A spokeswoman for the Assembly Commission, which runs Stormont, said its guidelines stated the building cannot be used for election purposes.
The guidelines were issued to all political parties and individual MLAs.
UKIP's David McNarry told journalist the party had sought and been granted permission to use Parliament Buildings.
"We're only following in the footsteps of others. We made an application to do what we're doing," Mr McNarry said at the launch.
In a statement to the BBC, the Assembly Commission spokeswoman said: "The use of Parliament Buildings as a venue for election purposes is not permitted under the Use of Assembly Resources During the 2015 General Election guidelines which were issued to all political parties and individual MLAs on 2 April 2015.
"The Northern Ireland Assembly Commission is aware that the guidelines may have been breached and is currently reviewing the matter."
UKIP is believed to have broken Northern Ireland Assembly rules by using Stormont's Parliament Buildings for its election manifesto launch.