The first phase of the new Uffizi project, an expansion and modernisation of Florences grand museum, opened in early March with six new rooms, the relocation of some masterpieces, an academic bookshop and a new exit at the rear of the museum.

The project, which will eventually double the exhibition space in the galleries, includes a controversial loggia for the new exit, designed by Japanese architect Arata Isozaki. The proposal has fuelled a cultural debate in Italy over the use of modern architecture in historic city centres. Meanwhile the construction of the loggia has been delayed and thrown into doubt by the discovery of mediaeval and possibly Roman foundations below the site.

The 60 million expansion will give the Uffizis collections of renaissance art more room, taking over the newly renovated first floor, which was home to the state archives until 1987. The first five rooms on the newly opened floor display paintings by Caravaggio and Guido Reni among others.

When the work is finished (not for another three years at least) the Uffizi will have increased its show space from 7,000 to 15,000 square metres; at the same time, the number of exhibits on display will increase from 1,500 to about 2,000.

One of the new rooms opened in March was the first floor hall parallel to the Arno river, connecting the east and west corridors of the galleries. In this room three items have been reunited after nearly 300 years. An ancient Greek vase and two 16th-century bronze sculptures were displayed together at the Villa Medici in Rome before being brought to Florence in the 18th century and separated in the Uffizi.

The hall itself is one of the jewels of the renovation, a place where you can see how the architecture and the surrounding city work together, says Antonio Godoli, the architect who is overseeing the expansion. "This room is very important because here you get a very good feeling of the concept of the Uffizi building," he said, standing in the sun-drenched hall overlooking the city and the river. "From here you look at the old town and at the same time its open to the outside, to the hills. Were beside the river because Vasari chose a very strategic place in the town. You feel this looking around. The idea of this museum is not a closed museum, but part of the town."

Giorgio Vasari designed the administrative offices, or uffizi, in 1560 for the Medici family, which then ruled Florence. As early as 1581, the top floor was organised for royal visitors to view the familys extensive art collection.

The Uffizi which last year received 1.5 million visitors contains one of Europes most important art collections. It is also one of the most unassuming exibition spaces, with graceful galleries looking onto a courtyard of arcades, but with no grand entrance or exit. Isozakis proposed exit, a splash of modernism in the heart of renaissance Florence, was supposed to give it an iconic identity, along the lines of the glass pyramid at the Louvre in Paris. Although embraced by Florences mayor, the museum administration and the then centre-left government when it was presented in 1998, the new exit plan also has its critics who decry the use of modern architecture to gussy up a historic building.

In 1990, Giovanni Michelucci, then 99, the architect chosen by Mussolini to design Florences train station in 1933, proposed a grand new glass prism exit for the Uffizi, leading out of the back of the museum. In his writings Michelucci referred to the need for "something which would become a landmark, and highlight the importance of the artistic and historical heritage (of the Uffizi)."

Micheluccis plan for the back exit was never realised, but in 1998 it inspired a design competition for an exit. Isozaki won with his concept for a huge fan-shaped loggia made of stone, steel and polycarbonate that would cover the whole square, allowing light to filter through while protecting the square. In his proposal, Isozaki said the design would create a new urban space for Florence. "The loggia will become an instrument for the museum to communicate with the city," he wrote.

"Its a very interesting design. Its very light, very open," said Godoli, as he stood at the newly opened but empty back exit, where Isozakis loggia should be built. "We still think we will make it in a few years, but its a problem because we have to see what is down there." As he spoke, Godoli pointed over the railing to the workers digging around unearthed mediaeval foundations. He explained: "The father of this plan was the old left-wing government. The new government of Silvio Berlusconi has said that this exit is not correct in an old place like this. Well see what happens."

Giuliano Urbani, the culture minister, referred to the uncertain future of the exit in February. "Some questions are still waiting to be resolved, such as the completion of the archaeological excavations and the assessment of the need and the way to build the loggia on Piazza de Castellani, the square at the back of Uffizi, according to Isozakis project," said Urbani.

Cristiano Brughitta, spokesperson for the central administration of Italys architectural heritage, said the excavations, which are expected to end this summer, might force modifications to the design of the exit. "Some important mediaeval foundations have been found and in a lower stratum there are what might be signs of a Roman theatre," said Brughitta. "Isozaki remains the winner of the competition and no one denies that."

For now the new exit leads visitors out to a construction site on a concrete ramp, winding past the excavation on one side and service vehicles and piles of gravel on the other, to Piazza de Castellani, newly renamed Piazza del Grano, in honour of the old vegetable and grain market that used to be located there.