The 34ha stretch of riverside land was completely rejuvenated for the five-month festival, which attracted 3.8 million visitors.
However, the site closed after two years and by the time of its acquisition by Langtree in 2004 it had become badly neglected.
The commercial property specialist signed a 10-year development agreement with the council but the recession and a public inquiry put paid to plans to develop housing. Langtree has carried out remediation works including restoring the Chinese and Japanese gardens and lake as well as the creation of a woodland walk.
In March a report to the council cabinet recommended purchasing the site because it presents "unique opportunities" for cultural events, concerts, exhibitions and local engagement.
"Examples could be holding an artist-in-residence programme working in partnership with the universities and using the site as a vibrant outdoor classroom with activities for schoolchildren through to adults enabling them to experience the diverse horticulture of the site," the report said.
It would also be possible to develop the Festival Gardens as a "unique and world-class cultural destination". The council is exploring those options as well as plans to create a high-quality housing development on the land.