The building was not listed by Heritage NZ but was protected under the council's District Plan. The trust had worked closely with Heritage NZ on the design and materials, for example by continuing Scott's use of white concrete blocks, native timber and black wooden posts. Rimu sarking from the old theatre had been re-used throughout the building.
Client rep Larry Jacobson, of Paihia, said ensuring the expansion complemented Scott's original design was a priority.
"Hopefully he'd be happy with what we've done if he was still alive today," Mr Jacobson said.
The project had been completed in a tight timeframe and under budget, which was a credit to the builders.
HB Architecture and Henwood Builders also worked together on the new Museum of Waitangi, the Treaty Grounds gateway and an award-winning toilet block. The visitors centre expansion cost about $1.7m.
The next project planned at the Treaty Grounds was a re-interpretation of the Treaty House itself. Work is expected to start in February and take a year to complete.
Last month the Waitangi National Trust took out the Maori cultural tourism title in the New Zealand Tourism Awards.