Pre-slab is the single most important plumbing decision point on a new build, because almost everything set out at this stage is cast into concrete and lives there for the life of the house. Moving a toilet or a floor waste after the slab is poured means cutting and re-pouring concrete, which is expensive, slow and structurally messy. Get it right before the pour and you never think about it again. We walk every owner-builder and every builder client through this before the concrete truck books in.
Why pre-slab is locked in forever
The drainage for every wet area runs under the slab. Toilet stacks, shower wastes, bath wastes, basin wastes, floor wastes, laundry drains and the sewer line all sit in the ground before the concrete goes on top. Once the slab cures, that pipework is encased. The only way to change a buried drain afterwards is to cut the slab open, modify the pipe, and re-pour. On a finished house with tiling and cabinetry over the top, that is a major job. The lesson is simple, decide before the pour. We cover the full stage sequence in the new build plumbing stages guide.
The decisions that must be locked before the pour
- Every wet-area location. Bathrooms, ensuites, powder rooms, laundry, kitchen, outdoor kitchen, butler's pantry. Each one needs drainage set out under the slab.
- Toilet stack positions. The toilet drain is the least forgiving fixture to move later. Where the pan sits is set by the stack position in the slab.
- Drainage falls. Drains need a minimum fall to run under gravity to the sewer. AS/NZS 3500 sets the minimum grades. The falls are built into the under-slab layout and cannot be adjusted later.
- Floor waste positions. Bathroom and laundry floor wastes are cast in. Their position relative to the shower and the tile fall matters for drainage and for tiling.
- Shower and bath waste positions. Set by the under-slab drainage. A late decision to move a shower means cutting the slab.
- Sewer connection point. Where the house drainage ties into the council sewer main or runs to the septic / AWTS field. This drives the whole drainage layout.
- Water service entry. Where the cold water service comes in from the meter, and where the pressure-limiting valve sits.
- Through-slab penetrations. Every point where a pipe rises through the slab into a wall or floor. These are sleeved before the pour. A missed penetration means core-drilling the cured slab later.
- Gully and inspection-opening positions. Overflow relief gullies and inspection openings need to sit in accessible, code-compliant spots.
The future-proofing decisions worth making now
The cheapest time to rough in for something you might want later is before the slab. If there is any chance of a future ensuite, outdoor shower, pool, second laundry, outdoor kitchen or granny flat, stubbing a capped drain and water point now costs very little. Retrofitting it later costs many times more because the slab has to be cut. We always ask the owner the future-intentions question at pre-slab, because most people have not thought about it and most builders do not ask.
- Future ensuite or bathroom. A capped stub in the right spot saves a slab cut later.
- Outdoor shower or outdoor kitchen. Common Gold Coast additions. Cheap to stub now.
- Pool. Even if the pool is a later project, a conduit and water point roughed in saves trenching across finished landscaping.
- Rainwater tank to internal use. If you might connect a tank to toilets or laundry later, the change-over plumbing is far easier roughed in now.
- Hot water unit relocation room. Make sure the HWU position suits the layout and has space for a future swap to a different type.
What the licensed plumber does at pre-slab
Pre-slab plumbing is licensed, notifiable work in Queensland. Your licensed plumber reads the architectural and hydraulic plans, sets out every drain, marks the through-slab penetrations, lays the drainage to the minimum fall required under AS/NZS 3500, and connects to the sewer main or sets up the run to the septic / AWTS. Before the slab is poured, a council plumbing inspector or the certifier's hydraulic inspection signs off the drainage. The pour cannot legally proceed until that inspection passes. We go into the inspection sequence in what plumbing inspections a new build needs in QLD.
The pre-slab walkthrough we recommend
Our strong recommendation on every build is a pre-slab walkthrough with the plans in hand, the plumber, and the owner present. Half an hour standing on the prepared site, confirming where every toilet, shower, basin and floor waste sits, and asking the future-intentions question, prevents years of regret. Most builders skip it because it costs them half a day. Most owners do not know to ask for it. We invite both, and we hand over a photographic record of every drain run with measurements before the slab goes on. That record is gold for any future renovation, because when you need to know exactly where the toilet stack runs through the slab, the photos show it.
Common pre-slab mistakes we see
- Late layout changes. The owner moves a wall or a bathroom after pre-slab set-out. Every change after the drainage is laid means re-doing under-slab work.
- Skipping the future-intentions conversation. The owner decides on an outdoor kitchen or ensuite a year later and faces a slab cut that could have been a cheap stub.
- Tight floor-waste positions. A floor waste set without coordinating the tile fall leaves water pooling on the finished floor.
- Insufficient fall. A drain laid without the required grade runs slow and blocks. The falls have to be right before the pour.
- Forgotten penetrations. A pipe rise that was not sleeved before the pour means core-drilling the cured slab, which is messy and can hit reinforcement.
Production builds vs custom builds
On a production-style build the layout is fixed by the builder's standard plan, so the pre-slab decisions are largely made for you. There is less room to move, but also less risk of a late change blowing the layout up. On a custom build you have full control, which means you also carry the responsibility to lock the layout before pre-slab. The more custom the build, the more important the pre-slab walkthrough becomes, because there are more decisions and more chances to leave something out.
How much do pre-slab changes cost
Pre-slab work is part of the standard new-build plumbing scope and is quoted as part of the whole job. The cost varies with the size of the house and the number of wet areas, so the honest answer is to get a quote off your plans. What is not cheap is changing pre-slab work after the fact. Cutting a slab to relocate a drain on a finished house can run into several thousand dollars once you count the concrete work, the reinstatement of tiling and cabinetry, and the lost time. The whole point of getting pre-slab right is that it costs nothing extra to do it properly the first time, and a fortune to fix later.
Get your plans reviewed before you book the pour
The best thing any owner-builder or homeowner can do is have a licensed plumber review the hydraulic and architectural plans before the slab is booked. We do this for builder-managed and owner-builder jobs across the Gold Coast. We read the plans, flag anything that will cause a problem, confirm the wet-area locations against how you actually want to live in the house, and make sure nothing future-proofing-wise is being left out. For the full picture of how a new build flows from pre-slab to handover, read our new build plumbing guide, see our new build plumbing service, or get in touch and we will go over your plans before the concrete books in.