tolerance

I have received a stone resin shower tray and as best as I can measure, one corner is 3 mm lower than the other three (across the top of the tray edge) and I don’t know whether this is an issue or not? Are these things normally completely flat, or is it the norm that they are out of flatness by...

I'm converting a car port which used to house the front door (with side light) so there will now need to be a standard 30" internal door there, but there also needs be a step down in to the new room as levels differ. Is a standard 32" door frame that includes a (preferably hardwood) threshold...