Architect Sarah Jefferys designed a home renovation for her family that mixes the old and new

Architect Sarah Jefferys has designed a house in West Cornwall, Conn., for her family and another family to share that has both traditional and modernist features.

Half of the home is an old stone cottage, something that looks like it jumped from the pages of a Jane Austen novel. Connected to that is a geometric addition, with one box cantilevered off the other and big windows.

From the outside, it looks like two completely different houses which just happen to be located right up against each other, but not awkwardly.

“The cottage appealed to me, but the site lent itself to a modern design,” says Ms. Jefferys, who founded New York City-based Sarah Jefferys Architecture, known for energy-efficient townhouses that blend the old and the new.

The size of the property allowed the couple’s three sons, now ages 11, 17 and 19, to play baseball and ice hockey at home. Here, Callum, 11, is on the zip line.

Getting to the final design took a lot of cardboard models, says Ms. Jefferys. Earlier drawings included a glass box, but in the end she preferred there be no barrier.

While the home’s old and new sides are somewhat discordant on the outside, they are fully integrated inside, with the design flowing congruously from one room to the next.

There is a common color palette, with a dominance of wood floors and white walls punctuated by bright wallpapers, furniture and pillows.

On the modern end, the first floor is an open kitchen with pine cabinets and Carrara marble counters.

The cabinets in the dining room of the old section of the house are painted blue.

"I love traditional architecture, but I’ll never replicate it—I’ll always design modern architecture,” says Ms. Jefferys.

As her 11-year-old son Callum puts it, “we like two things mixed together.”