From as early as 1259, the state openings of parliamentary occasions were held in the King's private apartment at Westminster, the Painted Chamber.
Simon de Montfort
It is not clear where Parliament under Simon de Montfort met in 1265 when they got to Westminster. It may have met in St Paul's for some of its debates and discussions, or used the Chapter House of Westminster Abbey.
The Westminster parliaments of Edward I sat in the Painted Chamber, or in the adjacent White Chamber (which eventually became the House of Lords), or sometimes in the Chapter House or Refectory at Westminster Abbey.