The call to the builder will be a waste of time I would think. The house may be 3 years old but the regulations that apply to it are the regulations in force at the time of application to building control.
I’m taking NI regs here but I believe the Scottish regs to be the same. Prior to 2014 the domestic heating compliance guide was the document used for guidance. In it a two zone system could be used and no need for TRVs in other rooms. In a house under 150m2 TRVs can be used along with a single zone.
The domestic building services compliance guide is the current guidance, each jurisdiction will have introduced it on a different date. Using it all rooms should have TRVs except reference rooms (those with room stats) houses under 150m2 are ok with one zone. Larger houses need two.