The formerly disused space now provides a guest room and study for the north-London home, which is owned by a couple who both work in the creative industries – one in photography, and the other in television.
"The clients wanted it to function as an extension of the house, but there was also to be a feeling that you were going on holiday when you went upstairs," explained Martin.
"They wanted it to be a study area, a living/sleeping area and a general hangout space for the family."
The space has a T-shaped plan, so it made sense to divide it into three rooms.
The south-facing area is the widest of the three wings, so this became the living space. A compact bathroom was created on the east side, while the west wing was converted into a study.
To create an extra level of privacy for this third room, it was concealed behind a moving wall with an integrated bookshelf.
To enter, residents simple push one side of the wall and it slides back on runners embedded in the floor and ceiling.
Martin describes this addition as "a new twist on the hidden library door". It allows the study to become a separated space for quiet working.
Both the walls and the angled ceilings have been lined with rough-sawn wooden boards, which were whitewashed to keep the space looking bright and airy. This detail refers to the aesthetic of a beach hut or chalet – a nod to the idea of being on holiday.
This is complemented by the joinery. As well as the bookshelf door, there are built-in shelves and cupboards in the living space, while the study features a bespoke oak desk.
"We designed all the joinery ourselves, but what was nice on this project was that the clients were very specific with their requirements," the architect told Dezeen.
"For example, the drawers of the study were sized to fit photo archive boxes, and the living room unit was designed to house a stereo and vinyl collection. With the more complex items, such as the sliding shelving unit, we worked with a joiner to ensure the design worked."
In the bathroom, square teal-coloured tiles line the walls of the wet areas.
Although these details are all very contemporary, Martin insists that the style is in keeping with the original Arts and Crafts character of the house, located just a few streets away from the Hampstead Garden Suburb – the district planned by Raymond Unwin at the start of the 20th century.
"The clients have respected the Arts and Crafts features of the house," he said.
"The loft space is more contemporary however the dark oak used for the desktop and flooring together with the sanitaryware, furnishings and use of the same paint colour mean that the space is not incongruous with the rest of the house, but complements it very nicely."
There are five skylights in total, ensuring there is plenty of natural light. Some vintage lighting fixtures were also installed, both in the study and the living area.
Photography is by Richard Chivers.