One of the most confusing areas of the JCT building contracts is how variations are valued. Clause 5 of the Standard Building Contract sets out the rules, but it’s written in classic JCT legal style — long sentences, heavy jargon, and lots of cross-references. So here’s a clean, practical breakdown of the valuation rules that every QS needs to understand.

What Are the JCT Valuation Rules?

The valuation rules determine how to price any change to the works, including:

• additional work
• omitted work
• substituted work
• changes in quality, quantity, or conditions
• work covered by Approximate Quantities

Whenever the Employer issues an instruction, these rules decide how the Contractor gets paid for that change.

5.3.1 — The Valuation Rules

Original Text:

To the extent that a Valuation relates to the execution of additional or substituted work which can properly be valued by measurement or to the execution of work for which an Approximate Quantity is included in Contract Bills and subject to clause 5.7 in the case of CDP Works, such work shall be measured and shall be valued in accordance with the following rules:

Translation:

If you’re pricing extra or changed work that can be measured (or work that had an Approximate Quantity in the contract), then you must measure it and value it using the rules below.

5.3.1.1 — Similar work, similar conditions, similar quantity

Original Text:

To the extent that a Valuation relates to the execution of additional or substituted work which can properly be valued by measurement or to the execution of work for which an Approximate Quantity is included in Contract Bills and subject to clause 5.7 in the case of CDP Works, such work shall be measured and shall be valued in accordance with the following rules:

Translation:
If work is similar, same conditions, no big quantity change

• then use the existing contract rate exactly as it is.
• No adjustments. No new prices.

Example:
Extra 10m of the same skirting in the same room → use the skirting rate in the contract.

5.3.1.2 — Similar work, but different conditions or much larger/smaller quantity

Original Text:

Where the additional or substituted work is of similar character to work set out in the Contract Documents but is not executed under similar conditions thereto and/or significantly changes its quantity, the rates and prices for the work set out in the Priced Document shall be the basis for determining the valuation and the Valuation shall include a fair allowance for such difference in conditions and/or quantity;

Translation:

If work is similar, but conditions are different or quantities have significantly changed then

• Start with the contract rate
• then adjust it fairly for the different conditions or larger/smaller quantity.

Example:
Same blockwork, but now at height on scaffolding → use the blockwork rate + fair uplift.

5.3.1.3 — Completely new or different work

Original Text :

Where the additional or substituted work is not of similar character to work set out in the Priced Document, the work shall be valued at fair rates and prices;

Translation:

If work is not similar to anything in the contract then

• create a new fair rate or price.

This typically involves:

• New build-ups
• Subcontract quotations
• Market rates
• Benchmarking

Example:
A steel beam added when the contract includes no steelwork → new rate must be created.

5.3.1.4 — Approximate Quantity was reasonably accurate

Original Text:

Where the Approximate Quantity is a reasonably accurate forecast of the quantity of work required the rate or price for the Approximate Quantity shall determine the valuation;

Translation:

If the approximate quantity was a good estimate then

• use the original rate.

Example:
AQ: 100m
Actual: 105m
→ Use the original rate.

5.3.1.5 — Approximate Quantity was inaccurate

Original:

Where the Approximate Quantity is not a reasonably accurate forecast of the quantity of work required, the rate or price for that Approximate Quantity shall be the basis for determining the valuation and the Valuation shall include a fair allowance for such difference in quantity.

The AQ was not a realistic estimate.

Translation:

The AQ was not a realistic estimate then

• Start with the original rate,
• Then adjust it fairly because the actual quantity is very different.

Example:
AQ: 100m
Actual: 350m
→ Rate may need fair adjustment.

Important Note

5.3.1.4 and 5.3.1.5 only apply if the work hasn’t changed except in quantity. The AQ rules only apply when the work is the same, just more or less of it.
Not when the nature of the work changes.

In Practical QS Terms

Here’s the valuation hierarchy in one sentence:

Start with contract rates; if conditions or quantities change, adjust them; if the work is totally new, create new fair rates; and treat Approximate Quantities according to how accurate they were.

Why These Rules Matter

Understanding these rules gives you:

• Confidence in valuation meetings
• Stronger justification for variation pricing
• Ability to challenge unfair adjustments
• Clarity when dealing with Employers, Contractors or Subcontractors

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