Raising to legal height - how much value would it add?

We've got a 2 bedroom house in Paddington, Brisbane. It has been raised and built under but was NOT raised to legal height. So the downstairs rooms used for bedroom, lounge etc are technically only storage areas.

I'd appreciate any comments or feedback on the following questions.

1. Can you re-raise a house that already been built under, or are you effectively having to demolish the downstairs and start again?

2. What sort of increase in value would there be from raising to legal height and making it officially a 3 bedroom house? Would it be worth doing?

You can raise a house that has already been raised, but that would mean demolishing the outside walls and starting again once raised. You should already have the right steel under from the first lift. For a good view, or if the end value would be worth the cost, it could be viable.

I would be asking a few local agents "what it is worth now with two "storage" rooms downstairs" and "how much would it be worth if it was legal height"?

Depending on how much extra value the finished house would be, you can then judge whether it is worth doing.
What is the ceiling height? The minimum habitable height is 2.4m in the building code of australia, but if it is under by a small amount a certifier might consider accepting it as an 'alternative solution' to the BCA. I mean something like 2.3 or 2.35m.

You would need to get a building approval from a certifier to convert the existing non-habitable area to a habitable area to do this. You would also need to confirm that the slab was built as per AS2870, i.e. adequate reinforcement and vapour barriers. Did you have any plans at the time when the slab was poured? If not you might have a hard time convincing the certifier the slab will be ok. In which case you'd need an engineer's certificate to state the slab is adequate.

Anyway I didn't really answer your question about does this add value, rather how you would go about getting it legal without raising.
Our son tried the avenue of having the slab certified to make their downstairs room classed as legal height, but without any information about the slab, this ran quickly into a dead end. Nobody was prepared to sign off on a slab they didn't have any information about.