Correct lay out is bath then basin at end of bath. So waste could run under bath but would then need a small boxing after the bath to the basin. So guessing under the floor would be neater?
Under floor would be neater, as mentioned though, run the pipe parallel with joists, do not notch and run 40mm pipe across the joists. Recommend you use an anti vac or waterless trap on the basin to avoid the bath pulling on the trap.
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