LOOKS SIMPLE, RIGHT? The facade of this 100-year-old Colonial on Cleveland Lane gives no clue to the roomy addition the owners added to the rear of the house. Decorated with antiques they have collected for years, the property is one of six on the Historical Society of Princeton’s House Tour this Saturday.

Fourteen years ago, Kirk and Sandy Williamson were living happily in a mid-19th century house in their native Ohio when he was recruited by Goldman Sachs for a job in New York. The couple were reluctant to uproot their children and leave the home they had meticulously restored and furnished with antiques they had been collecting for years.

But the offer was too good to refuse. After living briefly in Tribeca, Lawrenceville, and in the Fieldwood Estates development in Princeton, they knew it was time to find a house that fit their taste.

“Our passion is antiques, as you can see,” said Ms. Williamson, sitting with her husband in the living room of the oldest part of the house they now occupy on Cleveland Lane. The 1915 Colonial is among six open to the public this Saturday as part of the Historical Society of Princeton’s annual House Tour, from 10 a.m. to 4 p.m.

“We were at Fieldwood for two years when we started getting the itch,” Mr. Williamson said. “We started looking, and we wondered if we could ever find something in the western section of town. Then this came up.”

Most gawkers cruising down Cleveland Lane to check out the real estate don’t notice number 11. Set back from the street and modest-looking compared to its neighbors, the house is easy to miss. But the Williamsons had a feeling about it. With three children, they knew they would have to put on an addition and make major changes to the 2,100-square-foot house while remaining as respectful as possible to its history and style. Today enlarged to 5,600 square feet, the bulk of the house is nearly invisible from the street.

Built a century ago, the house was owned first by Princeton University professor Earl Baldwin Smith and his wife Ruth Hall. Mr. Smith specialized in ancient and medieval architecture and was an author, as was his son, Lacey Baldwin Smith. According to information from the Historical Society, the house was considered as a possible residence for Albert Einstein when he came to the Institute for Advanced Study in 1933. The Williamsons bought the property in 2004 from the family of Sidney and Louise (Rosenblatt) Ratner, who had lived there from 1950.

It was an addition from that era that the Williamsons removed as the first phase of work on the house. They hired Princeton architect T. Jeffery Clarke to design the project, which involved adding attic dormers and making the space livable for the family. “We took out the radiators and the old electric and put in new utilities,” said Mr. Williamson, who is on the board of the Historical Society. “We found someone in Rhode Island who could restore the old windows, so we could keep them. We made a few little changes, but most of the woodwork is original.”

The house had four bedrooms and three bathrooms when the couple bought it. During the first phase of the project, two of the bathrooms were removed. The small, original bedrooms now house a guest room and a combined bed and sitting room for the Williamsons’ son. The master bedroom, and the expansive quarters for each daughter, are located in the addition.

Since the house is on a narrow lot of about 55 feet in width, the second phase of the project — the large addition to the rear of the house — involved significant zoning issues. “After problems with some of the neighbors who were opposed to the addition, we actually put the house on the market at one point,” Mr. Williamson said. “But most of the neighbors were fine with it, and really supportive.”

After some tweaks and changes, the rezoning was unanimously approved in 2010. The couple hired Schulte Restorations as contractors after considering several candidates, and they immediately got to work. The project was completed in 2011.

“It unfolds in front of you,” Ms. Williamson said of the addition, which stretches from front to back. The couple fell in love with tiger maple, using it for cabinetry throughout the house. The centerpiece of the spacious, blue-and-white-tiled master bath is a soaking tub they bought back in Ohio. The tub was one of several furnishings — antique and reproduction — that the Williamsons were storing in three separate units during the renovation and addition process. In addition to the tub, there were the antique farmhouse sinks now in the kitchen and basement level suite, the red Aga stove, and more.

“I was on a first-name basis with the people at Buxton’s Boxes in Ewing,” jokes Mr. Williamson. “It was getting kind of crazy.”

His wife adds, “But it all came together and all the stuff we’d bought over the years, we’re using in the house. We knew exactly what we wanted, and it all worked out.”

The Historical Society of Princeton tour is Saturday, November 8 from 10 a.m. to 4 p.m. Tickets are $50. Children under 12 are not permitted; those 12-17 must be accompanied by an adult. Visit www.princeton history.org.