The house we're thinking of buying has a balcony area at the front above the hall and built in garage. This is all set back from the principle elevation of the house by 1.2m. If we were to extend the bedroom and landing forwards to take in this balcony, would we need planning permission? The footprint of the house would not change, nor would its height.

Have you seen this? http://www.planningportal.gov.uk/permission/house

You have to get permission for even creating a dormer extension which doesn't change height or footprint, but in a similar way can affect everyone elses view, light etc. So I'm guessing yes, you'll probably need it.

Yes, you'll need permission and also to comply with building regs, so you'll need an engineer report too.

I did the same last year and had to re-enforce my footings.

Generally anything above 2.4metres in height will require planning permission.