I have quite a large bathroom which currently has a stupidly small bath that I cannot fit in. I bought a spa bath to replace it thinking that I could just remove the old bath and replace it with the new one. However, I just had a builder come and tell me that I have to remove absolutely everything from the bathroom including all the floor tiles in order to fully waterproof and start from scratch. We do not need a new vanity or shower, they are perfectly good.

What I am wondering is, since the footprint of the new bath is bigger than the old one, can we not remove the old bath and the wall tiles and waterproof the area including up and over the existing floor tiles up to where the edge of the new bath frame would go? Then when the new bath has it's side tiles put on, the side and floor join will be sealed again. And/or would some sort of bath flashing help? Is it possible or is the buider correct?

Would this fit the Qld legislation for waterproofing?

Cupboard in piccie is freestanding.

Thanks in advance.