St Andrew's Healthcare, which plans to build the 132-bed facility in Cliftonville Road, has designed the building to reflect its current hospital building, which dates back to 1835.
Members of the West Northamptonshire Development Corporation (WND
C), last night gave planning permission for the scheme.
But the group's deputy chairman, John Weir, raised concerns about the classical style of the new facility. He said: "As a town we should be really proud of St Andrew's and this is a real example of Northampton moving forward. But in terms of the building, it's very domineering.
"However, it should be welcomed and it has my full support."
The new building has been designed to reflect the original hospital, which was designed by George Wallet in 1835.
It will provide medium-secure accommodation for men in eight wards and act as both an extension to the existing hospital and replace some current facilities.
Inside the building there will be a swimming pool, a library and a gym as well as education suites, activity rooms and a hairdressing salon.
Secure external courtyards laid out as gardens will also be provided.
The new centre, which will be built on the former Cliftonville Middle School site, will provide about 200 jobs.
To make up for playing fields which will be lost because of the development, the charity will give £200,000 towards the provision of facilities elsewhere in the town.
