Use a first-principles rate build-up for high-value or site-specific items, and use a rate library for routine, repeatable items that you rebase and check before pricing. This split reflects guidance from RICS, NRM2 and BCIS. QuantiFlow reads construction drawings and produces a draft bill of quantities for a quantity surveyor to review and sign off.
TL;DR:
- Build up high value, bespoke, or site constrained items from labour, plant, and materials, documenting gang hours, wastage, assumptions, overheads, and profit for tender review.
- Before using a routine library rate, confirm its base date and index, apply a regional factor, and check whether overheads, profit, and preliminaries are included.
- Mark each rate’s source in the bill and add an approximate resource split to high value library rates, giving reviewers assumptions to check.
- NRM 2 notes that major capital projects often favor first principles pricing for transparency, while routine maintenance and repeat orders can use checked library benchmarks.
Table of Contents
- What a rate build-up and a rate library rate actually are
- How to decide between a build-up and a library rate
- Practical steps to build a first-principles unit rate
- How to rebase and adjust a library rate for your project
- Hybrid workflows: combining both methods in one estimate
- Recording and presenting rates in an NRM2-aligned bill of quantities
- How a drafting platform fits into this workflow
- FAQ
- Sources
What a rate build-up and a rate library rate actually are
A first-principles rate build-up starts from nothing and reconstructs the cost of an item from its component resources. You list the labour gang and the hours it needs, the plant and its hire or operating cost, the materials with an allowance for wastage, then assemble these into a net unit cost before adding overheads and profit. RICS guidance on commercial management of construction describes this approach as giving granularity into labour hours and material quantities, which is exactly why it suits anything unusual or risky.
A rate library rate, by contrast, is a pre-determined unit cost drawn from historical market data, often published and periodically updated. The same RICS guidance frames library rates as a time-efficient benchmark for standard work items rather than a resource-by-resource reconstruction. Some libraries publish all-inclusive rates that already fold in overheads and profit; others expect you to add preliminaries and margin yourself, so the definition behind the number matters as much as the number.
A simple resource split illustrates the contrast. A build-up sets out the labour, plant and material costs separately, with overheads and profit added as appropriate. A library rate collapses all of that into a single figure, which is faster to apply but tells you nothing about what is inside it unless the documentation says so.
How to decide between a build-up and a library rate
The decision turns on a handful of practical criteria rather than a fixed rule for every item in the bill.
- Item value and contract risk: high-value items or those with significant pricing risk deserve a build-up so you can defend the figure at tender and during variations.
- Site constraints: restricted access, unusual ground conditions or bespoke details rarely match a published rate, so build them up from first principles.
- Repeatability: routine items that recur across many projects, such as standard brickwork or common excavation, are well served by a library rate.
- Data availability and time: when programme pressure is tight and a reliable library rate exists, using it frees time for the items that genuinely need a build-up.
NRM 2: Detailed measurement for building works notes that first-principles build-ups are often preferred for major capital projects, where transparency and the ability to adjust for site-specific risk matter more than speed. For routine maintenance or repeat-order work, a library rate can provide a benchmark, but check that it suits the project. Whichever library you use, BCIS guidance is clear that the rate still needs rebasing before it goes anywhere near a live estimate, a step covered in detail below.
Practical steps to build a first-principles unit rate
Building a defensible rate from scratch follows a consistent sequence, whatever the trade.
- Define the item precisely, including its NRM2 description, unit and any site-specific conditions that affect how it will be executed.
- Take off the resources needed: labour trades, plant items and materials, each quantified against the item.
- Establish the gang and hours logic, setting out how many operatives of each trade are needed and how long the item takes to complete. CESMM4 gives worked examples of this gang-based approach for civil engineering items, and the same logic transfers to building work.
- Cost the plant, using hire rates or internal plant charges for the hours or days required.
- Price the materials, adding a wastage allowance appropriate to the material and method of fixing.
- Assemble the net resource cost by summing labour, plant and materials for the item.
- Add preliminaries, overheads and profit as a percentage or fixed addition, consistent with how the rest of the bill is priced.
To keep the rate auditable, record the base date, the resource split between labour, plant and materials, and any assumptions behind gang size or wastage percentage. A rate with no supporting notes is difficult to defend if a contractor queries it during the tender period or a valuation later in the contract.
How to rebase and adjust a library rate for your project
A library rate is only a starting point. BCIS notes that rates are typically linked to indices such as the Tender Price Index or the General Building Cost Index so subscribers can rebase them to current market conditions rather than relying on a figure that may be several years out of date.
Before a library rate goes into your bill, confirm the following:
- Base date and index: identify when the rate was struck and which index applies to bring it to your current pricing date.
- Location factor: apply a regional adjustment, since a national-average rate rarely matches local labour and material costs.
- Scope of inclusions: check whether the rate is genuinely all-inclusive or whether preliminaries, overheads and profit still need adding.
- Resource mix assumed: look at the methodology notes to see what labour, plant and material split the rate assumes, since an unusual specification may not match it at all.
Treating a library rate as plug-and-play without these checks is one of the more common sources of estimating error, and it is precisely the gap that a first-principles build-up avoids by forcing you to state every assumption yourself.
Hybrid workflows: combining both methods in one estimate
RICS guidance describes surveyors using a hybrid approach, with library rates for stable, repetitive items and first-principles build-ups for high-value or high-risk items such as bespoke cladding or constrained-site foundations. Flagging which items were built up and which were drawn from a library, with a short note in the bill or an appendix, makes the logic visible to anyone reviewing the tender later.
Pro tip: Add a short resource split, even an approximate one, to any library-derived rate you include in a high-value bill; it gives reviewers something to check against rather than a single opaque figure.
This hybrid discipline also supports dispute avoidance. RICS guidance on commercial management of construction points to transparency as the main advantage of a build-up precisely because it lets stakeholders verify what each item rate contains, which matters most where a contractor is likely to query pricing during tender review.
Recording and presenting rates in an NRM2-aligned bill of quantities
NRM 2 sets out how a bill of quantities should be structured, and rate justification should map onto that structure rather than sit in a separate, disconnected file. Our guide to NRM2 takeoff rules for audit-ready BoQs covers the measurement side of this in more depth.
At minimum, keep a note of the base date, the index used for any rebasing, the resource split behind each build-up, and the inclusions or exclusions assumed for each library rate. Preliminaries and prime cost sums need their own treatment in the pricing schedule rather than being buried inside unit rates; our piece on NRM2 preliminaries and contractor breakdowns sets out how to separate these clearly. A rate register or appendix, linked from the main bill, gives reviewers a single place to check every rate's origin.
How a drafting platform fits into this workflow
QuantiFlow reads construction drawings and produces a draft bill of quantities for you to review and sign off, which gives you a structured starting point whichever pricing method you plan to apply. You can review the draft bill of quantities and decide whether a library rate or a first-principles build-up is appropriate for each item.
QuantiFlow is in development, with Solo at £69 per month and Business at £299 per month. Visit the QuantiFlow plans page to see current details.
FAQ
What is the main difference between a build-up and a library rate?
A first-principles build-up reconstructs a unit cost from labour, plant and material resources specific to the item, while a library rate is a pre-determined figure drawn from historical market data. RICS guidance frames build-ups as offering granularity and libraries as a faster benchmark for standard items.
When should I use a first-principles build-up instead of a library rate?
Use a build-up for high-value items, site-specific conditions or anything carrying significant contract risk, since it lets you justify every assumption at tender. NRM 2 notes this approach is often preferred for major capital projects for exactly this reason.
Do library rates need adjusting before I use them?
Yes. BCIS guidance explains that library rates are linked to indices such as the Tender Price Index so they can be rebased to current conditions, and you should also apply a location factor and check what the rate includes.
Can I mix both methods in the same bill of quantities?
Yes, and it is common practice to use library rates for routine, repeatable items and reserve build-ups for bespoke or high-risk items within the same bill. Flagging which items were built up helps anyone reviewing the tender understand the pricing logic.
Does QuantiFlow produce NRM2-compliant bills of quantities automatically?
QuantiFlow reads construction drawings and produces a draft bill of quantities for a quantity surveyor to review and sign off, so it supports your workflow rather than replacing your professional judgement. You remain responsible for checking measurement and pricing against NRM2 before the bill is issued.
Sources
Recommended
- Risk allowances in BoQs: the UK QS guide
- BoQ pricing for UK quantity surveyors: practical guide
- Control Price Risk with Approximate Quantities for UK QSs
- Provisional sums in BoQ: a practical guide for QSs
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.