Down District Council has moved into new premises at the former Downshire Hospital in Downpatrick.
The redevelopment cost £11m and its chamber has seats for 23 councillors.
In three years' time, the council is due to merge with Newry and Mourne, forming a new "supercouncil" with more than 40 members.
A decision has not yet been taken on where its headquarters will be located.
Down Council chairman Mickey Coogan said it would be up to the new body to decide.
"We have to look at accommodating meetings in Downpatrick, but also in Newry," he said.
"Newry is a city, there are a lot of council jobs there and I don't think anyone is going to let those jobs go lightly."
The council said members and staff from both councils were meeting every month to discuss practical issues, such as delivering bins, in a new bigger area that will stretch from Saintfield to Crossmaglen.
In a reorganisation of local government in Northern Ireland, known as the Review of Public Administration (RPA), 26 councils will become 11 "super councils".
Environment Minister Alex Attwood has been managing the changes to councils and has issued guidance about spending.
"I can understand why Down District Council spent money on new premises," he said.
"It created economic opportunities during the build and given the uncertainty about reorganisation, they had to make plans.
"I'd like to see councils spend money in the run down to RPA because that will be an economic driver. It'll provide jobs, it'll provide infrastructure that best serves the citizens on the far side of RPA."