Just what I was thinking Count!.................. Whattie, I'll do it for 9% lol
Unpleasant situation, but all too common. Lack of communication. I've always made it a rule when plans change, to make sure the client is fully aware of possible extras.......... even down to "worst case scenario".
The trouble is, architects, surveyors etc, are notoriously bad at PR (public relations).
It wasn't in the original contract, but you should have been made aware of possible costs once changes to the spec had been made.
It doesn't sound as though anyone is being purposefully naughty .......... just a tad naive.
Balconies, portal frames, room in a roof etc ................. they do take a lot of design and calculation. Impossible for me to comment without the detail, but it could easily be justified.
Any decent PM would be only too glad to get comparison quotes for you. I think that's the way forward.
Good luck with the project.