Purely hypothetical, but what happens when a leasehold building becomes uninhabitable?

General Discussion

I started thinking about this after a discussion over how long to extend a lease to.
Using mine as an example, it's a flat in a 1930s house conversion. There's 70 years left on the lease. Let's assume that in 20 years the mortgage will be paid and it'll wholly belong to me. Let's also assume that in 25 years the Gov't introduces further legislation that means it becomes legally uninhabitable, yet I still have 50 years left on my lease. The only solution is to knock the whole building down and rebuild.

Who pays for the rebuilding (which includes 3 other flats)?
What say do I have in the design and layout, such as "I'd like mine to be 20% larger than now".
Would there be any advantage to me extending the lease now, when the building will not last to the end of my present lease?

Interested to read your thoughts....it must have already happened to several already.
PtP