Councillor Emlyn Dole's wife, Gwenda Owen, received permission to partially convert stone barns at Capel Ifan Farm in Pontyberem last year.
But an investigation by planning officers found "substantial demolition of the original building".
Planning committee members will visit the site before making a decision.
A report put before the committee on Thursday also said the proposal has changed from being "a conversion of an existing barn" to a "new building development for business and holiday purposes".
The new structure may have to be removed if retrospective planning permission is refused.
A renovated barn belonging to Carmarthenshire council's leader could be removed after being modified beyond planning permission.