Residents on Sunday rallied outside the proposed site of the development at 71 Constitution Avenue, claiming the height of the building would approach 30 metres when the National Capital Plan limited buildings on Constitution Avenue to 25 metres.

The National Capital Authority is the consent authority for the building and says it has yet to make a decision on works approval.

The $50 million project on the former headquarters of the Hindmarsh company, on Constitution Avenue, includes 71 apartments, seven terrace houses fronting Getting Crescent, two ground-level commercial spaces and 187 parking spaces.

The existing three-storey building on the site would be demolished. Consultation on the project opened on November 24 and closed last Friday.

"The NCA will consider feedback received and prepare a consultation report prior to finalising its consideration of the proposal. The consultation report will be published following a decision on the works approval application," a spokesperson said.

The authority confirmed to The Canberra Times that "as per the National Capital Plan, buildings on Constitution Avenue have a height restriction of 25 metres above adjacent kerb levels".

A consultant's report for Hindmarsh says the proposed development has "habitable spaces up to 25m
above the natural ground level of the site and "the height of the adjacent kerb level changes across the site".

Hindmarsh ACT development manager Greg Smith said the building rose up with the topography, but he believed met the restrictions.

"The building fronting Constitution Avenue meets the 25 metres," he said.

Campbell residents' spokesman Mark Anderson said the residents did not oppose a new building being constructed on the site but were against it exceeding 25 metres due, they said, to the addition of "two penthouses, and rooftop equipment and lift overruns".

"There are rules and we want developers to adhere to the rules and we want the NCA to enforce them," Mr Anderson said.

The consultants' report said the height of the building had to take into account minimum floor level clearances to account for a 1-in-100-year flood. The residents argue other buildings have met that challenge without exceeding 25 metres.

The residents have called on the NCA not to approve the development as is, saying it will set a precedent and erode confidence in the planning rules overseen by the authority.

The consultant's report also refers to the developer having discussions with the NCA "during design development to arrive at a building solution".

The residents  were "seriously concerned" that  meant the NCA had "already agreed to the height increase, in direct conflict with the National Capital Plan for which it is the steward and gatekeeper".

A spokesperson for the authority said no decisions had been made.

"As with all major developments, a number of pre-application meetings were held with the applicant," the spokesperson said.

"These meetings are advisory only and it is up to the applicant if they wish to accept NCA advice during those meetings. The planning report was prepared by the applicant, it is not an NCA endorsed document.

"The NCA has not provided ‘support in principle’ for the design. The NCA will undertake a full assessment of the proposal and consider community views upon completion of public consultation."

Mr Smith said there had been consultation but not collusion with the NCA. They wanted a superior design, and part of that was setting back the buildings and providing an atrium to let in natural light.

"Generally, you'll consultant with people at the NCA on one or two occasions to check on what was permissible and where the design was going. We've met with the NCA on about five occasions," he said.

Mr Smith said he understood Campbell residents were worried about the changing nature of their neighbourhood, but he believed the development would ultimately bring life to the area.

"It will be a fantastic outcome for the area," he said.

Mr Smith said the residents also needed to acknowledge that the height of the townhouses fronting Getting Crescent was  far less than the 12m allowed, "commensurate with the low-density suburban
character of the street".

"We're 2.5m to 3m below the 12m permissible on Getting Crescent, so if we were to 12m, you wouldn't even see the buildings behind the development," Mr Smith said.

"The height of what's on Constitution Avenue is far negated by what's on Getting Crescent. You can't pick and choose what you like. It's a holistic development.

"When you're looking at it fitting in with the surrounding area, townhouses on Getting Crescent are far superior to apartments."

The consultant's report for Hindmarsh said the redevelopment of the site would provide activity
and a sense of place for the area, "consistent with Walter Burley Griffin’s original plans for
Constitution Avenue as a grand boulevard".

Mr Smith said should the development be approved, demolition could start in May and occupation commence from March, 2021.

"There is a high demand for this product," he said.