I have changed my back bedroom into a bathroom and vise versa but the joist in the original bathroom were not doubled up although it was a smaller floor space, I would say the volume of the jacuzzi is about the same size as two bath tubs together and these new houses have nothing to them studded...
i have just had a double jacuzzi installed upstairs i estimate that it weighs around 100kg empty and again estimate it holds about 90-100 gallon of water then 2 people 100-120kg
my house is a new build built about 6-7 years ago
the joists under the floor boards are 8"deep x2"wide about 10"...