I have a difficulty with the idea that its only now this issue has come to light.
Its pretty basic stuff - you apply for, and get, Planning Permission. Then you build in accordance with the Planning Permission and with Building Regulations.
If this was a self build, you'd have been hands on throughout the process.
If it was a more hands off approach, you'd still have been watching the building and been engaging with the process.
Until you can be a little more giving with the facts, no-one can tell you if the builder and engineer are liable although I'd hazard a guess that they're not.
I'm also pretty sure that you now know to what extent the house is unsaleable and whether an application for retention permission is called for.
mf