The Switch Room, a small building near the Power House, has been used as accommodation for visiting artists to the glassworks.

It is on the ACT Heritage Register, which says demolition "shall not be permitted, other than in exceptional circumstances, including circumstances in which the buildings are structurally unsound and beyond economic repair or where there are significant public health and safety reasons to warrant demolition" and unless there is no alternative.

While the Fitters' Workshop and Power House are to be preserved, the Switch Room is in line to be demolished to make way for a parking building as part of the redevelopment of the site surrounding the glassworks at the Kingston Foreshore.

National Trust chairman Eric Martin said he had been told the demolition was approved in 2014, but his group had been unaware and not consulted.

The public heritage citation still listed the Switch Room as protected, which wrongly reassured the public.

"You think it's being protected because it's on the Heritage Register but in fact the register is not up to date," Mr Martin said.

The National Trust was "totally opposed" to its demolition and had seen no rationale for the move.

"Once again heritage not is not being given adequate or due recognition in terms of this potential change, and the information available to the public appears to be totally confused," he said.

This week, the government announced that a Geocon-led consortium had won the tender to redevelop the area, which is the last remaining block on the Kingston Foreshore.

The project includes arts facilities, apartments, possibly a hotel, a carparking building, and office, shopping and restaurant space, but details of Geocon's winning bid have not been released.

The heritage citation says the Power House provided power to Canberra until 1929, then was brought online again for periods between 1936 and 1942 when the Burrinjuck Dam was being repaired, and again between 1948 and 1957. The Switch Room was built in the later period.

ACT Heritage Council chairman David Flannery referred inquires on Thursday to the planning directorate. A Land Development Agency spokesman responded that designs had not been finalised and were still be negotiated with Geocon.

In 2014, conditional demolition approval had been sought to make way for a carpark "to provide potential tender participants with options".

"If the preferred tenderer's final design requires the removal of the switch room, Heritage Council endorsement of the design of the new structured carpark is required," the agency said.

The approval requires demolition by October 2, 2019. It also requires the protection of a large gum on the site. The decision says the demolition was conditionally supported by the Heritage Council.

The Kingston Barton Residents Group has tried unsuccessfully to find details of the 2014 decision, and this week was forced to lodge a formal Freedom of Information request for a copy of the development approval (which has since been provided to The Canberra Times).

Committee member Nick Swain said the Switch Room was the only remnant of the postwar power generation phase and a "delightful" building that should be retained.

"It's been carefully designed to not look like a switch room," he said, suggesting it should be possible to incorporate it in the development.

"Repurposing such a building should be a trivial challenge to the syndicate given their experience with New Acton," he said, referring to the involvement of architects Fender Katsalidis in the New Acton development.

The residents group had met with the Land Development Agency in January, but the agency had been very guarded in the detail it would release, beyond saying that eight different contracts would be negotiated for the complex project and "just trust us, it'll be lovely", he said.

ACT Heritage wrote to Mr Swain at the end of January, telling him that the Heritage Register would not be amended until the building was demolished,

Asked whether the building would be demolished a Geocon spokeswoman said negotiations with the government would determine the final design.