Hi
We've recently had a small extension downstairs. The builders say they did not touch the main drain pipe that runs down the wall outside the house from the toilet and sinks, and have just boxed it in with mdf but whenever the toilet is used a really strong sewer smell comes into the house form the attic, does anyone have any idea what this might be? Thanks

Sewer drain and vent outside the house, must be European. Does the piping extend up above the roof line? Is there an attic vent nearby? Maybe the addition changed air flow and sewer gas being drawn back into attic through a vent.

There is a chance that your builder didn't extend your plumbing vent above the roof. As a result, your sewer gas vents into your attic. It should be easy to trace and easy to correct. Just extend your existing (short) vent through the roof.