We see roughly 30-50 bathroom reno jobs per year across the Gold Coast, plumbing-only scope or full project management. Most go well. The ones that go badly share a small set of mistakes that are predictable and preventable. Here are the 10 we see most often, what they cost when they happen, and how to avoid them.
This is written for homeowners planning a bathroom reno. The plumber, tiler, waterproofer and joiner should all know these things. The homeowner does not always know, and the cost falls on the homeowner when something is missed.
Mistake 1, budget chrome-plated brass tapware in a coastal home
Bathroom-internal salt exposure is much lower than outdoor, but the cheap chrome plating on budget tapware (often imported, sometimes labelled with Australian brand names but built to thin spec) pits within 5-8 years in coastal homes. The same tap lasts 20+ years inland.
Cost when it happens: $500-2,000 to replace failing tapware mid-life.
How to avoid: spec quality brass tapware (Phoenix Vivid, Sussex Voda, Astra Walker, Gareth Ashton) for coastal bathrooms. Worth the marginal cost for double the life.
Mistake 2, drainage falls cut to the AS3500 minimum
AS3500 minimum drainage fall is 1:60. Some plumbers cut to 1:80 or even 1:100 to make a tight layout work. Initially fine, but over years of soap scum and mineral buildup, the marginal fall becomes a chronic slow drain.
Cost when it happens: $1,200-3,500 to re-set drainage to proper fall in a future reno.
How to avoid: confirm at quote stage that drainage will be set to 1:60 minimum, ideally 1:50 for shower drains.
Mistake 3, waterproofing done by the cheapest trade
Bathroom waterproofing is a separately licensed trade in Queensland for good reason, failures lead to expensive structural damage. The cheapest waterproofer in a bathroom reno quote is rarely the right choice. Look for proper documentation (waterproofing certificates, photos of the membrane during install), and use someone with at least 5 years experience.
Cost when it happens: $5,000-20,000 to rip out and redo a bathroom that failed because of poor waterproofing.
How to avoid: use a waterproofer with track record. We coordinate with several licensed waterproofers we trust. Strata-grade waterproofing (with full documentation) for apartments specifically.
Mistake 4, niche taps on a wall that needs to be removed
Mixer in a tiled niche above the basin (rather than wall-protruding) looks premium. But access for cartridge replacement in 15 years requires removing tiles. Plan for future maintenance access.
Cost when it happens: $800-1,500 to remove and replace tiles to access a niche-mounted mixer for cartridge service.
How to avoid: use traditional wall-mounted or basin-mounted mixers where future serviceability matters. Or accept that niche taps will require tile removal at end of life.
Mistake 5, wrong centres for chosen tapware
Wall-mounted bath spouts and mixers have published centres (the distance between the wall holes for hot and cold supply). These vary by brand and model. Setting out 150mm centres for a tap that needs 130mm centres = the tap doesn't fit. Setting out generic centres when the chosen tap is a specific brand = misalignment.
Cost when it happens: $400-1,500 to re-position pipes after tiling has been done.
How to avoid: finalise tapware selection before plumbing rough-in. Provide the plumber with manufacturer specs for the chosen models. Coordinate with the tiler on fixture set-outs.
Mistake 6, ignored pressure-limiting valve
Mandatory under AS3500-2003. Limits incoming water pressure to safe range (typically 500 kPa). Gold Coast mains pressure runs 600-900 kPa unregulated. Without PLV, every new fitting in your bathroom fails 30-50% faster. Cartridge wear accelerates, mixer life shortens, flexi hose fatigue happens sooner.
Cost when it happens: shortened life on every fixture. Compounds into thousands over the bathroom's life.
How to avoid: confirm PLV is present and operational at the meter. Replace at the 10-year mark, $400-700.
Mistake 7, freestanding bath with no in-wall plumbing planning
Freestanding baths are visually stunning. They also have specific plumbing requirements, the supply and waste come up through the floor at very specific positions. Get the position wrong by 50mm and the bath doesn't sit correctly. Forget to plan a way to isolate the supply for future maintenance and you cannot replace the spout without partial floor demolition.
Cost when it happens: $1,500-5,000 to reposition or rework freestanding bath plumbing.
How to avoid: confirm exact bath model before plumbing rough-in. Plan for an under-floor isolation valve accessible via removable floor panel or service hatch.
Mistake 8, skipped pre-sheet pressure test
Pressure-testing the rough-in before the plasterer closes the walls catches every leak. Skip this step and any leak gets found after tiling, which means tearing out tile and waterproofing to access. Some shortcut plumbers skip the test to save time.
Cost when it happens: $1,500-6,000 to remediate a leak found after tiling.
How to avoid: confirm in writing at quote stage that pressure test is included. Be present during the test if you want to verify.
Mistake 9, no consideration for ventilation
Modern bathrooms with frameless glass showers and limited natural ventilation generate significant humidity. Inadequate ventilation leads to mould, sealant failure, paint failure, and eventually structural damage to substrates. Bathroom ventilation is electrical scope, but plumbing layout affects how easily ventilation can be installed.
Cost when it happens: $2,000-8,000 in mould remediation and finish replacement over 5-10 years.
How to avoid: coordinate exhaust fan location with electrician at rough-in stage. Ducted exhaust to outside (not just to roof void) is the right spec. Run fan during shower use and 15-20 minutes afterward.
Mistake 10, no future-maintenance access planning
Bathrooms with in-wall mixer cisterns, wall-hung toilets with in-wall cisterns, recessed shower mixers, and concealed cisterns all need maintenance access. Plan a service hatch or removable panel during construction. Forgetting to plan = future maintenance requires destruction of walls or tiles.
Cost when it happens: $1,000-4,000 to access concealed plumbing for service.
How to avoid: identify every concealed plumbing component at planning stage. Plan a service panel or hatch for each. Document the location for future plumbers.
Bonus mistake, wrong waste outlet on the bath
Bath wastes come in different sizes (40mm, 50mm) and configurations (bottom outlet, side outlet, overflow). Plumbing the wrong waste position results in a bath that does not fit the planned floor layout. Sounds simple but happens.
Bonus mistake, double vanity with single drain trap
Two basins draining into a single trap looks tidy but if one basin develops a slow drain (hair buildup), both basins are affected and the trap is harder to service. Separate trap per basin is cheaper to maintain.
Bonus mistake, hot water unit installed on bathroom exterior wall in coastal home
Sometimes the bathroom reno includes hot water unit replacement. Mounting the new HWU on the bathroom's exterior wall (especially east-facing in coastal homes) shortens its life by 30-40% versus a sheltered wall. Choose location during the reno.
How to spec a bathroom reno that avoids these mistakes
The pattern across all these mistakes is the same: decisions made by individual trades in isolation, without coordination. The fix is project management that links the trades. Either:
- Engage a builder or bathroom specialist who coordinates the trades
- Engage a project-managing plumber who runs the reno end-to-end (we offer this)
- DIY project management with thorough planning and trade communication
The non-PM approach (engaging trades direct and hoping coordination happens) is where most of the above mistakes originate.
Pre-construction checklist to avoid most mistakes
- Finalise all fixture selections before plumbing rough-in
- Confirm PLV is present and operational
- Brand-specific manufacturer specs provided to plumber
- Drainage fall confirmed in writing at 1:60 minimum
- Waterproofer selected with documentation requirements
- Pre-sheet pressure test confirmed in plumbing scope
- Ventilation specification coordinated with electrician
- Service access planned for every concealed component
- Coastal tapware spec upgraded for salt exposure
- HWU location and replacement timing confirmed if relevant
What we do to prevent these mistakes
Every reno we run goes through pre-construction review covering all the above. We push back at quote stage on any spec we believe will fail. Pre-sheet pressure test is standard, not optional. Coordination with waterproofer and tiler is built into our project workflow. Service access is documented in the as-built drawings.
Bathroom renos are most of what we do. Engage us for plumbing-only with detailed scope, or for full project management. Phone 0472 657 042.
Common questions
How can I tell if my plumbing rough-in was done properly without tearing into walls?+
Is it worth paying more for a project-managed reno vs DIY coordination?+
What is the single most expensive mistake on this list?+
Should I avoid concealed cisterns and in-wall fittings to make future maintenance easier?+
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