I agree the chances are that the garage ceiling/bedroom floor does not have any insulation in it and also perhaps neither does the roof space.
Missing insulation in new homes is commonplace and the energy performance certificate is often a tick box desktop exercise.
A thermal imaging camera or a professional thermal imaging survey will confirm this.
Give Paul Buckingham a call.
https://www.sustainablelifestyles.co.uk/ Sustainable Lifestyles Ltd Energy Efficient Healthy Homes
Tel. 07802935827 E-mail paul@sustainablelifestyles.co.uk
Bellway have a lot to answer for here!