Illegal rooftop additions to two historic palazzi in the heart of Valletta are "mutilating" the capital city's roofscape, NGO Din l-Art Ħelwa has said. 

Developers have illegally added a new floor to the building at 167 Merchants Street and built a rooftop structure "far larger" than permitted at a palazzo just a few doors down, at 176 Merchants Street, the NGO said. 

"Din l-Art Helwa remains concerned by the flagrant nature of these abusive works, which erode the Outstanding Universal Value of Valletta and threaten its status as a UNESCO World Heritage Site," it said in a statement. 

The NGO urged authorities to take swift and decisive action to safeguard the capital city's architectural heritage, saying the contractor responsible should be made to reverse the works and be held responsible for any damage caused. 

Any attempts by contractors to have the works retroactively sanctioned should be summarily rejected, it added. 

The palazzo at 167 Merchants Street was once known as Casa Roselli Massa and was the home of philanthropist Pietro Roselli and his wife Alusietta Massa. The building has a historical connection with the church of the Jesuit College, which eventually became the University of Malta.

The addition at 176 Merchants Street. Photo: Din l-Art Ħelwa