PLANS to replace Liberty Hall with a 20-storey building are due to be lodged with Dublin City Council within weeks.
It is understood the designs are being finalised and could be submitted to the local authority as early as December 10.
The ambitious blueprint, which makes provision for a 'sky pod' offering panoramic views of Dublin Bay, was unveiled by Siptu in early October.
However, the planning application is still not ready.
Siptu unveiled details of the proposed development of its headquarters at an annual conference marking the trade union's centenary.
It has promised that the new Liberty Hall would have a "wow factor" second to none in the capital.
Designed by Gilroy McMahon Architects, who were also responsible for Croke Park, the facilities would include a 300-seat theatre at lower-ground level.
If the project is approved, it will also have a heritage centre, illustrating Siptu's history and 15 floors of open-plan office space.
The proposed building would have three storeys more than Liberty Hall and would be 24m higher.
General Secretary of Siptu Joe O'Flynn said work on the development could begin in mid-2011 if planning approval is secured.
A glass-fronted 'sky lift' would take visitors to an enclosed observation terrace, or 'sky pod', at the top, according to the outline plans.
Des McMahon, of Gilroy McMahon Architects, said the 'sky pod' was likely to attract 250,000 visitors annually, generating good revenue for Siptu.
From the terrace, it would be possible to look down on the GPO in O'Connell Street and other historical buildings.
Mr McMahon said one of the principal objectives was to create "an appropriate, energising and inspiring" workplace for Siptu's staff.
Mr O'Flynn said he hoped the replacement building for Liberty Hall will become as familiar a part of the Dublin skyline as the existing building.
"Siptu is conscious of the union's place in the community ... this venture will preserve the union's long association with the current historical site," he said.