Building and Planning regs??

[Runningman]

I am considering buying a flat which was modified about 6 years ago. It is a freestanding freehold property attached to a semi-detached house. Originally it was all on the first floor with access via exterior brick steps.
It also had an underpass to the rear garden.
About 6 years ago six feet of the underpass was walled off to provide a hallway, and interior stairs were installed through to the first floor. In addition a front door was installed for access to the newly created hall.
The original entrance door at the top of the brick steps was bricked up.
The problem is I'm fairly sure no planning or building regs were applied for, although it looks like a good solid job was done.
What I need to know is -
1. Is there a time limit for planning and building regs, after which as an owner of a property you are not going to be persued by the planners etc.?
2. If I bought the property would here be any issues with selling it on at some point?

Any advice would be most welcome.