What Is a Fixed Price Building Contract?

Table of Contents

• Defining a Fixed Price Building Contract
• Fixed Price Versus Cost Plus: How the Two Differ
• What Is Included in the Contract Sum
  • Understanding Prime Cost and Provisional Sums
• Variations and When a Builder Can Change the Price
  • The variation mechanism, step by step
  • What a variation should contain
  • Variations the owner did not request
  • Managing variations after the fixed price is signed
  • Statutory and contractual limits
• Risks, Inflation and Material Cost Escalation Clauses
  • What a rise-and-fall clause actually does
  • Fixed price versus adjustable price
  • Testing whether an escalation clause is fair
  • How to protect yourself before signing
  • Other risks to look for
• How to Review a Fixed Price Contract Before You Sign
• Booking a Fixed Price Quote With Archibuilt
• Frequently Asked Questions

Defining a Fixed Price Building Contract

A fixed price building contract is an agreement where the builder commits to a set total price for the work described. A fixed price model is designed so homeowners know the cost before construction starts.

In practice, the contract is a package: the price, scope of works, plans and specifications, payment schedule, and rules for changes. If any are vague, the fixed price stops meaning much.

Key parts of a fixed price contract:

• Contract sum: the total price for the agreed work
• Scope of works: a list of what the builder will and will not do
• Plans and specifications: the materials, finishes and sizes to be used

Fixed Price Versus Cost Plus: How the Two Differ

With a fixed price contract, the builder carries the risk of cost overruns.

Here is how they compare:

A fixed price suits most homeowners with finished plans and a set budget. Cost plus can work while the design is still changing, but needs close tracking and regular cost reports.

Watch out: a fixed price is only as firm as its scope. If plans are incomplete, the builder may price missing items later as variations.

What Is Included in the Contract Sum

The contract sum covers everything the scope of works names: labour, standard materials, site setup, waste removal, and the builder’s margin. It should also list allowances for items not yet chosen, such as tiles or light fittings.

Ask for a clear breakdown listing each trade and major material. A line item that says only “kitchen” with no detail is hard to check.

Common inclusions to confirm:

• Site preparation and temporary services
• Structural and trade work described in the plans
• Materials and fixtures listed in the specifications

Understanding Prime Cost and Provisional Sums

Some items cannot be fixed until you choose them. Two common terms cover this.

A prime cost item is an item where the contract sets a budget amount, such as a basin, a tap set or a light fitting.

A provisional sum is an estimate for work that cannot yet be fully defined, such as ground conditions or hidden structural repairs.

These sums are normal but reduce the certainty of a fixed price.

Variations and When a Builder Can Change the Price

A variation is any change to the work, the contract sum, or the time for completion after signing, a new window, a moved wall, an upgraded floor covering, or a delay caused by a late owner selection. Variations are common on renovations and where most disputes start.

A builder can usually change the price only in a few situations:

• You request a change to the agreed work
• The contract allows for a specific, known risk, such as a provisional sum
• Unforeseen conditions are found, and the contract sets out how they are handled

The variation mechanism, step by step

A well-drafted contract sets out a repeatable process. The typical sequence is:

1. Request in writing. The owner or builder identifies the proposed change and puts it in writing, with a sketch, selection sheet, or written instruction.
2. Priced before work starts. The builder prepares a written variation stating the change, price, and any effect on the programme and other trades.
3. Approval in writing. The owner signs or approves the variation as the contract requires. Verbal approval is the most common source of dispute.
4. Recorded and reflected. The variation is added to the contract documents, the contract sum adjusted, and the payment schedule updated if needed.

Disputes that reach adjudication or the courts are rarely about the size of a variation. They are about whether it was authorised at all, or whether the price was agreed before the work was done. A written variation signed before work starts removes both questions.

What a variation should contain

A variation that will survive scrutiny usually states:

• A description of the change, cross-referenced to the plans or specifications
• The price, broken down by trade or material where possible
• Any extension of time and the reason for it

If a variation is priced as a lump sum with no breakdown, ask for the breakdown. A builder who cannot explain how a variation price was reached is a builder to question.

Variations the owner did not request

Some variations arise without an owner request. Examples include:

• A provisional sum being adjusted once the work is priced or completed
• A prime cost item being adjusted once the owner makes a selection
• A latent condition, such as rock or contaminated soil, discovered after excavation

In each case, the contract should say how the adjustment is calculated and what evidence the builder must provide.

Managing variations after the fixed price is signed

The fixed price is fixed only for the scope described. Once the contract is signed, the practical discipline is:

• Keep a single variation register, numbered and dated
• Never let work proceed on a verbal instruction, even a small one
• Ask for the price before the work, not after

Watch out: a series of small on-site changes approved verbally, each individually modest, can together add a significant amount to the final account. By the time the invoice arrives, the owner has no written record of what was agreed.

Statutory and contractual limits

Residential building contracts are regulated at national level, and rules differ. Variation requirements and deposit limits differ between different parts of the UK, so check the rules that apply where your project is located.

If a variation is disputed, the contract’s dispute resolution clause governs what happens next, usually notice, a period for discussion, then referral to a regulator, adjudicator, or court.

Risks, Inflation and Material Cost Escalation Clauses

A fixed price protects you from most day-to-day cost changes, but not always from large market shifts. Many contracts include an escalation clause, often called a rise-and-fall clause, allowing costs to be passed on in defined circumstances. This is the most misunderstood part of a fixed price contract, and where the certainty the owner thought they had can quietly disappear.

What a rise-and-fall clause actually does

A rise-and-fall clause adjusts the contract sum after signing to reflect movements in the cost of specified inputs. The clause should identify:

• Which costs can be adjusted, for example timber, steel, concrete, glazing, or specific imported components
• The reference point, the date or index against which the movement is measured
• The evidence required, supplier invoices, quotes, or a published index

A clause that allows any price change at any time, for any reason, is not really a fixed price contract. A clause that limits adjustments to documented, specific materials, measured against a stated reference point, is far more reasonable.

Fixed price versus adjustable price

In practice, three arrangements appear in residential contracts:

• True fixed price, no adjustment for cost movements; the builder carries the risk. This is what most homeowners think they are signing.
• Fixed price with rise-and-fall, the base price is fixed, but specified inputs can be adjusted under the clause. This is common in periods of high volatility.
• Cost plus with a guaranteed maximum price, the owner pays actual costs up to a cap, and the builder and owner share or allocate costs above it according to the contract.

Ask the builder which of these the contract represents and to point to the clause that says so. If the answer is vague, the contract is vague.

Testing whether an escalation clause is fair

A practical test is to ask four questions of the clause:

1. Does it name the specific materials or inputs that can be adjusted, or is it open-ended?
2. Does it require the builder to provide evidence, such as supplier invoices or a published index?
3. Is there a cap, a threshold, or a sharing mechanism, or does the owner bear the whole movement?
4. Can the price move down as well as up, or is the adjustment one-way?

A clause that fails all four is one to negotiate or walk away from. A clause that passes all four is a managed risk rather than an open-ended one.

How to protect yourself before signing

Most practitioners find that the best protection is not a legal argument after the fact but a set of practical steps before signing:

• Ask for the escalation clause to be highlighted and explained in writing
• Request a schedule of the specific materials subject to adjustment
• Ask what reference point or index will be used, and check that it is publicly verifiable

If the clause is complex, an independent building consultant or solicitor can review it before you sign. That cost is almost always less than the cost of a disputed adjustment.

Other risks to look for

Escalation is not the only risk in a fixed price contract. Others include:

• Unclear payment milestones, progress claims should tie to completed work, not to calendar dates alone
• Liquidated damages, check whether the contract states a rate for late completion, and whether it is capped
• Defects liability period, confirm the length and what it covers

Watch out: a contract silent on escalation is not necessarily safer than one with a clause. Silence leaves the position to be argued later, and the argument usually favours the party with the better records.

How to Review a Fixed Price Contract Before You Sign

Take your time with the contract. A short checklist helps you catch the most important points.

• The contract sum is clearly stated
• The scope of works matches your plans and specifications
• All prime cost items and provisional sums are listed with amounts

Before you choose a builder, check their licence, insurance and past work.

If something is unclear, ask for it in writing. A reputable builder will explain each term and update the document where needed.

Booking a Fixed Price Quote With Archibuilt

The best way to start is with a detailed quote.

To request a quote:

1. Share your plans, sketches or a brief description of the project
2. Discuss your budget and the finishes you have in mind
3. Review the scope of works and allowances together
4. Receive a written fixed price proposal to compare

For current prices or a quote, visit the Archibuilt website. When ready, ask for a renovation consultation before anything is signed.

Frequently asked questions

Does a fixed price contract mean the cost can never change?
No.

What should I check first in a contract?
Start with the contract sum, scope of works and variation process.

Last Updated: 10 November 2026

Add archibuilt.net.au as a preferred source on Google →