the stand-alone office, for working from home.

The plan is to take their time completing a compound that will expand over many years. Construction of the main house, Mr. Bearak said, is “probably 15 to 20 years down the road.”

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