Smell from ensuite shower drain.

New build property. > We moved in in October and have had sewage smell in our ensuite ever since. I suspect the waste from the toilet is backing up into the shower waste as they are within 20cm of each other and the toilet is the first on the pipe run. > they have connected the sink/bath/shower in line and then to the toilet soil pipe inside the house . > I have put air intake valves on the bath and sink waste trying to stop the vacuum from the toilet waste emptying the traps . > > I wonder if you could confirm that the builders were correct to connect everything together inside the property? And if this is inline with building regulations. > > The plumber the builder uses is not someone I have much confidence in having had our new house flooded twice since we moved in because of his poor workmanship so if I can I would prefer to sort the matter independently if possible. > > My understanding is that the only way to resolve the issue properly is to separate the bath/sink/shower waste from the toilet and run it through the external wall separately. > > Your advice would be greatly appreciated as the smell is driving us mad.