Stephen Kavanaugh of the London firm Inside Out Architecture undertook this renovation of a 2,583-square-foot apartment in Clerkenwell, in central London, on behalf of a couple.

Kavanaugh said he was taken in by the “dramatic geometry” of the crisscrossing concrete beams in the existing apartment’s ceilings. But he noticed immediately that the walls that divided the flat into rooms did so in a way that cut arbitrarily across the beams, muting their effect.

It struck Kavanaugh that the solution would be to strip away years-old drywall to expose more of the apartment’s concrete shell. “A number of spaces—including a TV room, two bedrooms, separate family and guest bathrooms, a utility room and an adaptable guest bedroom—were then ‘inserted’ into this hollow shell,” Kavanaugh says. “These inserts came in the form of numerous bespoke joinery pieces, designed with a light touch and simple, smooth finishes to contrast with, and hence emphasize, the strength of the textured concrete structure.”

Where new walls and custom joinery pieces were inserted, they were designed to stop short of the concrete beams—a touch that Kavanaugh says treats the interior walls “as something secondary to the structure,” contributing to the open, loft-like feel of the space. “Despite their simple expression,” Kavanaugh says, “the joinery pieces house a wealth of concealed functions, including fold-out beds, integrated radiators, storage units, kitchen appliances, glazed screens, curtain recesses, sliding partitions, and the entire family bathroom. All of the joinery in the project is bespoke, made to suit.”

The project was completed in February 2013 at a cost of $584,000 to $675,000.