Work on the Grade II listed 1930s building was given the nod during a meeting of the planning committee.
It will include the removal and replacement of the front desk, removal of the raised platforms in the Gordon Room, removal of some non-original walls and partitions, new carpets, lighting and replacement ceilings where necessary, internal fitting for the kitchens and shower rooms, and a combination of secondary glazing and replacement windows.
James Appleton, head of planning, told the meeting: “Generally, the alterations do not affect the historic layout of the town hall – and care has been taken to ensure that is the case – and key architectural features are enhanced as part of the works.”
The aim of the work is to improve the building’s thermal performance – how well it retains heat – while making more efficient use of space so that staff moving from nearby Portland House can be accommodated.
A council spokesman said an overall budget of £1.5m had been approved in December, though there will be extra IT costs.
They added: “This investment will enable our NHS partners to share our accommodation in Portland House.
“This will generate an annual saving of £364,000, paying back the investment in under six years.”