Control Price Risk with Approximate Quantities for UK QSs

Approximate quantities are measured items priced against estimated rather than firm dimensions, used when there is not enough design detail to produce a full bill, but the project needs an early price or an early start. They suit budget-setting and fast-track procurement, not final design certainty. The trade-off is real: the employer carries more cost risk, so the tender documents must set out exactly how and when those quantities will be remeasured and reported.

TL;DR:

• Approximate quantities are based on early estimates with remeasurement required upon final design completion, increasing final cost uncertainty.
• Preparing approximate bills involves documenting assumptions, clearly labeling items, and establishing remeasurement procedures and evidence sources before tendering.
• Contractors can be appointed early, but final prices depend on remeasured quantities, which require ongoing tracking and dispute management through clear documentation.
• Contingency sums should be tied to specific triggers like remeasurement milestones or design freezes to effectively manage variation risks.
• Use a structured workflow, including peer reviews and assumption logs, to ensure accuracy and consistency when producing or reviewing approximate quantities for tenders.

Table of Contents

• What approximate quantities are, and how they differ from firm quantities and provisional sums
• How approximate quantities are prepared, measured and remeasured
• Contractual and procurement implications for tendering and payment
• Managing risk: contingencies, provisional allowances and avoiding disputes
• Practical checklist for producing and tendering approximate quantities
• Tool-assisted drafting for approximate bills of quantities
• Sources
• FAQ

What approximate quantities are, and how they differ from firm quantities and provisional sums

NRM2 describes an approximate bill of quantities as the appropriate route when there is insufficient design information to prepare a firm bill, on the understanding that quantities will be remeasured on completion. That single condition, remeasurement against the final design, is what separates approximate quantities from a firm bill, where the QS has measured the work exactly from completed drawings.

Provisional sums sit in a different category again. Quantity surveyors need to keep the distinction clear when categorising items: approximate quantities are measurable work with an uncertain extent, while provisional sums cover work that is not yet defined or specified at all. Our own guide to NRM2 explained goes into the measurement rules behind both.

The appeal of approximate quantities is speed: an employer can get contractors on site before the design is frozen. The cost is variation risk and the administrative overhead of tracking every remeasured item through to final account.

• Speed: contractors can be appointed and mobilised before drawings are complete.
• Variation risk: every quantity is provisional until remeasured, so final costs can move.
• Description accuracy still matters: a vague item description undermines pricing consistency even when the quantity itself is only an estimate.

How approximate quantities are prepared, measured and remeasured

Preparation starts the same way a firm take-off would, working from the best available drawings or a digital model, but the QS applies judgement to fill gaps that firm measurement would not allow. The output has to carry enough documentation that a contractor pricing the bill and a QS remeasuring it months later reach the same understanding of scope.

1. Take off quantities from the issued tender drawings or model, noting the revision and date used.
2. Record every assumption behind an estimated dimension or omitted detail directly against the item.
3. State units and inclusions or exclusions explicitly, so contractors price on the same basis.
4. Agree the remeasurement method and evidence source with the employer before tender issue.
5. Reconcile the final account using the same drawings or model extracts used at tender stage wherever possible.

Pro Tip: Date-stamp every take-off source you use. When a remeasurement dispute arises months later, being able to show exactly which drawing revision the original quantity came from settles most arguments quickly.

Final account procedures guidance notes that remeasurement is normally taken from the tender drawings or a digital model rather than from site, with site remeasurement treated as exceptional and something that must be justified and recorded when it does happen.

Contractual and procurement implications for tendering and payment

Standard forms treat approximate quantities as a starting point, not a fixed price. The JCT Standard Building Contract with Approximate Quantities states that the tender sum is indicative, with the final price reached through remeasurement and valuation of the actual work carried out. That shifts the payment mechanism away from a lump sum and towards a schedule that tracks quantities as they firm up.

Procurement guidance from the CQSA makes the trade-off explicit: bills of approximate quantities allow earlier contractor appointment but increase price uncertainty for the employer, because contractors are pricing estimated rather than measured work. Tender documents need to close down as many of those uncertainties as possible before contractors price.

• State the remeasurement method and the evidence source (drawings, model, or site records) in the tender documents.
• Set out the instruction route for design changes that will affect approximate items.
• Give firm deliverable dates for outstanding design information tied to provisional sums.
• Confirm how interim valuations will treat approximate items until remeasurement is complete.

Our guide on handling tender addendum quantities covers what to do when design changes arrive mid-tender and contractors have already priced against the original assumptions.

Managing risk: contingencies, provisional allowances and avoiding disputes

Contingency sums exist to absorb the uncertainty that approximate quantities introduce, but a contingency without a trigger point is just a number sitting in the cost plan. Tie contingency release to specific events, a design freeze date, a remeasurement milestone, or an instruction, rather than leaving it open for either party to draw down informally.

Provisional sums and prime cost sums work alongside approximate quantities in the final account, but they answer different questions: one covers undefined work, the other covers a specified item where the supplier or price is not yet fixed. Our note on prime cost sums sets out how each is adjusted at final account stage.

• Tie contingency drawdown to a named trigger, not a general allowance.
• Set firm deliverable dates for design information linked to provisional sums.
• Agree a monthly remeasurement and reporting timetable with the employer from the outset.

Guidance and sector commentary recommend fixing deliverable dates for design information tied to provisional sums, so they can be converted to fixed prices during the contract period rather than being left outstanding at final account. That single habit, converting provisional allowances as soon as design information allows, does more to prevent final account disputes than any amount of contingency.

Practical checklist for producing and tendering approximate quantities

Use this sequence when you are preparing a bill of approximate quantities or reviewing one issued by someone else.

1. Confirm approximate quantities are the right route: is there genuinely insufficient design detail for a firm bill?
2. Issue drawings, model extracts and a written statement of assumptions with the tender.
3. Label every approximate item, provisional sum and prime cost sum clearly and consistently.
4. Set out the remeasurement method, evidence source and reporting frequency in the tender documents.
5. Agree instruction routes for design changes before tender return.
6. Have a second QS peer-review item descriptions and units before issue.
7. Keep a running log of assumption changes made during procurement, dated and attributed.

Pro Tip: Keep your assumption log in the same document as the take-off, not a separate file. When quantities are challenged at final account, the assumption sits next to the number it explains.

Consistent labelling matters as much as accurate measurement. Our workflow guide on keeping tender quantities within a defined tolerance sets out QA steps that apply equally to firm and approximate bills.

Tool-assisted drafting for approximate bills of quantities

Producing a bill of approximate quantities still means working through dense PDF drawings item by item, checking descriptions and cross-referencing dimensions against every revision. Some software reads construction drawings and produces a draft Bill of Quantities for a quantity surveyor to review and sign off, including approximate items where the design detail is not yet firm.

It does not remove the QS's professional judgement at any stage, and it is not a substitute for the assumption-logging and remeasurement discipline set out above: it produces a starting draft that a QS checks, edits and finalises. The platform is in development. You can review the current plans, Solo, Business and Enterprise, at Quantiflow.

Sources

For measurement rules, consult NRM2 at cost-plan stage. For remeasurement and final account, use RICS final account procedures. For contract wording, check JCT SBC/AQ. For procurement practice, see the CQSA guide and Designing Buildings.

• NRM 2: Detailed measurement for building works (RICS)
• Procurement guide - bills of approximate quantities (CQSA)

FAQ

How much does a QS charge per hour?

Quantity surveyor fees are typically quoted as a project percentage or a fixed fee rather than a standard hourly rate, and figures vary by firm, region and project complexity. Ask a chartered surveyor or your local RICS-regulated firm for a quote against your specific scope.

Can you provide an example of a bill of quantities?

A bill of quantities lists measured work items grouped by trade or element, each with a description, unit and quantity for contractors to price. NRM2 sets out the standard structure and item descriptions used in UK measurement practice, and a full worked example is best reviewed directly against that document.

What is the best estimating software for small builders in the UK?

There is no single best tool: small builders typically choose between spreadsheet-based estimating, dedicated take-off software and platforms that draft output from drawings, depending on scope and budget. QuantiFlow is one option that reads drawings and produces a draft bill for review, but the right choice depends on the size and complexity of the work.

How do you calculate preliminaries?

Preliminaries are calculated by identifying the site-wide and project management costs that sit outside the measured work items, such as supervision, site accommodation and temporary services, then pricing each against the programme duration. NRM1 and NRM2 set out the standard categories used to structure preliminaries within a UK bill of quantities.

Are approximate quantities the same as provisional sums?

No. Approximate quantities are measurable items with an uncertain extent, while provisional sums cover work that is not yet defined or specified at all. Both get adjusted at final account, but through different mechanisms.

Recommended

• Risk allowances in BoQs: the UK QS guide
• Stop Final Account Disputes: NRM2 Prime Cost Sums for UK QSs
• Provisional sums in BoQ: a practical guide for QSs
• BoQ pricing for UK quantity surveyors: practical guide

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.