We are selling 2 places and moving into one larger one.
We've found somewhere and made an offer. It was owned by one lady her whole life and once her kids left home she started letting the rooms to language students and built extra extensions everywhere - 2 downstairs and one upstairs, all plastic roofed, not thick enough walls/insulation. Not sure what the foundations are but doubt those are done properly either. They also go out further than any other extension on the street and no one else has the 1st floor extension they have done. It's a conservation area too.
On the property form the sellers ticked 'no extensions' 
and no issues with building control.
The solicitor has told us that as they are older than 10 yrs it should not be an issue. We would like to rebuild them properly so there is no point getting indemnity insurance as we will be making the planners aware.
We spoke to an architect whose firm also does planning about rebuilding them and they think the council will be fine as it's improving the look of the building. Problem i am concerned about is that the planners say no and then insist we pull them down for not meeting regs/potentially being unsafe in their eyes (despite standing for decades).
No one can tell us if this is a likely outcome or not because there is this 10 year rule but once they are aware, it does appear they can take action but I can't find any criteria about when they do.
Has anyone experienced this?