It sounds like you'll be creating a wet bath. How will you be protecting the structure from water? Tile and grout alone won't keep water from getting into adjacent spaces. Do you have permits in place to add the additional fixtures?

That's a powder room, not a bath. 30'' for the toilet and 30'' for a code compliant vanity with the required 15'' from center to wall leaves zero room for any shower. Even a super small pedestal sink wouldn't comply with the required clearances. The math does not work. No municipality would give a permit for this. Adding this unpermitted kludge will actually devalue the home.

Where I live we do not have a lot of the codes you are quoting so I don't think this is everywhere. Like we have no codes for vanities or distance from centers they don't even make sense. If they have permits this is probably the case where they live,

The minimum distance from the centerline of the lavatory to a wall is 15". (IPC 405.3.1)

The minimum distance between a wall and the edge of a free standing or wall-hung lavatory is 4". (IRC R 307.2)

These are internationally adopted building codes. You may want to read up on the required spacing of other fixtures as well. Here's a good illustrated link that shows this graphically so it's easy to understand.Starcraft Builders Rules of Good Bathroom Design