Use of a crane

OR

Build a tub in place with tiles and backerboard

OR

Find a cast iron tub, have it cut in half to move in, then reweld in place.

Simple! (Not.)

Have it in the garden ?!

Remove the window frame and have it lifted in with a crane...thats what my friend had to do when their super expensive/huge white leather sofa would not go through the door. I joke not!

Clancie might have hit upon the solution. I have seen cranes lift pianos to an upper floor of a brownstone in New York.

When you absolutely, positively , have no other way to get thatoversized Japanese soaking tub into the new addition LOL

Thanks all for the suggestions. Our windows might be a little too narrow for the crane option unless I can find a really narrow tub. Anybody have experience with pouring fiberglass onsite???

Movers have lifts that can simply lift it to the height of your window and then they manually will pull it in. You can ask a moving company.

I was referring to tub, not fiberglass.

I would recommend staying away from 'poured in place' or 'made on site' tub solutions. Never saw one that I liked or that did not develop issues.
If window is too narrow - what a great opportunity to get a new, larger window as part of your remodel.
Good luck!

LOL @ Mega Builders.. you made my afternoon. Sawzall time! LOL