Named Casa 2, the building was conceived by the Lisbon-based studio as a standalone "garden pavilion". It contains four ensuite bedrooms, a kitchen, library and communal living room across two floors, set within a large garden at the rear of the existing home.
Almost the entire southern end of the concrete building is wrapped in tall glass doors framed in oxidised brass, allowing the living area to blend with the garden in summer.
In the colder months, the space functions as a "winter garden" centred around a fireplace in the western corner.
"[Casa 2's] communal space is a generous, double-height winter garden, which ensures the transition between the intimacy of the rooms and the outdoor space," said Bak Gordon Arquitectos.
This contrast between openness and intimacy is reflected in the annexe's materials, with both the exterior and the living space finished with exposed pigmented concrete.
In the bedrooms and library, this rougher material is swapped for carpeted floors, Tadelakt plaster walls, fitted wooden furniture and terrazzo bathrooms.
The bedroom block has a trapezoidal shape, creating two small pockets of garden space at either side of the outbuilding, which the rooms overlook through deep-set windows on both the ground and first floors.
"The volume rests on the ground with specific geometry, committed to the multiple environments that gravitate around it," described Bak Gordon Arquitectos.
A staircase behind the living room leads up to the library, which houses a bookcase and small seating area illuminated by a skylight as well as an internal window that looks down onto the living space.
"The kitchen and the library are small spaces directly related to the circulation and characterised by their thermally-modified wooden cabinets that contrast with the Tadelakt-finished walls and the concrete floor," the studio added.
Two further skylights illuminate the centre of the plan, while two circular windows sit on the ceiling of each ensuite bathroom.
The annexe is surrounded by raised planted beds and two paved pathways, which connect to a route that surrounds the existing home and leads to an adjacent garage.
A white wall lined with trees surrounds the site boundary and a raised swimming pool sits at the centre of the garden.
Bak Gordon Arquitectos has completed several homes across Portugal, including an apartment renovation in Lisbon that introduced a small courtyard space and a home in the Alentejo region with an elongated swimming pool.
The photography is by Francisco Nogueira.