Once earmarked by the Land Development Agency as a possible site for redevelopment, the cavernous, 1940s-era building on Wentworth Avenue has since been added to the ACT Heritage Register and the prime real estate once and for all been declared off-limits.

There are also suggestions the building will be opened for use across seven days, not just on the weekends.

Three antique dealers who operate out of the Burley Griffin Antique Centre in the northern annex - an add-on from the 1980s at the back of the complex - were told by letter on Wednesday that they would have to vacate the premises by early January.

The removal of the northern annex is being billed by the LDA as a means to better connect the Old Bus Depot Markets with public space to be developed as part of the arts precinct and restore an old railway siding that used to carry coal to the powerhouse.

The replacement of the roof in the upper hall will take place between early January and late April and the markets will continue to use the lower hall and alternative spaces including the northern annex, the Fitter's Workshop and space outside.

The northern annex will then be demolished between May and August.

While the Land Development Agency had in 2009 suggested the bus depot site should be sold for redevelopment generating $15 million to $20 million in sales, its chief executive officer David Dawes said on Wednesday that proposal had since been abandoned.

"No, that's not on the horizon because what we want to do is go back and honour the original concept plan under the Colin Stewart masterplan where he had that arts precinct as well," Mr Dawes said.

"I think as we get more of that critical mass there, it becomes a really vibrant hub." He said the loss of the potential land sale had not been hard to swallow, saying the agency had instead focused on developing other parts of Kingston.

"Certainly we could generate a higher revenue if we were to sell it, but at the end of the day we would have had to replace [the building] in one way or another and that would cost quite a bit of money anyway," Mr Dawes said.

He said there was no safety issue with the asbestos sheeting in the upper hall roof which had been managed with an "encapsulated treatment" for several years. There was a desire to remove it now, before the building was handed over to the Community Services Directorate which includes the arts portfolio and has carriage of the development of the Kingston arts precinct.

"Obviously we can't transfer an asset that has an issue like an asbestos roof, we're just transferring a liability. So that's why we're embarking on this work," he said.

The former transport or bus depot, which dates from the 1940s, was once the base for the servicing of Canberra's buses and emergency service vehicles.

The Heritage Council twice refused to list the building on the Heritage Register but agreed to do so in 2010 after acknowledging its fully welded rigid portal frame and links to Canberra's early transport history were significant. Mr Dawes said the three antique dealers in the more modern northern annex had been operating on month-to-month leases for several years and the agency would help them to find new premises.

They had to leave the northern annex by early January to allow market stalls to move in while the roof was replaced in the upper hall.

Mr Dawes said the refurbishment also meant certainty for the award-winning markets, removing any possibility of them being relocated permanently.

"I think there would be an uproar in the Canberra community which feels it owns the bus depot markets," he said.

Mr Dawes said once the improvements were done, there was capacity for using the space throughout the week, not just on the weekends, such as for night markets, food stalls and receptions.