Hi. Not sure if anyone will see this but I was searching around for this exact topic and found this so felt it my duty to respond with my findings
I have a new build (~2018), north and north/east facing rooms/bathrooms have always been cold, like ice-boxes in the winter.
After just making do and then going through covid, this year I'd had enough and thought this cannot be right! After reading through the replies here and asking around I was about to pay for someone to come round to inspect our wall insulation. I thought I'll just go into the roof first to see if I can see anything obvious..... well, I noticed a few gaps in the roof insulation and some insulation just randomly lose on top of another covered area. I simply "fluffed up" here and there and moved the lose bits to cover gaps. Honestly I'm amazed at the difference. I've bitten the bullet and bought a Hive thermostat and a few smart radiator valves to put into those cold rooms and to have easier control over adding heat to those rooms when needed (i.e. guests), but just simply fluffing up in the roof has already made a huge difference and I'm amazed.
So - if you have a new build and its passed building regs, aside from a major screw up I'd be surprised if there was anything majorly wrong with your wall insulation. If a room is north facing, and has a window on the north and east sides like mine then it will feel colder than other rooms. BUT, I would recommend checking in your roof before forking out for any inspections etc.
I've gone extra in going for smart heating but that is a personal choice and technically not completely required. I'm hoping though it makes putting the heating on more efficient and I will have easier control of what comes on when and to what temp.
That's my two pennies on this subject - hope it helps someone
