Northampton Borough Council has announced the public part of the museum in Guidhall Road will close on Friday, February 19, when work will start to dismantle the displays so objects can be put into storage.
The project, which was due to start in 2014, will involve an extension of the current site between the former Gaol Block and Guildhall Road sections of County Hall.
The first phase of the extension, £8 million of which will be funded through the sale of the Sekhemka statue once on display at the museum, will provide more exhibition space and the creation of a corporate hire facility.
The council’s cabinet member for community engagement, Councillor Anna King, said: “At a time when other museums are closing down or cutting back, we are making a huge investment in ours, which will help boost its fortunes and protect it for the future.
“During the extension and renovation, we will be running an appeal to expand and enhance the collection, and we have already been approached with donations linked directly with Northampton’s heritage.”
The museum is set to reopen in autumn 2018.
Contractors are set to be appointed to the project in the new year and a planning application will be submitted shortly after, expected to go before the planning committee in late spring or early summer.
Later phases of the work, including the complete renovation of County Hall’s old Gaol Block, are expected to be paid for through “fundraising activity and grants,” the council says.
The authority also says there will be “pop-up” events and open days at the museum during the renovation, but most of the school activities, along with some of the museum’s general activities, will be transferred temporarily to Abington Park Museum.