I've just signed a contract on a highset property similar to that described here. Downstairs is built in and advertised as a rumpus, study etc, so habitable.

The agent told us (without us asking) that the ceiling was below legal height downstairs, and that most highsets in the area are similar.
Our solicitor then expressed concern that the council could kick up a stink if we were to a) use the downstairs area "habitably", or in the future, sold the house advertising it as habitable. Possibly even fine us, or force us to rectify the issue.

Note that we have our building inspection next week and we'll ask that question specifically and get them to verify whether it is or isn't legal height.

I guess the biggest concerns are:
– will we be liable for a fine?
– will we be facing a battle to sell down the track, ie not being able to advertise the downstairs rumpus/study/living/bathroom/kitchen etc?
– is it possible to fix, and how much is it going to cost?