Portuguese architect Álvaro Siza has added an angular white-concrete extension to the Monastery of Leça do Balio in Porto to provide "a new landmark" for the site.

Designed by Siza with landscape architect Sidonio Pardal, the extension provides additional space for visitors to the site, which is the headquarters of the cultural entity Foundation Livraria Lello.

"The addition wants to adjust to the history and actual atmosphere of the monastery without the pretence of innovative arrogance," Pritzker Prize-winner Siza told Dezeen.

"The white concrete used in the chapel reflects its position of great autonomy," he added.

The 399-square-metre extension is enclosed by four concrete walls, containing a geometric building and a spacious gravel courtyard.

Rising above the border walls, a pair of towers measuring 12 and 14 metres in height mark out the extension.

The main building is formed of two angled, symmetrical volumes, which rise at their fronts to form lightwells. A sheltered entrance sits between them, while additional doors are provided at their corners.

Inside, the extension features a stripped-back interior defined by exposed concrete walls, which are contrasted by wooden doors and gravel flooring that extends from the courtyard.

Openings punctured into the tower and roof draw light inside the extension to create a play of light and shadow across the interior.

In the courtyard, Siza has designed a 1.8-metre-tall concrete sculpture named Wayfarer. It is intended as a "poetic tribute to the pilgrim's inner journey", referencing the structure's proximity to Camino de Santiago – or Way of Saint James – which is a series of pilgrimages leading to the tomb of St James.

"I was asked to design a building project that would highlight the importance of Leça do Balio in the Way of Saint James," Siza said.

"A new landmark in the building that occupies part of a geometric garden belonging to the monastery and has a connection with the church itself."

Accompanying the extension was the transformation of an existing on-site residence into a cultural venue, led by Foundation Livraria Lello.

Connected to Siza's extension by gravel paths, the on-site residence functions as a "multipurpose nucleus" and cafeteria for the local community, and has been updated with new window frames, wooden doors and granite details.

Other recent projects by Pritzker prize-winning architect Siza include an angular expansion to the Serralves Museum and a ceramic mural for a rustic restaurant interior.

The photography is by Alexandre Delmar.