Consider the whole building envelope
A comfortable extension needs coordinated floor, wall and roof construction. Connections to the existing house, frames and foundations matter just as much as insulation within the wall.
Ask for project-specific performance figures and calculations. One insulation value does not describe the whole room. Requirements and performance must match the design and chosen construction.
Fresh air is part of comfort
A well-insulated room also needs a suitable ventilation plan. Consider use, occupancy and existing ventilation. An opening window alone does not solve every situation.
Think about warm days too
Extensive glazing can heat a sunny room significantly. Discuss orientation, glazing, shading and heat removal during design, when changes are easier than after finishing.
Match the heating to the house
Whether underfloor heating or a radiator is suitable depends on heat loss, the existing system and floor construction. Have the required capacity assessed before making a final choice.
Ask about performance at the junctions
Ask how the new and existing floors meet and how roof edges and frames are sealed. Drawings and installation must agree at material junctions. Record who will commission ventilation and heating at handover. The insulation package alone does not show whether the new room will have adequate ventilation and even heating.