If I could get some thoughts on the below that would be good:

I'm in the process of buying a terraced house where the owners have
knocked through the lounge/dining room to create one large space.
Doesn't look like Building Regs approval was sought, my building
survey noted this should be checked etc.

Looks like the thorugh lounge space was created 25+ years ago by the
owner, looks like an RSJ has been used but I'm not in a position to
know if it's supported correctly or the beam is the correct size.

Would be council grant this retrospectively, or is this just planning
permission?

How would this be handled the seller got a structural engineer to
confirm its was good for 20 years ago but not for today?

Any thoughts/opinions would be appreciated.

I will give the Building Controls office of my council a call tomorrow
to see what they say.