Had a extension built and now can smell a horrible sewage smell

Drain pipes and sewer drains should be separate. the Builder should of redirected the new drain, the old drain should of been removed and a new drain connected, the drain pipe should connect to a P-Trap or Soak away a very simple job to carry out this may seem obvious, but the 2 main rules are, when changing or adding drains they must run downhill (fall) it only needs to a slight fall but must be continuous, poor installed guttering will become full and overflow. Not having drain done correctly will lead to overflowing, blockages and nasty smells, U bends, back in let gulleys, P Traps are designed to stop smells coming out from the drain as the water that keeps in the Bend (trap) prevents smells

When the extension was built was the local building control officer involved ?? They should’ve checked all works completed including the drains.... Were the drains replaced when your builder built over them?? They should have been ...