⚡ TL;DR — Quick Summary
- “Fixed price” contracts in WA still legally allow the final price to rise through Prime Cost (PC) items and Provisional Sums (PS).
- PC items are allowances for products you haven’t chosen yet (tapware, tiles, appliances); you pay the difference when you upgrade.
- Provisional Sums cover work that can’t be fully scoped at signing, with site works being the most common blowout.
- The Home Building Contracts Act 1991 bans rise-and-fall clauses but explicitly permits PC and PS estimates.
- Builders must not understate allowances, but “reasonably cost” is open to interpretation and often set to the cheapest option on the market.
- The homeowners who finish on budget are those who understood what was actually fixed before they signed.
“Fixed price contract” is a very reassuring phrase in the Perth home building market. It implies certainty — you sign at a number, you build to that number, you settle at that number. For most homeowners, it’s a major reason for choosing a project builder.
The reality is more complicated, unfortunately. Almost every fixed price contract in WA contains two categories of allowance: Prime Cost (PC) items and Provisional Sums (PS), which legally let the final price climb above the contracted figure. Sometimes by a few thousand dollars. Sometimes by tens of thousands.
This isn’t a scam. It’s how the system is designed to work. But it’s also where most Perth cost blowouts happen, and most homeowners don’t fully understand the mechanism until they’re already paying for it.
What “Fixed Price” Actually Means in WA
In Western Australia, residential building contracts are governed by the Home Building Contracts Act 1991. The Act bans “rise and fall” clauses. This means your builder cannot pass on general material or labour price increases mid-build. That part of the price genuinely is fixed.
What’s not fixed is the cost of items and work that weren’t fully defined at the time of signing. The Act explicitly allows builders to include PC items and Provisional Sums as estimates, with the final cost adjusted up (or down) once the actual selection or work is complete.
Important: A more accurate description of a “fixed price” contract is this: the labour, the structure, and the standard inclusions are fixed. The estimated allowances are not.
Related: Fixed Price Builds: The Key to a Stress-Free Build | A Guide to Home Building in Western Australia
Prime Cost Items (The Selection Allowances)
A Prime Cost item is a dollar allowance included in your contract for products you haven’t yet chosen, such as tapware, door hardware, light fittings, kitchen appliances, floor tiles, and carpets. The builder estimates a figure, includes it in the contract sum, and you make the actual selection later, usually at a colour and electrical appointment.
🕑 Common PC Items in a Perth Build
🚿 Tapware
Bathroom and kitchen taps, mixers, shower roses
💡 Light Fittings
All internal and external light fixtures
🍳 Kitchen Appliances
Oven, cooktop, rangehood, dishwasher
🏠 Floor Coverings
Tiles, carpet, timber or vinyl flooring
🚪 Door Hardware
Handles, hinges, locks, and entry sets
Here’s how the maths plays out. Your contract has a $500 PC allowance for kitchen tapware. You walk into the supplier showroom, fall in love with a $1,200 mixer, and a variation is processed. You pay the $700 difference, plus, in many cases, a builder’s margin on top of that difference, which can add another 15–25%.
⚠️ The PC Variance Maths
Contract Tapware Allowance
$500 PCThe figure included in your signed contract.
Your Actual Selection
$1,200 mixerA reasonable, mid-range choice at the showroom.
What You Actually Pay
$700+ diff +15–25% marginThe gap plus the builder’s margin on the variation.
Multiply this across tapware, light fittings, tiles, appliances, and floor coverings. A $5,000 cumulative PC variance can become a $7,500 hit to your final price. None of it breaches the contract.
Provisional Sums (The Unknown Works)
Provisional Sums work the same way conceptually but cover work rather than products. They’re used for items the builder genuinely can’t quote accurately at signing because the scope isn’t fully defined.
🔨 Common Provisional Sums in a Perth Build
🏗️ Site Works
Excavation, retaining, sand pad, drainage — the most common PS blowout in Perth
⚡ Service Connections
Power, water, and sewer connections from the street
🌳 Tree Removal
Removal of existing trees on the block, including stumps
🛣️ Driveways & Crossovers
Often excluded or provisionally summed, rarely fully fixed
🏚️ Demolition
Knock-down of existing structure if applicable
The most common Provisional Sum in a Perth build is site works. Your block looks clean and sandy on the surface. Site works are budgeted at $18,000. Excavation begins, and the crew hits a layer of limestone reef that needs hammering out, plus a buried tree stump from a previous demolition. The actual site works come in at $34,000. That $16,000 gap is added to your contract price as a variation. Legally compliant, contractually permitted, and entirely on you.
Warning: If a soil test hasn’t been done before signing, any site works Provisional Sum is essentially a guess. Always ask your builder whether a soil test has been completed and how the PS estimate was derived.
Related: Home Building Expenses Over Budget | Steps to Building a House in Perth
The Legal Protection You Probably Didn’t Know You Had
Under the Home Building Contracts Act 1991, a builder must estimate PC items and Provisional Sums at or above the lowest amount these items could reasonably cost. The Act explicitly states that the cost must not be understated.
In plain language: a builder can’t deliberately lowball allowances to make the headline contract figure look more competitive. If they do, that’s a contractual breach you can take to Building and Energy (formerly the Building Commission).
What the Law Says
Allowances must be estimated at or above the lowest amount these items could reasonably cost. Understating is a contractual breach.
What “Reasonably” Means in Practice
Open to interpretation. A $440 PC for an entire bathroom’s tapware may technically comply while pointing to the cheapest fixtures on the market.
The Practical Gap
An allowance can be technically compliant and practically misleading at the same time. This is exactly where independent eyes on the contract make the difference.
Where Cost Blowouts Actually Come From
In our experience reviewing Perth building contracts as brokers, cost overruns usually fall into four predictable patterns.
📈 The Four Cost Blowout Patterns
🪟 Underweight tile and tapware allowances. Builders quote to a price point that no real homeowner shops at. The PC item complies with the Act, but everyone in the industry knows you’ll exceed it.
🌏 Site works estimated for the best case. A standard sand pad is assumed for a block that hasn’t been properly tested. When ground conditions differ, the variation lands.
💸 Margin stacked on variations. Some builders apply their full markup to the difference between the PC allowance and the final cost. Others apply it to the entire upgraded amount. The contract usually permits this, but it’s rarely highlighted at signing.
📋 Items absent from the contract entirely. Driveways, fencing, landscaping, retaining, flyscreens, clotheslines, letterboxes: none of these may appear in your “fixed price” at all. If they’re not listed, they’re not included.
Related: Risks of Cheap Builders in Perth | Volume Builders vs Custom Builders Perth
How to Read a Perth Building Contract Honestly
Before you sign, work through these checks:
📋 Pre-Signing Contract Checklist
Phase 1: Audit the Allowances
Find every PC item and write down the allowance. Then go to a supplier showroom and price what you’d actually pick. The gap between the two is your real upgrade budget.
Identify every Provisional Sum. Ask how the estimate was derived. For site works, ask whether a soil test has been done; if not, the figure is essentially a guess.
Phase 2: Check the Margin and Scope
Check the variation margin. What percentage does the builder add on top of PC and PS variations? It will be in the contract, often 15–22%.
Cross-check inclusions and exclusions. Make a list of what’s not in the contract. Assume nothing is included unless it’s explicitly listed.
Phase 3: Ask the Right Question
Ask for the realistic completion price, not the contract price. A good builder will give you an honest answer. A nervous one won’t.
Related: Questions to Ask Builders Before Signing a Contract | How to Choose a Builder in Perth
How a Building Broker Exposes the Real Number
📞 When to Get a Broker to Review Your Contract
- Your builder can’t tell you how the site works Provisional Sum was derived
- The combined PC allowances add up to less than $10,000 for an entire home
- The contract doesn’t mention driveways, fencing, or landscaping anywhere
- You’ve been quoted a variation margin above 20%
- You’re comparing two contracts and can’t work out why one is cheaper
- You’ve never seen a Perth building contract before
At Better Way 2 Build, we read fixed price contracts the way an experienced industry buyer reads them: looking for the underweight allowances, the missing scope, the variation margins, and the Provisional Sums most likely to blow out.
We benchmark PC items across our panel of builders, so we can tell you when a tapware or tiling allowance is genuinely tight versus genuinely realistic. We flag site works estimates that don’t reflect the actual block. We make sure inclusions and exclusions are itemised in plain English before you commit. Because our service costs you nothing, there’s no penalty for getting experienced eyes on the contract before you sign.
Related: What Is a Building Broker and Why Should You Use One | What We Do at Better Way 2 Build
The Honest Definition of Fixed Price
A genuinely fair fixed price contract isn’t one with no PC items or Provisional Sums; those are sometimes unavoidable. A fair contract is one where the allowances are realistic, the variation margin is reasonable, and the scope is fully transparent.
Most contracts you’ll see in Perth are technically legal but practically optimistic. The headline price gets you in the door. The real price emerges over the next six to twelve months as PC items get selected and Provisional Sums get reconciled.
✅ What a Fair Fixed Price Contract Looks Like
✅ Realistic PC allowances — benchmarked against what homeowners actually select, not the cheapest spec product on the market.
✅ Evidence-based Provisional Sums — derived from a soil test or site assessment, not a best-case assumption.
✅ Reasonable variation margin — clearly stated, not buried, and applied consistently.
❌ No PC items or PS at all — not realistic for most Perth builds. Treat claims of a 100% fixed price with caution unless every item is fully specified.
The homeowners who finish on budget aren’t the ones who signed the cheapest contract. They’re the ones who understood what was actually fixed before they signed.
Better Way 2 Build — Perth’s Building Brokers
We review fixed price contracts before you sign, benchmark PC allowances, flag Provisional Sum risks, and make sure you know what your build will actually cost. Our service costs you nothing.
Serving Perth Metro | First Home Buyers | Investment Properties | Custom Homes
If you’d like an experienced broker to walk through a contract with you before you commit, book a free consultation. We’ll show you where the real numbers sit and what your final cost is likely to look like, not just what the front page says.
Better Way 2 Build | Building Broker Services | Serving Perth Metro and Greater Western Australia
