Let’s posit that fish gotta swim. Birds gotta fly. Teachers gotta teach, doctors gotta heal, lawyers gotta advocate, or at least they gotta parse contracts closely.

And architects? Pretty clearly, they gotta model, and design, and then they gotta build.

And if you are an architect who has parents who already live nearby, who are aging, who are finding formidable obstacles in the vast spaces of their own house, what you gotta do is build a new house for them.

Make it be next door. Make it be easily accessible to people who are likely to face increasing levels of physical challenge as they age, but who have exacting standards of aesthetics. (And who are you kidding? You have that sense of aesthetics yourself — clearly you got it from somewhere.) Make the accommodations be barely visible. And then stand back and enjoy the multigenerational setting you’ve created.

Or at least that’s what you do if you are Robert Gross of Englewood.

Mr. Gross and his family live in a lovely corner of Englewood, on a street with big, rambling, occasionally jaw-droppingly-beautiful old houses set back on lush lawns and hidden by huge old trees and exuberant gardens. There are some exceptions to that general rule; some new houses are perhaps showier than conventional taste would dictate, and his own house is gorgeous but modern.

Next to his house is a brand-new modern one, all sharp angles, glowing wood, and glass.

There’s a history that underlies this story.

The Grosses’ block and the neighboring ones used to be part of one estate. The house that used to stand next to theirs was a stable that was home to the big house’s horses; later additions, at various times, added both to its charm and to its architectural incoherence.

When he and his wife, Abigail Hepner Gross, and their three sons moved to Englewood in 2006, the Stellars owned the house next door.

Dr. Stanley Stellar and his wife, Rosetta van Gelder Stellar, and their two children moved into Englewood — first to the less large-scale west side in 1946, and then onto the East Hill in 1953.

The Stellar’s house

The Stellar’s house

“The main house was built in around 1906, 1907, some time like that,” their daughter, Susan Stellar, who now lives in Detroit, said. “Some intermediate owner added a garage, which, by the time my parents bought the house, had been converted to a living room — hence the really small windows on the east side, and a big picture window where the garage door had been. And another garage was added on the east end. My mother always liked to say that the house originally was just three garages.

“And then my parents added a library in the basement — that was the only basement in the house, because the rest of it was on a slab, because it hadn’t been a house.

“It was very idiosyncratic,” she added.

That matters — we’ll come back to it.

Dr. Stanley Stellar and his wife, Rosetta van Gelder Stellar.

Dr. Stanley Stellar and his wife, Rosetta van Gelder Stellar.

The Stellars were Jewish, although neither was particularly connected to the community. Rosetta Stellar originally was from Holland, where much of her family was slaughtered during the Holocaust. “She never really felt that she was Jewish,” even though she was, her daughter said. Dr. Stellar came from a more traditional background but it remained background to him.

He had been a doctor, fresh out of medical school, during World War II, taking care of the wounded in the European theater, in Belgium and northern France. “He had a dog tag that said that he was Jewish, but he felt that because he didn’t practice the religion, he wasn’t really Jewish, but still people kept on calling him Jewish,” Ms. Stellar said.

“He said that he felt it wasn’t safe to be Jewish.”

The Englewood into which they moved had not yet become Jewish. It still had vestiges of the anti-Semitism that flowered with famous aviator and American-Firster Charles Lindbergh, who had married into the locally influential Morrow family. It was not anti-Semitic, not really, but it was not yet philo-Semitic either.

Dr. Stellar was a neurosurgeon, a field that was just beginning to develop when he was young; he opened his practice in 1949, worked in a few hospitals, and ended his career, decades later, after many years at St. Barnabas in Livingston.

As Dr. Stellar told him the story, Mr. Gross reported that the surgeon had applied for a job at Columbia Presbyterian and was told that it would be easier for him to succeed there were he Protestant rather than Jewish. So that’s what he did. The Stellars — two parents, two children — all converted.

“I was 10 years old when one day they said we were going to the church,” Ms. Stellar said. She was talking about First Presbyterian in Englewood. “So we went to church. We went from nothing to a church. My parents wanted to experience a culture beyond the Jewish culture.

“I always felt like I didn’t fit in, and this helped. It was definitely an experience.”

Later, the neighborhood became increasingly and then overwhelmingly Jewish; the Stellars never hid their background. In fact, Ms. Stellar said, “We always got the Jewish Standard in the mail.”

By the time the Grosses moved next door, the Stellars’ conversion, nominal as it was, was deep in the past. Rosetta died in 2011 and Stanley died in 2012; he was 98 and she was 101. So by the time they met, the Stellars were old.

“We were very friendly,” Mr. Gross said. “We did what neighbors do.” The Grosses became the people the Stellars called when they needed help; they were emotionally close to their children, but they were physically close to the Grosses.

When Dr. Stellar died, their daughter, recognizing the value of the relationship, decided to give first refusal of the house to the Grosses. And the Grosses jumped at the chance. They wanted his parents, Pearl and Jack, next door.

The older Grosses lived just a few blocks away, “but I knew that next door would be better,” Mr. Gross said.

Pearl Weiss Gross and Jack Gross both were born in prewar Europe. She was born in a small town in Hungary; when she was 5 years old, the family moved to Budapest, “and that saved my life,” she said. Everyone in her hometown, Debrecen, was deported. Her family ended up in a safe house, under the protection of Raoul Wallenberg and the Swedish government. After 1945, the family wandered to Paris, to Germany, and then, in 1949, to the United States, settling in Borough Park, in Brooklyn.

Jack Gross’s family all was murdered in the Holocaust, a time that he chooses not to discuss.

When Pearl and Jack married, they moved first to Montreal, where they lived for nine years and had their three sons, and then, after a short stay in Queens, to Allentown, Pennsylvania, where Mr. Gross, the youngest of their sons, grew up.

The Grosses were builders, mainly in Pennsylvania. Jack Gross started small but ended up as a very successful businessman; Ms. Gross, who worked with him, concentrated on managing rental properties. They both still work, at least part-time. “My dad wanted to retire years ago, but retiring just wasn’t in his DNA.” Neither was it in his mother’s. She still drives them to Pennsylvania every week or so.

In 1993, Pearl and Jack Gross moved to Englewood, where he built a big house for the two of them. They became active members of the Jewish community there — like all their children and grandchildren, they are modern Orthodox — and they lived in that house happily until the late spring of this year, when they moved to the new one.

Most but not all of the Gross family have fun at Robert and Abigail’s house.

Most but not all of the Gross family have fun at Robert and Abigail’s house.

The problem, Robert Gross said, and his mother agreed, was that it was becoming too big for them to navigate easily, and it seemed likely that age wouldn’t make the long distance between rooms shrink to any appreciable degree. So when the house next door went on the market, his next move seemed obvious.

At first, Pearl Gross wasn’t entirely comfortable with the idea. Might it possibly be too close to her son and his family? “But my daughter-in-law approached me, and she said, ‘Mom, this house is for sale. Why don’t you buy it, so we can live next door to each other?’ And I said, ‘Honestly, do you really want to live next door to your mother-in-law?’ And she said, ‘Honestly, do you really want to live next door to your daughter-in-law?’”

The answer to both questions was yes.

“She has her own life, they all have their own lives, and we have our own independent lives,” Pearl Gross said.

And there was another reason for the families to live next to each other. “I grew up with grandparents I didn’t really know,” Robert Gross said. “We didn’t speak the same language really, and they were older and died when I was young, and there were many cultural differences. It was a limited relationship.

“It always struck me that it would be such a blessing to be able to know your grandparents not by getting in a car occasionally but by coming over to take some potatoes or asparagus.

“It means so much to me to have the relationship with my parents — comfortable, loving, and with them day-to-day being the people we see.

“We think that is the blessing of living next door.”

So the decision to live next door to each other was made, but what about the house?

The Stellars’ house was wonderful in its way, but it was not a good house to age in. When the different parts were cobbled together, they were not made level, so there were many steps, many unevennesses, many lurking dangers. Also, “there was a lot of damage from termites and moisture,” Ms. Stellar said. It was not going to provide the comfortable shelter that Mr. Gross wanted for his parents.

So he decided to tear it down and build a new one.

The almost trompe l’oeil aspect to the house Mr. Gross designed is that it is both designed to help deal with the restrictions of age and to hide it.

To begin with, the house is not big, at least not by neighborhood standards. But because the sun bountifully, goldenly lights all of it on all but the grayest day, it seems much larger than it is. The top floor, which is lovely but conventional, has bedrooms and bathrooms for visitors, but Pearl and Jack Gross live on the ground floor. The kitchen, the dining room, and their bedroom are all just steps from each other, although again that fact is not apparent.

Architect Robert Gross designed this house in Englewood for his parents.

Architect Robert Gross designed this house in Englewood for his parents.

The outside of the house features a built-in bench that allows you to sit and wait to be picked up. It’s a great help for someone who would prefer not to stand for too long, but it also is a marvelous place to sit and just look out at the lovely green world right in front.

It also has a chain dangling from the roof. Its function is to channel rainwater and so to function in place of a gutter; its appearance, so strikingly unlike the rest of the house, is deeply appealing.

The new house, still unfurnished here, with bright light and sharp shadows.

The new house, still unfurnished here, with bright light and sharp shadows.

Inside, Mr. Gross used computer models to maximize the way shadows intersect the light. They are sharp and the light is pure; the dark lines move throughout the day, marking the sun’s passage. (Not overwhelmingly so, though; the house is full of details that are visible if you want to see them and add to the background sense of comfort and pleasure if you do not.)

Three generations — Pearl and Jack Gross sit in their new house; behind them, Abigail and Robert stand with their sons, Darius, Judah, and Max.

Three generations — Pearl and Jack Gross sit in their new house; behind them, Abigail and Robert stand with their sons, Darius, Judah, and Max.

The bathroom, full of features anyone would want at any age, also includes such details as grab bars that are camouflaged as towel holders. The camouflage, it is important to add, never is cutesy, it simply is that if one feature can do double duty, it does.

Whenever it is, sometime far in the future, that this house comes to be sold, it can go either to people who want to take advantage of its special features or people who do not want them but are drawn by its beauty.

It’s been just a month or so since the older Grosses moved next door to the younger Grosses, but so far, so good, they report.

“I used to live in a house that my husband built, and now I am living in a house that my son designed,” Ms. Gross said. “I think that says something about us.”