OP Most posters will already know what I am saying below, but you say that this is your first house purchase. So a few basics:
To the best of my knowledge, an indemnity policy won't help you very much. All they usually do is protect you from Council prosecution for going against their rules. The indemity insurers won't pay for new/remedial building work, for example. hoa.org.uk/advice/guides-for-homeowners/i-am-buying/what-is-indemnity-insurance/
You can ask Council Building Control (usually linked to Planning Department) to come round and inspect the attic. If they think that it meets Building Regulations for rooms that are to be lived/slept in they can issue a certificate saying so. That's what previous posters mean by 'sign off'.
(Building Regs cover things like roof insulation, strength of loft-floor joists, ceiling height, size and position of windows (for fire escape), width and slope and headroom and position of stairs, firedoors and landing space at top or bottom of stairs, etc etc.
Building Control will have a published scale of fees on the Council website - I'd expect a few hundred pounds.)
But an attic is not a room to be used for living/sleeping. It's just storage space.
And as other posters have said, that's how the house is being advertised. The sellers are making it clear that the attic and dormer does not meet Building Regulations as a living/sleeping room,. They are doing nothing wrong.
However, if you choose use the attic as living or sleeping space, you might find that it invalidates your household building insurance.
If you are worried about the quality of the building work, you need to ask a surveyor whether the attic and dormer are structurally safe. If you've already had a survey done, and the surveyor raised no issues, then the attic is very probably safe as an attic but not as a living/sleeping room.
Is the dormer at the back or the front of the builing? If at the front, in some (not all) areas, planning permission might be required. But only your local planners can tell you that. Most council planners have a great deal of useful info on their websites. But you can also pay for a quick consultation; again, their website will tell you how to contact them.