5 bedroom Cottage but 2 bedrooms are not habitable ?

[Doggit]

They could have lost the documentation, but sorry, there's no way that building control would ever have signed that off; you can't cut the purlins, and not support them, and you couldn't support them without increasing the height of the ceiling joists or the floor. They may well have been used as bedrooms for the servants, but you'd need to find evidence of existing stairs etc that show they were were installed when the house was built, plus there would have been windows as well, so look for evidence of them.

But once they started putting in the dormers, and cutting the purlins, they would have needed to apply for both planning permission, and a building control notice, so have a chat with the local council, and just quietly ask them if any work has been applied for, hopefully without raising too many suspicions.

Now you may be getting a brush off from your solicitors, as they should have asked about any building works, and then got the documentation to support it, (not just taken the vendors word for it) - which they don't seem to have done. They can't tell you that they may well have been used as maids bedrooms, as that's not their job; it's to ascertain that everything has been done properly, and that you aren't left with a lemon. You may need to use another solicitor against them, which won't be easy.