Mr. Dungan isn't a billionaire: He's an architect. The homes belong to his clients, who became friends during the design and construction process. 'I just call and say I'm going down there, and they say 'Have fun',' says Mr. Dungan, of Birmingham firm Dungan Nequette. 'It's a great perk.'

Since Michelangelo, architects have had patrons who finance their work. But as time progressed, the relationship between architects and clients became closer to that of equals, especially at the high end. Fallingwater, the famed home perched over a waterfall, developed from the friendship between businessman Edgar J. Kaufmann and Frank Lloyd Wright. Letters between the two men illustrate the closeness of their relationship, and how Mr. Kaufmann overcame his reservations to allow Mr. Wright to design the unique house.

As a result of that closeness, architects often become friends with their clients, which can lead to all manner of perks. That can sweeten what isn't always a very high-paying profession. The median pay for architects in the U.S. was $73,090 in 2012, with the best-paid 10% in the profession earning about $118,230. About 48% of residential architects' business comes from repeat clients, according to a 2012 survey by the American Institute of Architects.

Designing a house can take years. That means countless hours spent together, often over meals and in the clients' homes, with discussions about personal subjects like sleeping habits.

'It's such an intense, psychological experience to do a residence together. You learn their idiosyncrasies,' says Stephan Jaklitsch of New York firm Jaklitsch/Gardner Architects. Mr. Jaklitsch has worked on nine homes for Robert Duffy, president of Marc Jacobs International. They include a 6,000-square-foot house in Provincetown, Mass.; a number of multimillion-dollar Manhattan townhouses and, currently, a restoration of the historic Astor Tea House, which was built in 1948 by Vincent Astor in Rhinebeck, N.Y. (Mr. Duffy has since sold all but two of the homes.)

The two met in 1999, introduced by a contractor. That relationship led to Mr. Jaklitsch designing more than 50 stores for Marc Jacobs around the world. Mr. Jaklitsch says Mr. Duffy has become such a good friend they'll often take spontaneous side trips for fun. 'We'll be at dinner in Lisbon and somehow we find ourselves flying to Marrakesh the next day,' he says. On a business trip to Singapore they ended up checking out the Louis Kahn National Assembly Building of Bangladesh in Dhaka, India. Mr. Duffy says the relationship is 'based on mutual respect and trust in each other's abilities to achieve the same goals.'

Luxury travel is another perk. Garland Lasater, Jr. and his wife Mollie used frequent flyer miles to fly Ted Flato, of Lake Flato Architects in San Antonio, and his wife to Japan for a week, where Mr. Flato designed a trip for themselves and the Lasaters to look at gardens and buildings for inspiration. 'You have the privilege of getting to know these incredibly talented people. It becomes more than a working relationship,' says Mr. Lasater, who is now the godfather of Mr. Flato's daughter. Mr. Lasater learned about the architect/client bond from his father, who was intimate friends with O'Neil Ford, known as the grandfather of Texas Modernism.

Miami architect Rene Gonzalez spent a week sailing with a client, Greek shipping executive Ion Varouxakis, in the Greek Islands on a yacht with five staff members. 'He wanted me to see how he lived,' says Mr. Gonzalez. The two went sightseeing together, and Mr. Gonzalez has an open invitation for a repeat trip.

Like any friendship, the relationship between architect and client isn't always smooth sailing. They can have differences of opinion and run into tough spots. Still, architects say it is rare that projects become so combative they walk away.

'Every client expands my life. You go into their world and you always learn something,' says Jim Olson, founding principal of the Seattle-based firm Olson Kundig Architects. 'But it's like family -- there are some you become closer to than others.' Mr. Olson spent his 70th birthday at Fallingwater with longtime clients and friends Nora and Vincent Petrucelli. Another client, in Korea, sent Mr. Olson and his wife on a weeklong vacation to historic temples and inns.

Joe Luna of Luna Design Group in Danvers, Mass., is passionate about small aircraft, so it was fortuitous that syndicated radio star John Garabedian liked a building that Mr. Luna had designed and interviewed him for home project priced around $2 million. Mr. Garabedian became a confidante, and Mr. Luna has gotten to fly on some of the 20 rare airplanes (including every early model of Piper) in Mr. Garabedian's collection.

Mr. Luna says that while he's happy when his clients become friends, that doesn't necessarily mean he wants his friends to become clients. 'There's always the expectation of if it's a friend, they should get a discount,' he notes, adding that he avoided doing work in his hometown for many years because 'I didn't want to be designing houses for the parents of the kids my kids played baseball with.'

Having a well-connected client can also mean benefits like opera, theater and concert tickets. Architect Chris Pardo recently caught the main acts at the music festival Coachella in the desert south of Palm Springs, Calif., and met the hip-hop duo Outkast backstage. Mr. Pardo is designing a home for one of the executives at Goldenvoice, the festival's producer and the two have become friends.

Steve Rugo designed Chicago three-star Michelin restaurant Alinea, in the process becoming good friends with co-owner and chef Grant Achatz and co-owner Nick Kokonas, whose home he then designed. As a result, Mr. Rugo says he can now get a table at pretty much any restaurant anywhere in the world. It's often a very special table, like the one on a private upstairs candlelit terrace he and his family were recently given at the tiny La Villa Mahana on Bora Bora, run by Damien Rinaldi Dovio, formerly the chef at L'Orangerie in Beverly Hills. 'There's always a table for me. I just say Grant said I should come here to eat,' he says.

There are benefits for the clients, too. Mr. Dungan, the Birmingham-based architect, says when he visits all those homes in Florida that he's designed, he also checks up on them and makes tweaks, and then incorporates what he learns in future designs. 'It's one thing to design a house. It's another to have to live with the decisions you've made,' he says. One night at a home in Alys Beach, he realized he'd designed a room without anywhere to put a cup of coffee or plug in for his cellphone while reading in bed. So he created a niche in the armoire and added an outlet.

The closer a client becomes, the easier it is to design for them, say architects Elissa Morgante and Fred Wilson of Chicago firm Morgante Wilson Architects. The couple met Jim and Ann-Marie Streibich 20 years ago when they did an addition to their home in Wilmette. It turned out their children went to the same nursery school. Eight years later, when the Streibiches asked their now-close friends to design an 8,000-square-foot home on Lake Michigan, Mr. Wilson says he and Ms. Morgante plotted the floor plan as if they were there as guests. They even designed themselves the ideal guest room, where they now stay about three times a year on vacation.

Mr. Streibich says he and his wife 'let [the architects] do what they want' with the design because they know them well enough to understand their aesthetics. As for visiting, Mr. Streibich says it is an open-door policy. That's a good thing, since Mr. Wilson plans on crashing at the couple's 4,000-square-foot Chicago penthouse after an occasional night on the town. 'If I get overserved and I need to sleep over, that is where I'll go,' he jokes.