Cutting a new doorway into garage

[broken biscuit]

We are exchanging this week on our new place. It's a 3 storey townhouse with internal garage. Unfortunately there is no door from the garage into the house. Therefore, one of my first jobs / projects will be to get a doorway cut.

I understand it will have to be a 'fire door', and that it will obviously require a lintel cutting into it. The wall is block (from the garage side at least) and I 'think' the wall is structural, running all the way up to the roof, rather than just an internal wall for the ground floor.

Will I need to acro prop the floors and wall to both sides of the doorway? I have someone who will do it (father in law, pretty handy at most DIY) but he seems to think it will require a mass of supports etc to brace the wall while the hole is cut and lintel goes in, whereas I am not sure.

Any ideas what getting someone in to do the job would be likely to cost? I am perfectly capable of fitting a door frame and hanging a door, I just dont fancy cutting holes and having walls collapse!