NEWQUAY, England — People who move to Nansledan, a new residential community in the southwestern corner of England, must abide by certain rules.

Homes and doors can be painted only certain colors, including pastel pink and eggshell blue. Local businesses are welcome to set up shop, but no fast-food chains, please. And don’t think of tampering with the dime-size holes in some of the bricks outside the houses: They are there to make homes for bees.

Also, don’t be surprised if you see Charles, the Prince of Wales, strolling down the street, admiring what he’s wrought.

Nansledan, which will eventually have about 4,000 homes, could be the most ambitious project undertaken in the 700 years of the Duchy of Cornwall, the patchwork of properties spread across England, covering more than 200 square miles, that provides an income to the Prince of Wales.

A dukedom within a kingdom, the duchy was created in 1337 by Edward III for his eldest son, Prince Edward (known after his death as the Black Prince, perhaps because of the color of his armor). Prince Edward became the first Duke of Cornwall and the Prince of Wales, but he was never king — he died, probably of an illness, at age 45 while his father was still on the throne.

Most of the time since then, the duchy has passed to the eldest surviving son of the monarch, who is also heir to the throne and holds the title of the Duke of Cornwall.

Since 1952, that has been Prince Charles. At age 71, he has been in charge of the duchy longer than anyone before him (thanks to his mother, still reigning at age 93).

The duchy, which receives rent from tenants that include farmers, homeowners and shopping centers, earned 21 million pounds, or about $28 million, for the year ending March 2019, and Charles shared some of that income with his two sons and their families.

That has become a sticking point lately. It’s unclear if duchy money will continue to help pay the bills for the prince’s younger son, Harry, and his wife, Meghan, also known as the Duke and Duchess of Sussex, who recently caused a royal earthquake when they said they would separate themselves from their traditional duties and move to North America for part of the year.

On a recent visit, few locals in Newquay, the seaside town adjacent to Nansledan, said that they paid much attention to the fact that tracts of land around them were providing money for Harry and Meghan.

“That connection doesn’t matter,” said Steph Maclaren, an artist. “They don’t have a bearing on anyone.”

Ask residents about the duchy’s big housing development, and they often respond with blank looks. “It just appeared,” is a common response.

The Duchy of Cornwall is often portrayed as a collection of picturesque organic farms with rushing streams. But it is also a vast real-estate holding company, with properties including Dartmoor Prison and a home improvement superstore in Milton Keynes, a town north of London.

Nansledan, the duchy says, has been inspired by Prince Charles’s philosophy on architecture and the environment. The prince is outspoken in his support for traditional housing styles and sustainable development.

In a treatise entitled “Housing Britain: A Call to Action,” the prince has written that, “We must demand better places that break the stranglehold of the conventional mold of monocultural housing estates and zoned developments that, up to now, have put the car at the center of the design process and not the pedestrian and thereby created an increasingly unsustainable environment.”

The streets of Nansledan are angled to discourage drivers from speeding and the layout is intended to allow residents to reach shops and schools without having to drive. Marketplaces, plazas and nature reserves have been built in for residents. Houses have bird boxes constructed into their walls to encourage nesting and the gardens have communal orchards.

Alex Eley said she jumped at the opportunity to set up shop there in October 2018 because she thought the Duchy of Cornwall would be supportive toward her catering business, which limits plastic and uses local products. Prince Charles made the duchy’s own farm entirely organic more than 30 years ago, and he has separately set up the Duchy Originals brand, which sells organic food. He spoke about the topic of sustainability at the World Economic Forum in Davos, Switzerland, this week.

“We’re shouting and preaching from the same hymn book,” Ms. Eley said.

When the duchy first tested the prince’s architectural principles in Poundbury, a village on the southern coast of England, it was ridiculed by some as a “feudal Disneyland.” But it flourished enough to lead to another test, on Tregunnel Hill in Newquay; and then to Nansledan, a 540-acre site that will eventually have almost twice as many homes as Poundbury. (Residents own their houses, but the land underneath still belongs to the duchy.)

As environmental concerns have become mainstream, the prince and his ethos of sustainability and long-term stewardship of the land have attracted less criticism, something he notes in his housing essay.

Although Poundbury was “derided by many at the time,” the prince wrote, “I am heartened that both the duchy’s and my foundation’s work is now gaining approval and shifting the tide of opinion.”

Residents and businesses who have moved to Nansledan (the name means broad valley in Cornish) are enthusiastic about their choice. They have already agreed to the rules of life on a duchy estate, including the requirement that residents wanting to change the color of their house or door must apply for permission from the duchy and conform to tints reminiscent of fishing cottages on the coast.

Buying a house in the development is like joining a club, said Tracey Nicholas, the project administrator for the Duchy of Cornwall in Nansledan. “These are the rules,” she says, flipping through the color charts.

And the rules that come with living on duchy-owned land are part of the attraction for some residents.

“You have to buy into that,” said Aaron Smith, a florist who moved from Staffordshire, about 275 miles away, to set up home and shop in Nansledan with his partner, Matt Drohan, six weeks ago.

“It can seem quite controlling,” Mr. Smith said, but he loved the “cookie-cutter feel” of the place, joking that he aspired to be like Bree Van De Kamp, the uptight perfectionist in the television series “Desperate Housewives.”

Residents’ commitment to the community has helped with maintenance too. The site has had problems with dog poop being left out and construction materials blowing away from the building sites by high coastal winds. So Mr. Smith organized a group to go litter picking that has been nicknamed Team Sparkle.

“It’s perfect,” Mr. Smith said with evident pride. “It’s so safe because everybody is watching.”

Mr. Smith is also a staunch royalist — it was the Duchy of Cornwall name that drew him to take a look at the development while on holiday.

To them, the royal family can do no wrong. Harry will be a “people’s prince, like Diana,” even if he moves away, said Mr. Smith.

Mr. Drohan is equally sympathetic. “You can’t blame them, the way they treat them,” Mr. Drohan said of the tabloids. “To be fair to Meghan, they are dreadful.”

Life in the community, where a four-bedroom home goes for about £400,000, is not without small frustrations. There is, for example, an issue with a pizza van.

The van would arrive on Fridays, park in the neighborhood, and sell pizzas baked in its wood-fired oven. It attracted quite a few customers, apparently, but a homeowner eventually complained. The duchy discussed matters with the van’s owner. The result: No more visits from the pizza van.

Residents who liked the van are “up in arms,” Mr. Smith said, and don’t want Friday-night pizzas to end. It is being discussed on the residents’ Facebook page. One resident approached the florists to discuss alternative parking spots for the van. A teenager getting his hair cut at the Nansledan barbershop said he hoped he would be able to take advantage of the van one last time.

Danny Murphy, who opened the barbershop almost two years ago, has had his own run-in with the rules. He wants to put up a single barber pole, but the Duchy has stipulated that he must have two poles and that they must be just red and white.

“They’re quite funny about what they want here,” Mr. Murphy said.

His fellow barber, Max Hoar, shrugged: “What they say goes.”