Who is your housebuilder?
"Sometimes!"
Air leakage tests get fiddled all the time if the new home fails! You need to get a thermal survey for your home as well as an independent air leakage test.
You will then know where to cold is getting in and the heat is getting out.
With this information and proof you should be able to force the builder/warranty provider to act!
All new homes have minimum temperatures for certain rooms.
Lounge & dining room 21 deg C
Bathroom 22 deg C
Hall, Kitchen, Bedrooms, Toilet 18 deg C
Finally, if the walls are cold I am surprised you haven't got a serious mould problem.
Be sure to position furniture away from walls so the air can circulate.
Next excuse the builder will say it is "caused by your lifestyle" drying clothers indoors, no ventilation, not using extract fans etc.