My house is 18 months old and there is a smell of sewage all the time in the utility and in the main bathroom. There are 4 ensuites and only one soil pipe in the centre of the house. I want to know what can be causing this

All of your vents have been tied back in the attic to one main vent that's run out the roof. Check the attic to see if one has been left open. Are the main bathroom and the utility room close or back to back? Is there a floor drain in the utility room?
Let me know, Tom

Did you trap all plumbing fixtures ? Do you have any floor drains ? If positive: is this drain primed ?