I hate new builds. They ought to be good but...
So your house needs an open vent. Basically.
Normally, ICs will be something like this:
View attachment 62437
The red line is the sewer, the blue ones is each house's soil stack to the nearest IC, and there will be a vent either at green dot at the head (house 3) or a separate vent pipe to the IC (yellow). My terminology may not be exact, but you get the idea. I'm showing conventional 600x450mm IC covers, but yours may be small plastic round things as it's a new property. Basically, there will/should be access wherever there is a branch or abrupt change in direction.
It was suggested a HepVo valve could be fitted under your sink in series with the existing trap. This will certainly work if the issue is pressure developing and forcing through the trap. It's a one-way valve. They are expensive but work well. It may shift the problem elsewhere though (possibily to a neighbour...)
What if you find your IC and lift the lid slightly to allow some air to exit? Make sure to cover with a wire mesh though (rats...). This should have a similar effect to the yellow pipe in the above drawing. It's not a permanent solution and it will make the immediate vicinity of the garden smelly, but if it means your house doesn't smell of drains - which, long term, isn't going to be that good for you - it may just keep you sane while you arrange a permanent solution (and may be a good way of convincing your idiot builder that he needs to vent the head of the drain... which, by the way, is in the level 2 plumbing course, so not exactly a difficult concept).