A specification change rarely moves one number. It can alter the measured quantity, the item description, the rate, the associated work and the programme, and it may or may not be a contractual variation. The reliable approach is to fix the document baseline, compare the revised clause with the original, trace every affected drawing, schedule and bill item, re-measure, then record the basis for the change. This guide sets out that sequence for UK quantity surveyors.
TL;DR
- Establish which document issue your original measurement used before comparing anything. Without that baseline, you cannot say what changed.
- Check the whole chain: the changed clause, every drawing and schedule it touches, the bill items, and the related labour, plant, specialist fees and preliminaries.
- A change is not automatically a variation. Check the contract for what counts as a change and who may instruct one.
- Value changed work from existing bill rates where the work is comparable, and build a fair rate where it is not.
- Keep the instruction, revised documents, quantified additions and omissions, and rate build-ups together, so every figure traces back to its source.
Table of Contents
- Why a specification change moves more than one quantity
- Step 1: Fix the baseline
- Step 2: Compare the revised specification with the original
- Step 3: Trace every affected drawing, schedule and item
- Step 4: Is it actually a variation?
- Step 5: Re-measure and record
- Step 6: Reflect the change in pricing and cost control
- Step 7: Keep the records that make the figure defensible
- A checklist for the next revised specification
- Where drafting tools fit
- FAQ
- Sources
Why a specification change moves more than one quantity
A bill of quantities is a coordinated list of work items, descriptions and quantities. RICS NRM2 describes it as the basis for tendering and for valuing executed and varied work. Because the description and the quantity sit together in each item, changing the specification can affect either or both.
RICS guidance on change control and management lists the ways a change can arise. It may replace labour, materials or plant, alter the location of work, amend the programme, alter scope, or increase the volume of an existing item. Only the last of those is a pure quantity change. The others often change the description, the rate or the sequence of work as well.
Three typical examples show the spread:
- A product swap. Moving from one cladding board to another of different dimensions may leave the area unchanged, but change the number of fixings, the support rails, the lead time and the rate.
- A performance upgrade. Raising an insulation requirement may increase thickness, which can alter reveal depths, fixing lengths, junction details and the quantity of finishing items.
- A finish change. Replacing a painted finish with a tiled one adds a substrate preparation item, adhesive, grout and movement joints, and may remove the decoration item entirely.
In each case, the visible change in the specification is the starting point of the measurement exercise, not the whole of it.
Step 1: Fix the baseline
Before you compare anything, record which document issue your original measurement used. That means drawing numbers and revisions, the specification issue, any schedules and any addenda that were incorporated at the time.
NRM2 expects the quantity surveyor or cost manager to measure from coordinated drawings, specifications and other project information. A specification change therefore means checking the changed requirement and every drawing or schedule it affects. If the baseline is unclear, you cannot tell whether a difference is a new change or something that was already in the issue you measured.
Practical points for the baseline:
- List the issue and date of each document used for the original bill.
- Note which documents were incorporated by addendum, and when.
- Confirm that the revised specification states what it supersedes.
- Keep superseded issues. You will need them when a rate or a quantity is questioned.
Our guide to drawing revision control for QS teams covers this in more detail, and the same discipline applies to specifications and schedules.
Keeping the document set coordinated
Most errors after a specification change come from a mismatch between documents: the specification says one thing and a schedule or detail still says another. Reliable document version control tools help the team work from the same current issue, so that you are not measuring from a superseded sheet while the designer has moved on. Whatever system you use, the test is the same: can anyone on the team say which issue each measured line came from?
Step 2: Compare the revised specification with the original
Read the revised clause against the original clause by clause. Do not rely on a revision cloud or a summary email, as these may miss consequential edits elsewhere in the section.
For each difference, classify what it affects:
| Type of effect | What it means | Typical consequence for the bill |
|---|---|---|
| Quantity | More or less of the same work | Quantity changes, same description and rate basis |
| Scope | Work added, removed or moved | New or omitted items, possibly new preliminaries |
| Quality | Different material, finish or standard | Revised description, usually a new rate |
| Performance | Different thermal, acoustic, fire or structural requirement | Revised description, possible knock-on to adjacent items |
| Responsibility | Design or specialist responsibility shifts | Fee, risk and insurance implications |
A single clause can fall into more than one row. Recording the classification makes the later pricing argument far easier, because it shows why you applied a bill rate to one item and a built-up rate to another.
Step 3: Trace every affected drawing, schedule and item
Next, find everything the revised clause touches. Work outward from the changed requirement:
- Drawings. Plans, sections, elevations and details that show the affected element. Check both where it is drawn and where it is annotated.
- Schedules. Door, window, finishes, ironmongery and sanitaryware schedules often repeat product references.
- Bill items. The item that carries the changed work, and any items that depend on it, such as fixings, supports, sealants, making good and protection.
- Interfaces. Where the changed element meets other trades, check thickness, depth, tolerances and sequencing.
- Design responsibility. Where a contractor or specialist designs part of the work, check whether the change moves that responsibility.
RICS guidance on change control points out that the primary measured work is only part of the picture. When a material, product or performance requirement changes, check the directly associated labour, materials, plant, specialist fees and preliminaries, and ask whether related details, interfaces, programme, risk, insurance or bonds are affected.
A useful working habit is to keep a change log with one row per affected item: the original reference, the revised reference, the nature of the change, and the drawing or clause that justifies it. This is the document you will lean on if someone asks why a line moved.
Our article on linking each BoQ line to its drawing revision describes the same principle for the initial bill.
Step 4: Is it actually a variation?
A change in specification does not automatically entitle anyone to payment. RICS guidance notes that a change is not always a contract variation, and that the contract typically defines what constitutes a change and who is authorised to make one.
Before valuing anything, check:
- Who instructed it. Was the instruction issued by the person the contract authorises?
- What the contract says. Does the clause allow the change, and how does it say the change is valued?
- Whether the bill already covered it. A revised document may clarify something the original description already captured.
- Whether it arose from an error in the bill. Different forms treat this differently.
On bill errors, the position depends on the contract. Under a fully priced bill of quantities contract, RICS Valuing change explains that the bills define the quality and quantity of work, and that errors in quantity or measurement method are corrected and treated as variations under the relevant contract. Under NEC, the NEC guidance on errors in bill quantities says that where Option B applies, the employer takes the risk or reward of quantities being higher or lower, and the quantities are remeasured, except where a compensation event causes the change. It also says that correcting an error, ambiguity or inconsistency in the bill is treated as a compensation event under the stated mechanism.
The practical point: read the contract form and its amendments first, then decide how the change is to be treated. If you are unsure of the contractual position, take advice from the contract administrator or a construction solicitor before committing a figure.
Step 5: Re-measure and record
NRM2 provides the standard rules for measuring building works. Use the same rules for the revised quantities as you used for the original, so that additions and omissions are like-for-like and the descriptions remain consistent with the bill.
What counts as the revised measurement depends on the type of bill:
- Firm bill. Quantities are fixed at tender. RICS NRM2 notes that tender price reliability increases with the accuracy of measured and described quantities, and that a firm bill is a strong basis for cost control because changes can be assessed against bill rates.
- Approximate quantities. NRM2 states that approximate bills are subject to remeasurement as built. Under RICS Valuing change, quantities are remeasured when the design is complete, progressive agreement of measurement is important, and bill rates may be used for variations, adjusted for quantity changes. Our piece on controlling price risk with approximate quantities goes further.
- Remeasurement contracts. Changes flow through the remeasured final account. See final account disputes on remeasurement contracts.
For every revised line, record:
- The document issue and clause or drawing reference.
- The measured dimensions, with the take-off method used.
- The additions and omissions, shown separately.
- The bill item reference it adjusts or replaces.
Additions and omissions shown separately matter. Netting them off hides the reason a total moved, and that is where most queries start.
A short worked example
Suppose the original specification called for one type of internal partition board, and the revised specification requires a higher-performance board with an additional layer on one side in corridor walls only.
- Baseline. Record the drawing and specification issues used for the original bill.
- Compare. Identify the corridor walls as the affected partitions, and note that other partition types are unchanged.
- Trace. Check the partition setting-out plan, the partition type schedule, the fire strategy drawing, and any doors or services penetrations in those walls.
- Measure. Omit the original board over the corridor wall area. Add the new board over the same area, plus the extra layer. Check fixings, taping and jointing, and any change to overall wall thickness that affects skirtings, door linings and floor finishes.
- Record. Log each addition and omission with its source reference and bill item.
The measured area of the wall may not change at all. The cost does, and so might the description of several adjoining items.
Step 6: Reflect the change in pricing and cost control
Once the revised quantities are agreed, price them. RICS Valuing change says that where changed work is not identical or similar to contract work, existing rates may be adjusted for differences in conditions, character, quantity or other contract-provided reasons. Where that is not possible, a fair rate or price is calculated. It also explains that a priced bill gives the quantity surveyor more information when valuing variations, because agreed rates for similar work are more likely to exist than under a lump sum without a bill.
A workable order of preference:
- Apply the existing bill rate where the work is the same or comparable.
- Adjust the bill rate where conditions, character or quantity differ, as the contract allows.
- Build a fair rate from labour, materials, plant and other costs where there is no basis in the bill.
RICS change control guidance says change substantiation should identify quantities or lump sums and provide a detailed breakdown of labour, material, plant and other costs. It also advises that each proposed change should be assessed for its reasons, cost, timescale and location impacts before acceptance or rejection.
Then carry the effect through:
- Update the estimate or bill.
- Update procurement, programme, risk and preliminaries.
- Update the cash-flow forecast.
- Record the change in the cost report.
RICS recommends maintaining a rolling final account that includes agreed changes and a considered allowance for changes not yet agreed. The RICS commercial management guidance describes the same idea: track the original estimate, agreed variations and a probability allowance for variations not yet agreed.
Timing changes the cost
The same specification change costs different amounts at different times. The same RICS commercial management guidance says design-change effects depend on project stage, location, accessibility and procurement maturity, and that later changes generally have greater potential impact, especially after related materials have been procured or fabricated. When you receive a revised specification, ask early what has already been ordered, fabricated or installed. That answer often matters more to the final figure than the measured quantity.
Step 7: Keep the records that make the figure defensible
A revised quantity is only as useful as the trail behind it. RICS guidance on change control recommends keeping, for each change:
- the formal instruction and its contractual basis;
- an explanation of the change;
- the revised drawings and specifications;
- quantified additions and omissions, with bill references;
- rate build-ups, quotations, orders and invoices;
- a detailed cost breakdown;
- relevant photographs;
- programme effects and current status.
RICS recommends that the quantity surveyor, project manager, employer's agent or contract administrator assembles and agrees this documentation with the employer and contractor, ordinarily monthly. A monthly rhythm keeps disputes small. A change agreed in the month it happens is far easier to settle than one reconstructed at final account.
If your changes arrive as tender addenda rather than post-contract instructions, the same logic applies in compressed form. Our seven-step triage for tender addendum quantities shows how to prioritise.
A checklist for the next revised specification
Use this as a short pre-submission check:
- Have I recorded the document issues used for the original measurement?
- Have I compared the revised clauses line by line with the originals?
- Have I checked every drawing and schedule that mentions the element?
- Have I checked associated labour, materials, plant, specialist fees and preliminaries?
- Have I confirmed the instruction is valid and the change is a variation under the contract?
- Are additions and omissions shown separately, each with a source reference?
- Is the rate basis stated: bill rate, adjusted rate or fair rate?
- Has the cost report and rolling final account been updated?
- Have I noted what has already been ordered or fabricated?
- Is the documentation agreed with the other parties?
Where drafting tools fit
We are building QuantiFlow to read construction drawings and produce a draft bill of quantities for a quantity surveyor to review and sign off. It is in development. The judgement in this guide, whether a change is a variation, which rate applies and what the contract allows, stays with the surveyor. You can find out more on the QuantiFlow site.
Better still for the reader, the habits above work with any tool or none: fix the baseline, trace the chain, separate additions from omissions, and keep the record.
FAQ
Does every specification change change the bill of quantities?
Not necessarily. A revised clause may clarify something the original description already covered, or change a performance requirement without altering a measured quantity. Compare the revised clause with the original bill description before deciding. If the description, quantity or associated work is unaffected, record that conclusion and its basis.
Which rates should I use for changed work?
Start with the existing bill rate where the work is the same or comparable. Where conditions, character or quantity differ, RICS Valuing change says existing rates may be adjusted. Where there is no basis in the bill, calculate a fair rate from labour, materials, plant and other costs, and keep the build-up.
Is a quantity change a variation?
It depends on the contract. On a remeasurement basis, quantity differences are normally dealt with through remeasurement rather than as instructed variations. NEC Option B, for example, places the quantity risk on the employer except where a compensation event causes the change. Always check the contract form and its amendments.
How often should change documentation be agreed?
RICS guidance says the quantity surveyor, project manager, employer's agent or contract administrator should assemble and agree the documentation with the employer and contractor, ordinarily monthly. Agreeing in the month of the change keeps the rolling final account current.
Sources
- RICS, New rules of measurement (NRM2), 2021
- RICS, NRM2 October 2022 update
- RICS, Change control and management, reissued March 2024
- RICS, Valuing change, reissued March 2025
- RICS, Commercial management of construction, 2016
- NEC, Errors in quantities in bill of quantities
Recommended
- Remeasure After Revisions: 5 NRM2 Steps for UK QSs
- Stop Final Account Disputes on Remeasurement Contracts: NRM2 BoQs for QSs
- Construction measurement quality control: the UK SME guide
- Keep Tender Quantities Within ±5%: NRM2 Workflow for UK QS
This article is for general information only and is not professional, legal or commercial advice. Quantity surveying decisions should be taken by a qualified professional with reference to the specific project, drawings and contract in question. Content is produced with AI assistance and reviewed before publication. QuantiFlow Ltd accepts no liability for reliance on it.