It has been one of the most controversial planning applications in Bath's history, prompting more than 1,000 letters of rejection or support at its peak.

But yesterday a raft of technical changes to the design of the Holburne Museum's famous glass-and-ceramic cube extension was voted through by council planners, with only nine letters of complaint.

Despite a groundswell of opposition from heritage campaigners, who saw it as a desecration of the Georgian listed building at the end of Great Pulteney Street, the gallery's expansion plans were granted last November at the second attempt.

But as investigative work for the £10-million scheme got under way, architects discovered a complication with the installation of an air-conditioning system, meaning proposals had to be re-submitted to the council's development control committee yesterday.

Excavations showed a vault, filled in during the past century, that was earmarked to house the essential air-conditioning plant could not be used without significant modification to what The museum was forced to redraft plans to create an extension to the existing basement to house the equipment.

The appearance of the approved scheme is unaffected by the inclusion of the basement, but the additional space means the kitchen can be located below the proposed cafe.

The original scheme also included a reflective pool from which the rear extension emerged, but new proposals omitted the pool in favour of a smooth granite surface.

Other slight changes included: a more glazed appearance to the neck connecting the old building and the new, no glass louvres in the roof, and the temporary removal of two gate posts so construction lorries can access the site without causing damage.

Yesterday the planning committee voted 8-3 to pass the changes, with one member abstaining, and 8-4 to pass the new listed building consent.

However, some councillors expressed concern at the loss of the water feature.

Councillor Paul Crossley said: "I'm really pleased the basement is going to be used and about the changes to the neck of the building, but I have a real sadness about the loss of the water feature, which at the time was billed as part of the concept.

"If we have to lose it, I hope we don't have to have the granite instead and I hope we can negotiate on that because I think that material is unacceptable."

Cllr John Bull said: "It's unfortunate this feature has been withdrawn and I hope we can emphasise the fact that we would like to see it reinstated."

However, Cllr Steve Willcox, who voted against the proposals, said: "I think this extension is wrong for this listed building and environment."

Officers and the museum are now negotiating about swapping the granite plinth for another material.

After the meeting the museum's director, Dr Xa Sturgis, said the alterations to increase the size of the new basement and relocate facilities made the best use of the space and minimised the intervention to the fabric of the Grade I listed building.

He said: "We are delighted to be able to move forward with this essential project.

"We are working constructively and well with planning officers to sort out the few remaining points of detail and are confident that we will be starting the major works early in the new year."

The museum is famous for its 18th-century British portraits and a collection of works connected with Bath in its Georgian heyday.

The extension has been designed by architect Eric Parry to increase display space by 50 per cent and was billed as a "do-or-die" scheme to ensure the museum's survival.

The final decision on the proposals still rests with the secretary of state because of the building's listed status.