The last comment was from andi and it might be your problem: He wrote: "I know there are all sorts of possibilities, but here is a possible simple one. (We had this problem). Check the beeswax seal around the toilet. Sometimes the smell can come through if the seal is old and needs replacing."
Hopefully you mentioned this to your builder before your one year warantee expired or that he will agree to fix it because it is a defect. This situation is not acceptable and you and your family might suffer ill health from sewer gas. Remember, one large component of sewer gas can be methane which can go BOOM and which can also cause symptoms similar to carbon monoxide poisoning. Also, you will end up breathing in fumes from whater else you might have dumped into your drains which can compound the health risks.