A five-storey building adjacent to the ACT parliament building will house 1700 public servants. It will have 20,000 square metres of space, leased by the government for 20 years, with an option of another five.

It would be built to meet the "WELL" rating of health for its occupants, with a gym, childcare centre, natural light, air-conditioning and cafe food designed around health. Capital Property's head of property, Richard Snow, said he hoped it would be the first to meet the rating in Canberra. It would also have a five-star energy rating and a 200 KW roof-mounted solar array.

A 12-storey building next to that and on the corner of Constitution Avenue and London Circuit, also with 20,000 square metres of space, is to house a 90-bed hotel and commercial office space - although Capital Property has also left open the possibility of apartments there.

The two buildings will have a joint three-level basement carpark, including 250 public carparks, slightly more than the 225 available in the open-air carpark at the moment. The ACT government will lease another 175 parks, with 220 more parks for other use.

The office block has been an on-again, off-again project for many years, first announced in 2010 as a government build, when it was to include a ministerial wing and a "sky bridge" to link it to the Assembly building. In 2012, the government decided instead on a block built and owned by the private sector, with the government as tenant.

In 2014, the block was back on the agenda, when the government called expressions of interest for what was then envisaged as an office block to house 3400 public servants. Eleven companies registered interest and six were shortlisted to submit detailed tenders.

Mr Barr said all six proposals had been high-quality, innovative and "financially attractive". The Capital Property Group's was "excellent value for money by balancing a competitive financial offer with a design scheme that is elegant in urban, architectural and interior design terms".

The development, including street level cafes and landscaped plazas, would be "a major boost to jobs and economic activity" and allow the ACT to substantially consolidate policy and administrative staff.

Mr Snow, said it had been about 10 years since the company had built in the city - the federal Industry department building in Akuna Street - with development in recent years focused at the airport and adjacent Brindabella Business Park.

This was was "a great opportunity for our group to go back into development in the city" with a benchmark building. "That's important to us," he said.

Contracts would be signed, then Capital Property would seek planning permission from the National Capital Authority - a process he expected to be "streamlined and non-contentious", given the authority had already seen the plans and it was designed to the authority's requirements - no more than 25 metres for the office block and meeting the maximum city height limit near Constitution Avenue.

He hoped construction could begin late next year, with both buildings ready by mid 2020, with the possibility the hotel block might be completed six to 12 months before the office block.

Capital Property was yet to settle an operator for the hotel, and whether it would be a 4.5 or 5 star hotel.

The Snow family's company, owner of the Canberra Airport, also bought the land to build the new suburb of Denman Prospect last year, where it is building 2000 homes.

Managing director Stephen Byron said plans for the 9000 sq m London Circuit site were focused on "enhancing the urban fabric" of that part of the city, and building a "dynamic new state-of-the-art workplace".

There would be a "Legislative Plaza" between the assembly building and the new office building, and a "retail laneway" between the two new buildings.

Mr Byron said the Capital Property Group's bid was the only one that had a Canberra-based company as both developer and the builder. The head contractor is Construction Control and the architect is Bates Smart.