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Welcome to the Bunnings Workshop community @GrantT. It's wonderful to have you join us, and many thanks for your question about installing a bath.
Has the bath plumbing already been installed, or are you able to remove the shell? The most cost-effective and straightforward solution would probably be to knock up a simple timber frame that sits under that section. I've added a rendering below to illustrate. Here's a handy step-by-step guide on How to build a bath frame that gives you a more comprehensive look.
Running another course of bricks is an option, but you'll need to remove the bath to do so. Here's another guide on How to build a brick wall. I suspect that if you look at the bath's installation guide, it would likely recommend that the bath be supported under all edges, so any filling of this gap needs to happen from underneath before being tiled over.
Let me mention one of our knowledgeable members @RSS to see if they have any advice.
Please let me know if you have any questions.
Mitchell
Thanks Mitch
Decided to box it in
Sounds good, @GrantT. It would be great if you could post some images of your solution, as they'll assist any of our members experiencing a similar issue.
Mitchell
@GrantT, @MitchellMc has recommended what I would do. The only other thing I would stress, is the waterproofing of that area. Water seeping under or pooling around this area will rot timber and cause mould quickly. Long term, you may run into other structural issues from poor waterproofing.
If the tub isn't supported correctly, it might move/twist, crack/break the tub itself and any plumbing connected to it. Also, the waterproofing may be affected if to much movement occurs.
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