Mr Armstrong confirmed last week his wife had received rental expenses from the Assembly as owner of the office building.

He's now likely to become embroiled in a wrangle over whether planning permission should have been sought for it.

The MLA, a UUP deputy whip at Stormont, last week said his prefab was a well-used "flagship" for the party and the roadside sign for it was "very noticeable".

A DoE Planning Service spokeswoman said: "The Divisional Planning Office has no record of planning permission being granted for a constituency office or an associated sign at this location. Enforcement action is not under way but the alleged breach will now be investigated by the division."

Mr Armstrong responded by saying the office was an extension to his home and he does not consider that planning permission is required.

The premises are located on lawns close to the MLA's house within the family farm at Coagh Road, Stewartstown. He said last week that his rental claims ceased at the end of 2007. According to the Assembly, the payments involved £350 per month.

MLAs are permitted under Assembly rules to rent their offices from family members.

"The office used to be run out of the house but now it's run from a prefabricated extension to the house," Mr Armstrong said.

The MLA once again defended the location of his office.

"We have to provide a service to constituents and they expect a good service," he said.

" If the office is run from the vicinity of where the MLA lives, it will be superior to one run from a nine to five office in the high street."

The DoE stated that it is dealing with a retrospective planning application for a "beauty resort" business at the same location.

Mr Armstrong said this related to a daughter's business located within the dwelling house. He expressed confidence that it would be approved.

" You are allowed to do a business of a small calibre from the house," he added.

The DoE spokeswoman said: "In relation to another business at this location, planning permission has not been granted.

"However, the division is currently processing a retrospective planning application for the, 'unauthorised change of use from domestic kitchen, study and store to health and beauty retreat' and a consent to display a sign for this business.

"A decision has not yet been made on these applications.

"The division does have a current enforcement case in relation to the unauthorised health and beauty retreat and associated signage at this location. Enforcement action is currently on hold pending the outcome of the retrospective planning application and the consent to display application."