The primary school was originally built in 1956, on the border between the Caselas neighbourhood and the Monsanto Forest Park, to service the newly constructed low-density housing scheme.
The school occupies a hillside site that links the forest along its upper edge with a district of three-storey apartment buildings further down.
The main additions are accommodated below the original building and help connect it with the neighbourhood.
"Building most of the new spaces at the lower level of the plot helps to keep the school's volume framed in the surroundings, without disregarding the existing building," Site Specific Arquitectura explained.
"At the same time, it creates an accessible roof in the extension of the porch, restoring the yard of the original typology."
On the upper level, two new buildings are introduced on either side of the older structure. A library and administrative spaces are located to the west of the main entrance, with the multipurpose room and support areas to the east.
Both of the new additions are entirely separate from the original building, but their roofs extend to lightly rest against its single-storey porch, providing continuous covered circulation.
Site Specific Arquitectura's interventions are drawn using basic geometric forms that are distinctly contemporary in their expression, but also complement the older building's more traditional design.
Primary school pupils using the improved spaces in the existing building also have direct access to a larger playground, as well as green spaces created on top of the kindergarten.
Staircases inside the new structures descend to a lower level where the activity rooms are connected to outdoor courtyards. These spaces, and the canteen that is also accommodated on this floor, have direct access to the playground.
"The distribution through the two floors and the existence of the patios on the lower floor creates the illusion of an extension confined to one floor," the architects added, "which follows the topography of the surroundings and allows the creation of generous and autonomous outdoor spaces with different characteristics."
Large holes cut into the roofs above the courtyards create a visual connection between the two levels, and ensure that plenty of natural light enters the interior through the windows below them.
Site Specific Arquitectura has previously converted a Portuguese warehouse into a monastery chapel.