Work on a new two-storey building at Stockton's Preston Park Museum has begun.
The extension will provide space for touring exhibitions and loaned artefacts.
Construction work started on Monday, with the building set to open in 2025. Enlargement of the south car park, behind Butterfly World, to provide almost 100 additional spaces will begin in the spring.
Stockton-on-Tees Mayor Councillor Jim Beall said the development would create "an inspiring visitor experience".
The extension is to the west of the existing Grade II listed Preston Hall, with the new building set to connect with the Victorian Street attraction.
There will be interactive galleries on the ground floor, while the first floor will house three large spaces equipped with humidity and temperature control systems to allow "unique and prestigious touring exhibits" to be presented.
Plans for the park, including a new café and toilets, are also being developed.
Councillor Steve Nelson, Cabinet Member for Health, Leisure and Culture at Stockton-on-Tees Borough Council, said: "This new extension will only enhance [the Hall's] current offer, bringing objects of importance and quality never before seen in the area.”
The council secured £20m from the government’s Levelling Up Fund for the project.