Top 5 Estimating Mistakes Plumbers Make (And How to Avoid Them)

Under-quoting or missing critical items is the fastest way to turn a profitable job into a loss. Here are the top 5 estimating mistakes we see most often in Australian plumbing jobs — with exact fixes from the PCA and AS/NZS 3500 series to keep your quotes accurate and compliant.

01Under-Quoting TMVs / Tempering Valves in Class 3 & NDIS Jobs

Mistake: Quoting standard tempering valves (50 °C) for all bathrooms in Class 3 dwellings (group homes, NDIS SDA, supported accommodation).

Why it hurts: the delivery limit for sanitary fixtures used for personal hygiene purposes is set by AS/NZS 3500.4:2025 Clause 1.11.1, and it is tighter for Class 3 buildings than for an ordinary house. Meeting the tighter limit means high-performance thermostatic mixing valves to AS 4032.3, not ordinary tempering valves. Confirm the figure for the building class you are pricing.

Fix: Allow 1 high-performance TMV per bathroom (not just accessible ones). Reece prices: $200–$500 each + commissioning ($200–$500 job total). Budget $1,000–$2,500 extra for 4–5 bathrooms.

02Forgetting Overflow Relief Gullies (ORG)

Mistake: Omitting the ORG on sewer-connected properties.

Why it hurts: AS/NZS 3500.2:2025 Clause 4.6.6 mandates at least one external ORG on every gravity sewer connection — spill level 150 mm below lowest fixture, 75 mm above ground. No ORG = major defect, council rejection, sewage backup inside.

Fix: Always include 1 ORG (e.g., Reece trap + riser + grate, $200–$600 total). Check Sydney Water/council approval — they require it for new/altered connections.

03Missing Fire-Rated Penetrations in Multi-Storey or Commercial Jobs

Mistake: No allowance for fire collars/wraps/mastic on pipes through fire-rated slabs/walls.

Why it hurts: NCC Volume One Specification 13 requires maintaining the FRL (e.g., -/120/90). Plastic pipes need collars; metal pipes need wraps + mastic. Missing = major defect at handover.

Fix: Plastic → 1 collar underside slab (e.g., Trafalgar FyreCOLLAR, Reece 3210044); walls → 1–2 collars. Metal → TWRAP roll (3210034) + mastic. Allow $150–$600 per penetration (material + labour).

04Wrong Hot Water Sizing (Especially in Large Homes)

Mistake: Quoting 250–315 L storage for 4–5 bathroom homes.

Why it hurts: AS/NZS 3500.4:2025 Table 3.1 suggests 315–400 L storage, but peak simultaneous demand (3 showers + bath + kitchen/laundry) can exceed tank capacity → cold showers & complaints.

Fix: Use one 32 L/min continuous flow gas (e.g., Rinnai Infinity 32) for unlimited hot water. Better for long runs/high demand than storage. Add recirculation if needed. Allow $3,500–$6,000 (unit + install).

05Skipping Isolation Valves (AS/NZS 3500.1 Clause 5.4)

Mistake: Omitting isolation valves, or pricing only the fixture ones.

Why it hurts: AS/NZS 3500.1:2025 Clause 5.4 requires accessible isolating valves in eleven locations, including several that quotes routinely miss: each pressure-limiting or pressure-reduction valve, each pumping apparatus, each storage tank inlet, each tank outlet over 50 L, each irrigation offtake, and immediately before every flexible hose assembly connected to a mixer valve, tap outlet or cistern. Fire services need four more under Clause 5.4.3. No valves = callbacks when servicing or leaks occur.

Fix: Count valves from Clause 5.4.2 rather than per bathroom. Every flexible hose to a mixer, tap or cistern needs one, and so does every TMV, pressure-limiting valve, tank inlet and pump. Quarter-turn DR brass runs about $20 to $60 each. On a four-bathroom house with the plant items included, allow $800 to $2,000 in materials.

06Tip for Plumbing Estimating

These five mistakes cost plumbers thousands in rework, delays, and lost reputation. Always cross-check against the latest PCA and AS/NZS 3500 2025 editions — and build a buffer for compliance items.

At SNZ Plumbing Estimating, we include every required item — TMVs, ORGs, fire collars, correct hot water sizing, and isolation valves — so your quotes are accurate and profitable.

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07Related reading

• Isolation Valves: Where They’re Mandatory & Why Quarter-Turn Ball Valves Win
• NDIS SDA Plumbing Requirements – Class 3 Dwellings 2025 Guide
• How to Size Hot Water Systems Correctly – Avoid Cold Showers in Large Homes
• Top Factors Driving Up Plumbing Estimates in 2025 (And How Homeowners Can Save)

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