Prime Minister Julia Gillard has given in-principle support to the ACT Legislative Assembly being granted the authority to determine its own size without approval from the Commonwealth.

Chief Minister Katy Gallagher plans to establish an expert working group to examine options for increasing the size of the Assembly from its current 17 members.

Ms Gallagher's preference is for 25 MLAs to be elected at the 2016 election, but another option would be to expand the chamber to 21.

The Legislative Assembly chamber in Civic's two-storey South Building was built to comfortably seat 25 MLAs but there is little if any spare office space.

Ms Gallagher said it was possible the Assembly building could be extended but it was unlikely a third-storey would be approved.

A more likely solution would be moving ministers out of the building, as occurs in many other Australian jurisdictions.

''One of the options is that you could move the executive somewhere,'' Ms Gallagher said.

''It could be into departments as they do in some other states where they have a sort of ministerial office, but you'd have to look at the options.''

Based on current pay rates, adding an extra eight MLAs to the Assembly would cost taxpayers more than $2 million in additional salary costs for members and their staff. This does not include the cost of providing taxpayer-funded cars to MLAs.

In the lead-up to the ACT election, the government abandoned plans for a new government office block next to Civic Square which would have included ministerial suites and a sky bridge to the Legislative Assembly.

Before the government considers accommodation for new MLAs, it needs to win approval to expand the Assembly. The federal government will introduce amendments to the ACT Self-Government Act, enabling a two-thirds majority of MLAs to vote to increase the size of the Assembly.

The present 17-member Assembly is made up of MLAs from one seven-member and two five-member electorates.

A 21-member Assembly would probably consist of MLAs from three seven-member electorates, while a 25-member chamber would have five-member delegations from five districts.

Minor parties and independents are more likely to win seats from seven-member divisions.

Despite this, Greens MLA Shane Rattenbury said he was open to the idea of a 25-member Assembly.

Liberal leader Zed Seselja said negotiations between his party and Labor would determine the future size of the Assembly.

''I think there are a number of other things that we do need to look at in this context,'' Mr Seselja said.

''I think you do need to look at the resourcing of individual members, we need to look at the resourcing of our committee offices. I've argued for a long time that that's been inadequate, so you shouldn't be first expanding the size of the Assembly without addressing those kinds of things as well.''

Ms Gallagher said she hoped the three parties in the Assembly could work together and agree on a plan for expansion.