Detailed Estimates, BOQs and Cost Summaries

Introduction

Estimating and costing convert drawings, specifications and construction methods into a forecast of money, materials, labour, plant and time. A detailed estimate is not merely a total price. It is a transparent measurement and pricing document that allows an owner to obtain approval, a contractor to plan work, and a quantity surveyor or engineer to control expenditure.

This chapter explains the preparation of item-rate estimates, bills of quantities (BOQs), schedules of rates, cost abstracts and complete estimate reports. It also explains contingencies, provisional sums, preliminaries and work-charged establishment. The principles apply to buildings, roads, drainage, water-supply works and other civil-engineering projects, subject to the measurement rules and regulatory requirements of the relevant jurisdiction.

Learning Objectives

After studying this chapter, a learner should be able to:

• distinguish preliminary, detailed and revised estimates;
• interpret drawings and specifications for measurement;
• prepare item descriptions, units, quantities and rates;
• compile a BOQ and an item-rate estimate;
• prepare a schedule of rates and a cost abstract;
• apply preliminaries, contingencies, provisional sums and work-charged establishment appropriately;
• check quantities, arithmetic, units and scope;
• assemble a professional estimate report and explain its limitations.

1. The Estimating System

A construction estimate normally develops through the following sequence:

Figure: The principal workflow for preparing a detailed estimate.

A preliminary estimate is prepared when design information is limited, often using floor-area, cubic-content, functional-unit or comparative rates. A detailed estimate is based on measured quantities for each work item. A revised estimate is prepared when the sanctioned cost is likely to be exceeded or the scope changes materially. A supplementary estimate covers additional work not included in the original scope.

The estimate must state its basis: drawing numbers and revisions, specification edition, measurement method, price level or date, taxes, location, currency, exclusions and assumptions. Without this information, a numerical total can be misleading.

2. Measurement and Item Description

2.1 Taking off quantities

Measurement begins with a systematic reading of drawings. The estimator identifies work sections, records dimensions, applies deductions and summarizes quantities. Typical sections include site clearance, excavation, filling, concrete, reinforcement, masonry, damp-proofing, roofing, finishes, doors and windows, services and external works.

For a rectangular quantity:

where Q is volume in m³, L is length in m, B is breadth in m and H is height or thickness in m. For an area, omit the third dimension: A = L × B. For a linear item, such as pipe or skirting, Q = L. The formula assumes compatible dimensions and a geometry represented adequately by the chosen shape.

For a concrete slab measuring 12 m × 8 m × 0.15 m:

Q = 12 × 8 × 0.15 = 14.40 m³

If an opening is deducted, its volume is subtracted only when the adopted measurement rules require it. Measurement conventions differ: some standards allow deductions below a specified size; others treat junctions, overlaps and waste through the rate. The governing standard—not personal habit—controls.

A take-off sheet should show item number, description, number, length, breadth, height or thickness, quantity, unit and remarks. Dimensions should be traceable to a drawing or site record. Separate measured work from assumptions.

2.2 Units and descriptions

Common units are m³ for excavation and concrete, m² for formwork and finishes, m for pipes and skirting, kg or tonne for reinforcement and structural steel, and number for doors, fixtures and fittings. A description should identify material, grade, size, location, workmanship and what is included. “Concrete” is inadequate; “reinforced concrete of specified grade, including placing, compacting and curing, excluding reinforcement and formwork” is more useful.

An item should be measurable, priced and verifiable. Excessively broad items conceal scope; excessively fragmented items create unnecessary administration. The BOQ should follow a logical construction sequence and use consistent terminology.

3. Item-Rate Estimates and BOQs

An item-rate estimate contains measured quantity, unit rate and amount for every item. The basic pricing relationship is:

where Aᵢ is the amount for item i, Qᵢ is its quantity and Rᵢ is the unit rate. The unit of Rᵢ must be currency per unit of Qᵢ, so Aᵢ is currency.

A BOQ is a structured list of these work items, generally issued for tendering. It enables different contractors to price the same measured scope and provides a basis for interim valuation and variation measurement. A BOQ is not a substitute for drawings, specifications or conditions of contract.

A typical BOQ contains:

Worked example: small concrete package

The following illustrative quantities are measured from assumed drawings.

The arithmetic is checked item by item: for example, 850 × 1.80 = 1,530.00. The example deliberately separates concrete, formwork and reinforcement because they have different measurement and cost behaviour.

4. Building Up Unit Rates

A unit rate should reflect the resources and obligations required to complete one measured unit. A general rate build-up is:

where R is the unit rate; M is material cost; L is labour cost; P is plant and equipment cost; O is subcontract or specialist cost; W is wastage or loss allowance where permitted; and H is allocated overheads and profit. Taxes, transport, testing, curing, loading and unloading must be included or excluded explicitly to avoid double counting.

For a material component:

where Cₘ is material cost per measured unit, qₘ is material consumption per measured unit, pₘ is purchase price per consumption unit and Tₘ is allocated transport and handling cost. Waste should be based on realistic procurement and site practice, not an automatic arbitrary percentage.

Labour may be derived from output:

where n is the number of workers, w is labour cost per worker-hour, t is hours worked and O is output in measured units. The formula assumes the crew and output are defined for the same conditions; productivity changes with access, weather, supervision, congestion and skill.

Plant cost may include ownership or hire, fuel, operator, maintenance and mobilization. Current quotations, approved schedules of rates, supplier invoices, historical data adjusted for location and date, and resource analysis are common sources. Rates should not be copied without checking market validity.

5. Schedules of Rates

A schedule of rates is a list of standard work items and their unit rates. It may be used for recurring maintenance, valuation of variations, framework contracts or preliminary budgeting. It normally contains item code, description, unit and rate, with rules explaining inclusions and exclusions.

A schedule is efficient but not universal. A rate developed for normal soil, accessible site conditions and ordinary working hours may be inappropriate for rock excavation, restricted access or night work. Rates should therefore state assumptions and be periodically reviewed for labour, material, fuel, transport and regulatory changes.

6. Cost Abstract and Estimate Summary

The cost abstract groups detailed items into meaningful work sections. It provides management with a concise view while retaining a link to the detailed sheets.

For the illustrative package, suppose preliminaries are 8% of measured work, contingencies are 5% of the subtotal after preliminaries, and no provisional sum or tax is included:

• Measured work = 4,709.00
• Preliminaries = 0.08 × 4,709.00 = 376.72
• Subtotal = 5,085.72
• Contingency = 0.05 × 5,085.72 = 254.286 ≈ 254.29
• Estimated total = 5,340.01

The stated bases are essential. A contingency applied only to measured work would produce a different result. Rounding should normally occur at the final presentation stage, while underlying calculations retain adequate precision.

A useful abstract may contain site preparation, substructure, frame, walls, finishes, services, external works, preliminaries, provisional sums, contingencies, taxes and grand total. It should reconcile exactly with detailed pages.

7. Preliminaries, Provisional Sums and Contingencies

Preliminaries

Preliminaries are project-level costs necessary to organize and operate the work but not normally measured as permanent construction quantities. They may include mobilization, site offices, temporary services, fencing, security, surveys, setting out, safety arrangements, insurance, bonds, testing, cleaning and demobilization. They can be priced as lump sums, time-related items or percentage-related items. The method should reflect actual cost behaviour and avoid duplication with measured rates.

Provisional sums

A provisional sum is an allowance for work or services whose scope or extent cannot reasonably be defined at tender stage. Examples include unforeseen utility diversion, specialist testing or nominated specialist work, depending on contract rules. It is not automatically a contractor’s profit and is not evidence that the work will definitely be required. When the scope becomes known, the sum should be replaced or adjusted in accordance with the contract, with supporting measurement and valuation.

Contingencies

Contingency is an allowance for identified uncertainty within the estimate’s defined scope, such as quantity development, minor design refinement or reasonably foreseeable conditions. It is not a permission to conceal omissions, poor measurement or unauthorized scope growth. The percentage or amount should be justified by project maturity, risk register and estimating policy. Client-held risk allowances and contractor risk pricing should be distinguished to prevent double counting.

8. Work-Charged Establishment

Work-charged establishment refers to personnel and associated costs charged directly to a particular project or work, rather than maintained as a permanent general establishment. It may include site engineers, supervisors, surveyors, clerks, safety staff and temporary technical support, where permitted by the governing accounting and contract system.

The estimator should define posts, duration, salary or wage, allowances, accommodation, travel, communication and associated equipment. For a monthly staff cost:

where Cₛ is staff cost, n is number of personnel, cₘ is cost per person per month and d is duration in months. If different personnel have different rates, calculate each category separately and sum the results. The cost must not be included again in preliminaries or overhead mark-ups unless the pricing rules expressly require it.

9. Preparing the Complete Estimate Report

A professional report commonly includes:

1. title page, project identification and estimating authority;
2. purpose, scope and description of the project;
3. drawings, specifications, site information and revision status;
4. measurement rules, units, currency and price date;
5. assumptions, exclusions and design information still outstanding;
6. detailed take-off and BOQ;
7. rate build-ups and source quotations;
8. work-charged establishment and preliminaries;
9. provisional sums, contingencies, taxes and escalation treatment;
10. cost abstract and grand total;
11. programme assumptions and cash-flow implications where required;
12. checking certificate, sign-off and appendices.

A checking procedure should include independent arithmetic checks, comparison of dimensions with drawings, review of omissions and duplication, reconciliation between BOQ and abstract, verification of units, confirmation of rate dates, and a reasonableness comparison with similar projects. Electronic spreadsheets and estimating software improve speed, but formulas, version control, locked cells, audit trails and independent review remain necessary.

10. Applied Case: Illustrative Community Classroom

Background

A local authority proposes a single classroom with a floor area of 72 m². The design is at developed-detail stage, and approval is required before tendering. This is a fictional illustrative case.

Problem

The project team has a drawing set, outline specification and recent supplier quotations but no cost report. It must establish an approval cost while identifying uncertainty.

Analysis and approach

The estimator first confirms the drawing revision and divides the work into substructure, superstructure, finishes, services and external works. Quantities are measured using the adopted standard. Each item receives a clear description and rate. A quotation for roofing is checked for delivery and tax status; labour productivity is compared with recent local projects.

Suppose the measured work sections are:

Preliminaries are assessed at 7%: 0.07 × 80,000 = 5,600. A defined but not yet designed laboratory testing service is included as a provisional sum of 2,000. A 4% design-development contingency is applied to measured work plus preliminaries: 0.04 × (80,000 + 5,600) = 3,424. Before tax, the approval estimate is therefore 80,000 + 5,600 + 2,000 + 3,424 = 91,024.

Outcome

The report identifies the total as an approval estimate, not a fixed contract price. It records that the provisional sum will be instructed and valued when defined, and that market prices are based on a stated date. The client can compare the total with available funding and decide whether to proceed to tender.

Learning points

The case demonstrates that a reliable estimate depends on scope definition, traceable measurement, justified allowances and explicit assumptions. A single percentage added to an unexplained total would provide less control and weaker accountability.

11. Limitations, Ethics and Contemporary Practice

No estimate can remove uncertainty. Inflation, exchange rates, supply disruption, abnormal ground, design changes, weather and productivity variation can alter final cost. The estimate should communicate uncertainty rather than imply false precision.

Ethically, an estimator must not manipulate quantities to obtain approval, hide exclusions, use obsolete rates knowingly, duplicate allowances or favour a supplier without justification. Confidential quotations and personal data must be handled responsibly. Public projects additionally require procurement rules, segregation of duties, records of approvals and auditability.

Digital take-off, BIM quantity extraction, cloud collaboration and AI-assisted checking can identify inconsistent descriptions, missing items and unusual rates. They do not replace professional judgment: models may contain omissions, wrong classifications or outdated revisions. AI-generated quantities or rates must be validated against drawings, specifications, site conditions and approved measurement rules. The responsible professional remains accountable for the submitted estimate.

Best-Practice Checklist

Before issue, confirm that:

• every item has a unique number, description, unit and quantity;
• dimensions can be traced to drawings or site records;
• measurement rules are stated and consistently applied;
• rates include the intended resources and exclude no necessary scope;
• preliminaries, provisional sums and contingencies have separate definitions;
• taxes, escalation and currency basis are explicit;
• detailed pages reconcile with the abstract;
• arithmetic, formulas and rounding have been independently checked;
• assumptions, exclusions, risks and drawing revisions are recorded;
• the report has date, author, checker and approval status.

Chapter Summary

Detailed estimating converts project information into measured work, resource-based rates and an auditable cost forecast. A BOQ presents defined items with units, quantities, rates and amounts; a schedule of rates provides reusable standard prices; and a cost abstract groups detailed information for decision-making. Preliminaries cover project organization, provisional sums allow for incompletely defined work, contingencies address justified uncertainty, and work-charged establishment records project-specific staff and support costs. Reliable reports state their basis, assumptions, exclusions, price date and measurement rules. Independent checking, ethical disclosure, version control and professional judgment are essential, particularly when digital tools or AI assist the process.

Key Terms

Detailed estimate: A quantity-based estimate prepared from measured work items.

Bill of quantities: A structured list of described work items, units and quantities for pricing and valuation.

Item rate: The price per unit of a measured work item.

Take-off: The process of extracting dimensions and quantities from project information.

Cost abstract: A summarized grouping of detailed estimated costs.

Schedule of rates: A list of standard work items and unit prices.

Preliminary estimate: An early estimate based on limited design information.

Revised estimate: An estimate updated when cost or scope changes materially.

Preliminaries: Project-level costs required to organize and operate construction.

Provisional sum: An allowance for work whose scope or extent is not sufficiently defined.

Contingency: An allowance for justified uncertainty within the stated scope.

Work-charged establishment: Project-specific personnel and support costs charged to the work.

Quantity: The measured extent of an item in its applicable unit.

Rate build-up: Analysis of materials, labour, plant, overheads and profit composing a rate.

Measurement rules: Standard conventions governing descriptions, units, deductions and inclusions.

Cost reconciliation: Agreement between detailed pages, section totals and the final abstract.

Price base date: The date to which rates and market prices relate.

Audit trail: Evidence showing how quantities, rates and approvals were developed.

Review Questions

Short Answer Questions

1. What is the principal difference between a preliminary estimate and a detailed estimate?
2. State three items commonly included in preliminaries.
3. Why must a BOQ state units for every measured item?
4. Define a provisional sum.
5. What is the purpose of a cost abstract?

Long Answer Questions

1. Explain the complete process of preparing an item-rate estimate from drawings to approval.
2. Describe how a unit rate may be built up from materials, labour, plant, overheads and profit.
3. Discuss the distinction between preliminaries, contingencies and provisional sums.
4. Explain the contents and quality-control requirements of a professional estimate report.
5. Describe the role, calculation and control of work-charged establishment.

Analytical / Application Questions

1. A project has measured work of 250,000 currency units. Preliminaries are 6% of measured work and contingency is 5% of measured work plus preliminaries. Calculate the total before tax and explain the basis of each allowance.
2. A BOQ contains excavation, concrete and backfill, but the drawings also show reinforcement and formwork. Identify the estimating risks and propose a checking procedure.
3. A contractor argues that a provisional sum should be treated as completed work in the tender total and also marked up again as profit. Evaluate this position using clear contract and costing principles.

Multiple Choice Questions with Answers

Question 1

A rectangular foundation trench is 10 m long, 0.6 m wide and 0.8 m deep. What is its excavation quantity?

A. 4.8 m³

B. 6.0 m³

C. 8.0 m³

D. 10.6 m³

Correct Answer: A. 4.8 m³

Question 2

Which document groups detailed estimated items into major work sections for management review?

A. Take-off sheet

B. Cost abstract

C. Site diary

D. Material test certificate

Correct Answer: B. Cost abstract

Question 3

A unit rate normally represents the cost of completing:

A. The whole project

B. One unit of a defined work item

C. Only the materials for a project

D. Only the contractor’s profit

Correct Answer: B. One unit of a defined work item

Question 4

Which allowance is intended for work whose scope cannot reasonably be defined at tender stage?

A. Contingency

B. Preliminary item

C. Provisional sum

D. Measured quantity

Correct Answer: C. Provisional sum

Question 5

Measured work is 4,709 currency units. If preliminaries are 8% of measured work, what are the preliminaries?

A. 376.72

B. 377.00

C. 4,332.28

D. 5,085.72

Correct Answer: A. 376.72

Question 6

Which practice best prevents double counting in an estimate?

A. Adding every allowance to every rate

B. Defining inclusions and exclusions for each cost component

C. Using only rounded rates

D. Removing all preliminaries

Correct Answer: B. Defining inclusions and exclusions for each cost component

Question 7

A schedule of rates is particularly useful for:

A. Recurring work and valuation of variations

B. Replacing all drawings and specifications

C. Recording weather observations only

D. Determining soil classification without investigation

Correct Answer: A. Recurring work and valuation of variations

Question 8

Which information is essential for interpreting an estimate’s prices?

A. The estimator’s preferred colour scheme

B. The price base date

C. The contractor’s lunch schedule

D. The number of pages in the report

Correct Answer: B. The price base date

Question 9

Work-charged establishment most directly refers to:

A. Permanent national taxation

B. Project-specific personnel and support costs

C. The cost of cement only

D. A deduction for defective work

Correct Answer: B. Project-specific personnel and support costs

Question 10

What is the most appropriate role of AI-assisted quantity checking?

A. To replace approval and professional responsibility

B. To validate outputs against drawings, specifications and measurement rules

C. To create undisclosed contingency allowances

D. To ignore drawing revisions

Correct Answer: B. To validate outputs against drawings, specifications and measurement rules