Johnston overhauled the maisonette, which occupies the lower two levels of a Victorian terrace house, for a couple and their two young children.
The architect aimed to maximise the space, bring in more natural light and add more storage to maintain an uncluttered appearance.
The architect swapped the layout around, placing sleeping areas on the upper level, and living spaces on the ground floor.
Prior to the alterations, the bedrooms were located on the lower floor and were dark and enclosed, while the upper floor kitchen and living room lacked a relationship with the rear external courtyard.
To bring in more natural light, the architect removed the rear external wall on the lower level to extend the residence. The addition is clad in a white render, matching the style of neighbouring houses.
Glass sliding doors, which are the full width and height of the space, now open to the courtyard garden, while large roof light allows light to flood into the main space.
Inside, on the ground floor, the existing partitions were demolished and replaced with a supporting steel structure concealed within the walls and floor to create the open-plan living, kitchen and dining room on the ground floor.
A compact plywood box hosting the kitchen and staircase is lined with storage, and includes a desk with adjacent shelving. A utility room and cloakroom are also tucked beneath the stairs.
A lower level study area and a half landing, which could not be raised due to the property above, is concealed behind the wooden volume.
White-painted walls feature throughout the property, along with neutral-toned materials like plywood, stainless steel, polished concrete and white plaster for the lower floor.
Contrasting dark timber flooring and white-built in cabinetry feature on the bedroom level.
"The restrained nature of the material palette is designed to complement the simplicity of the spaces," said the architect
"The aesthetic of the upper floor is also minimalist, with dark timber floors, white walls and simple white built-in joinery," she added. "Ceiling heights are generous and light floods in through the tall windows."
The furniture used to dress the apartment – styled for photography by Emma Archer – is intended to complement the material palette and includes a white dining table with matching chairs.
A black leather sofa with dark wooden legs, a wooden side table and a white rug add colour and texture to the lounge, which has polished concrete flooring.
In recent years the demand for contemporary house extensions in London has spiked, driven by a growing appreciation for design.
Recent London extensions include a board-marked concrete and glass addition to a home in the north of the capital and a sooty brick tower used to add extra room to another.
Photography is by Rory Gardiner.