A thermal bridge is a location where heat passes through a building assembly more readily than through surrounding areas. Material changes, geometry and junctions can contribute. Its implications need assessment within the complete construction rather than as an isolated surface feature.
How this term relates to your project
Floor edges, roof junctions and frame perimeters deserve attention in an extension. A well-insulated main surface may still have a weaker connection. Ask how the insulation line continues and how crossing components are detailed. A cold-feeling patch does not establish the cause by itself; air leakage or other conditions may also contribute.
Discuss observations with the relevant specialist rather than simply adding material to the visible surface. The response should suit structure, moisture behaviour and use. Photographs taken before finishes are closed help document installed junctions and support later investigation if the completed room behaves differently from expectations.