Some of the world's most innovative architects including Renzo Piano, who first came to fame with the Centre Georges Pompidou, will compete to develop the winning concept for a $450 million expansion of the Art Gallery of NSW.

The project, Sydney Modern, is expected to double the size of the gallery. It will also create a great cultural plaza that connects the gallery with the Domain and the Royal Botanic Gardens of NSW.

The gallery's director Michael Brand said on Tuesday he was excited so many leading architects would be "so inspired by our site and vision as to want to have a go at it".

The 12 firms, winnowed down from a list of about 30 firms suggested by experts, were chosen to represent diversity of size, location, culture and emerging talent. They include Herzog & de Meuron in Basel, who designed the Roche Tower and transformed the old Bankside power station in London into the Tate Modern, London's David Chipperfield Architects, Kazuyo Sejima and Ryue Nishizawa SANAA Tokyo who designed the new museum of contemporary art in New York, Kerry Hill Architects in Singapore and RMA Architects in Mumbai. 

Australian firms Candalepas Associates in Sydney and Sean Godsell Architects and Fender Katsalidis in Melbourne have been invited to compete.Sydneysider Angelo Candalepas probably has the most intimate knowledge of the site. "I've gone there as a primary school student and as a student at high school and university and I know the gallery very well," he said.

Each firm was sent the brief on an iPad that opened to show text, video, photos, site plans, and images of works in the gallery's collection.

While the details are confidential, Dr Brand said the only way to extend was via a land bridge north towards the harbour and across the Cahill Expressway via Mrs Macquarie's Road. This could use the land to the east of Mrs Macquarie's Road, which is owned by the Botanic Gardens, the site of an old naval reservoir . The gallery was looking at doubling its current 23,000 square metres and it was up to the architects to develop concepts that could go vertically or more horizontally, said Dr  Brand.

"We are not expanding into the gardens as some people have claimed," said Dr Brand. There was nothing where the naval reservoir was but two concrete slabs covered in grass.

Of the 12 firms, a final five will be selected by a jury of experts in January 2015. It will be an anonymous selection process where judges won't see the names on the concepts.Dr Brand stressed the process was designed to select a firm and not a final concept.