THE 20th Century Society has raised concerns about plans to alter "an important Brutalist" building in Bradford city centre.

Arndale House - opposite the Wool Exchange, in in the process of being converted from offices to flats, and as part of this a planning application to change the building's full length windows was submitted to Bradford Council.

Last month Bradford Civic Society objected to the plans - pointing out that Arndale House was the only building in the UK to have been designed by the architects's firm behind Seattle's Space Needle - John Graham and Partners.

Going beyond objecting to the plans, the group called for the building to be listed for its historic significance.

Now the 20th Century Society has joined the Civic Society and raised concerns about the proposals for the building - which dates back to 1965 and replaced the much loved Swan Arcade.

The application, by Xchange Development Company, calls for the building's full length, single glazed windows to be replaced with double glazed UPVC windows the developer says is more suitable for residential use.

Call for Bradford building that was designed by same company behind Seattle's Space Needle to be listed

The 20th Century Society campaigns to retain and protect buildings that are good examples of 20th Century architecture.

In response to the new application, the group has written to Bradford Council, saying: "Arndale House is an important Brutalist office building designed by a major, internationally-known American architectural practice.

"It is therefore considered a Non-Designated Heritage Asset of great significance.

"While the Society welcomes the conversion of offices for residential use, we are concerned about the current proposals.

"We believe the proposed removal of the office’s existing windows and their replacement with double-glazed openable UPVC units will cause substantial harm to a valuable NDHA.

"Arndale House is characterised by its regular pattern of single-paned and seemingly frameless windows.

"This appearance would be completely altered if the proposed windows were installed which have grey frames and are split into three panels.

"This proposal would not just harm the building’s aesthetic value but would involve the removal of much architecturally and historically significant primary fabric.

"The Society encourages the applicant to consider secondary glazing which will improve the energy efficiency of the windows without the need to replace original fabric.

"The Society strongly objects to the proposed changes to Arndale House’s windows which will cause substantial harm to an important Non-Designated Heritage Asset."

Bradford Council's own Design and Conservation officer, Hannah Meekings, has also voiced concerns.

In her report on the proposals, she said: "I have concerns about the visual impact of the proposed windows. The existing windows are, as far I can tell, original. They comprise of a single pane of glass and have a minimal frame (possibly metal edged) which is deeply recessed into the reveals. In my opinion, UPVC is not an appropriate material for the proposed frames and the manner in which the window has been subdivided and the incorporation of plastic inserts at the top and bottom will result in a significant visual impact which will be detrimental to the simplistic intended appearance of the building of which it’s fenestration is an important contributing element.

A decision on the application is expected in the New Year.