About five years ago, a young New York City couple—she’s a real estate executive, he’s a bond trader—decided that they wanted a beach house on Fire Island, a narrow spit of land off the southern coast of Long Island where he had spent his childhood summers. They found an architecturally undistinguished wooden cabin that was built sometime in the 1950s and whose greatest asset was its location high above the oceanfront. It measured only 1,400 square feet, which didn’t seem adequate for a couple with two young sons. They bought it with the idea of making radical changes and, if necessary, tearing it down and replacing it.

It was a major project, but the couple didn’t have to waste time finding someone they trusted to take it on. The wife had become good friends with Alexandra Angle at Wesleyan University in Connecticut, and they had remained close after graduation. Angle had gone on to become a successful interior designer, and the couple had happily hired her to design their homes in Manhattan and upstate New York. Asking her to transform their Fire Island home was a no-brainer.

The couple bought the house in February 2010. They asked Angle to make it livable for the summer so they could take advantage of the upcoming beach season, after which, they thought, they would take the next step: a complete renovation or a teardown. But the next step never came. The family was so delighted with Angle’s transformations that they gave their old house an indefinite reprieve.

To get the house ready for summer in just three months—and to avoid spending a lot of money on what was supposed to be a temporary fix—proved a challenge. “Everything was just really basic,” Angle says. “They wanted it really relaxed, relatively easy, and somewhat low-key.”

Angle used color as her not-so-secret weapon, employing it strategically on her canvas of white walls and ceilings, employing a sunny-bright palette that turned the once-drab dwelling into a visual feast. Her inspiration came from the colorful kāhili feather standards that she saw on a visit to the Bernice Pauahi Bishop Museum in Honolulu.

The main living area has floor-to-ceiling windows that offer a sweeping ocean view. Angle refreshed the built-in banquettes around the room’s perimeter with fabric from Liberty, buying rolls of extra yardage knowing that beach houses get a lot of wear and tear and that the cushions would eventually need to be re-covered.

The piano, a leftover from the home’s previous owners, got a fresh face when Angle had it painted white, flanking it with chairs from Källemo and a Flos floor lamp that echoes the lines of the chairs. A medicine chest that hangs over the piano is actually a bar—a custom version of one that Angle designed and produces. A wood-burning stove by Antonio Citterio with Toan Nguyen is essential for chilly, rainy days, and a Tropicalia Cocoon swinging nest chair by Patricia Urquiola for Moroso adds a whimsical twist. “The kids love it,” Angle says.

The kitchen required no extensive intervention; Angle simply removed all the cabinet doors and painted the interiors a startling red, and replaced the refrigerator, stove, and countertop. She chose linoleum from Armstrong for the floors throughout the home—a practical and inexpensive choice for a beach house. In the kitchen, the tiles are installed in an orange-and-white striped pattern.

The dining area, a contrast in all white, is furnished with Bertoia chairs surrounding a Kartell table. A print by the Dutch photographer Leo de Goede is the only art on the wall.

The color story continues in the dark hall, which Angle brightened by painting the doors leading to each of the four bedrooms a shade that coordinates with the palette of the room beyond. The bedrooms are all simply furnished and are mostly white, with vibrant linens, accessories, and lighting adding dashes of color here and there. The boys share one room for sleep and one for play.

The bathrooms, their age showing in decrepit fixtures and rusty water stains, needed the most work. Angle kept the existing plumbing but moved walls and had new fixtures installed. Both bathrooms now have boldly striped linoleum floors.

The family loves their beach retreat, and it shows; the house looks as good as it did when Angle finished it five years ago. An overhaul may have been the original plan, but it doesn’t seem to be in its future.