A shortlist of six architects firms has been drawn up to design a replacement for Liberty Hall in the centre of Dublin.
The winning candidate will be chosen at the end of this month after 34 architects firms expressed an interest in the project.
Last year Siptu announced that it wants to replace its headquarters, which was once the highest building in Ireland.
The building brief for the plan envisages a tower which will double the floor space from about 5,500 square metres to 11,000 square metres. That target could be achieved by building slightly higher than the present 16 stories while also building out.
The current Liberty Hall takes up only 40 per cent of the footprint of the site.
There are plans for an auditorium, a sky deck cafe and a union heritage centre.
Opened in 1965, Liberty Hall has been a controversial building since its inception and has now outlasted its usefulness.
Siptu advertised internationally in August and the initial list was whittled down from 34 to six last week.
A team of three consultant architects and two senior Siptu officials, general secretary Joe O'Flynn and the president Jack O'Connor, will decide the winning firm after interviews later this month.
The winner will be put to the union's national executive committee for approval next month.
The design stage will only begin after the winning bid is chosen.
Consultant architect Brian Hogan said: "Siptu would much prefer to appoint an architect on the basis of his past work, his experience and his reputation."
Mr Hogan said the trade union would work with that architect from the beginning "looking at the options available so that the union feels that it owns the concept".
Siptu hopes to have planning permission secured in the middle of next year.
All going to plan, Liberty Hall will be demolished in 2009 and the project will be completed 18 months after that.
No figure has been given by Siptu for the cost of replacing Liberty Hall, but the union envisages 4,000 square metres (or about 40 per cent of the total floor space) will be left for commercial development to help pay for it.
Mr O'Flynn said Siptu would consider a commercial partner for the project, but only if it was felt that one was needed.
"A number of commercial interests have visited us and spoken to us about getting involved in the project, but we have decided that all of the initial preparation will be done directly by ourselves, "he added.
"The preferred option is to complete the project by ourselves."
