The short answer
Moving a bathroom, kitchen or laundry means the existing drain almost never suits the new layout. Repositioning it properly involves a new branch, re-grading the run to correct falls, and reconnecting to the boundary trap. Doing that before the slab is poured costs a fraction of doing it after.
The constraint nobody draws on the plan
Renovation plans get drawn around light, views, and where the walls currently are. The drainage rarely gets a mention until someone works out that the new bathroom is nine metres from the existing sewer connection and two hundred millimetres too low to reach it by gravity.
That is not a disaster. It is just a cost, and it is much cheaper to know about it while the plan is still a drawing.
What repositioning a drain actually involves
On a recent drainage renovation the existing drain was repositioned and upgraded to suit a new layout. In practice that means:
Cutting in a new branch where the new fixtures need to connect, rather than running a long shallow line back to wherever the old connection happened to be.
Re-grading the run so the whole thing sits at the correct fall from the furthest fixture down to the boundary. Waste moves by gravity, and it needs enough slope to carry solids along with the water. Too flat and the water runs ahead while the solids stay put. Too steep does the same thing.
Reconnecting to the boundary trap so the system still vents properly. The boundary trap holds a water seal that keeps sewer gas out of the property, and if the reconnection is not made properly the system stops behaving the way it was designed to.
Water testing before backfill. Every run gets tested while it is still exposed. If it does not hold, it comes up now rather than through a finished floor later.
On another job, internal sewer points were relocated to suit a new layout and the boundary trap connection was completed as part of the same work. Same principle: the drainage got moved deliberately rather than extended hopefully.
Why the sequence matters so much
Rough-in happens before sheeting and waterproofing. At that stage, moving an outlet position is a matter of minutes. After the wall is closed, waterproofed and tiled, the same change means opening a finished wet area, which is the single most expensive place in a house to open.
The realistic order on a wet-area renovation:
- Layout confirmed, including where every fixture actually lands
- Drainage assessed against that layout, and re-graded or re-routed as needed
- Rough-in: waste, hot and cold points, appliance points
- Inspection and testing
- Sheeting and waterproofing
- Tiling
- Fit-off: tapware, fixtures, appliances
Step 2 is the one that gets skipped, and it is the one that costs the most to revisit.
Questions worth asking before you commit to a plan
- Where is the existing sewer connection, and what level is it at?
- Can the new fixture positions reach it by gravity, or does something need to change?
- Does the existing run have the fall it needs, or has it already been extended once before?
- Is there a boundary trap, and where?
- What is under the path between the new fixtures and the connection: slab, paving, a driveway, a tree?
That last one is often the real budget item. A drain that has to cross a driveway is a different job to one that crosses a garden bed.
The part worth spending on
Getting the drainage right is not craftsmanship for its own sake. It is the cheapest version of the job, because the alternative is paying someone to open up finished work and do it again. The rough-in stage is the only point at which this is inexpensive, and it lasts about a week.