BOQ Preliminaries in Construction: A Complete Guide for Quantity Surveyors and Estimators

A QS joined a commercial fit-out project in its third week. The tender had been submitted six weeks earlier, the contract had been awarded, and work had mobilised quickly. She was asked to take over the commercial management from a colleague who had moved to another project.

The first thing she did was open the preliminaries section of the priced BOQ and compare it against the current site costs. What she found was a gap she had not expected. The tender prelims had been priced carefully — site office costs, project staff, temporary utilities, security, scaffolding. All reasonable allowances based on the tendered programme. But the programme had slipped by four weeks during mobilisation. Additional trades had been brought in to recover time. The temporary generator had been upgraded to handle the increased electrical load. Two extra site supervisors had been appointed.

None of these changes had been formally captured as cost adjustments. The site team had dealt with each one operationally without connecting them back to the prelims budget. By the time she reviewed the costs in week three, the preliminaries section was already running at a rate that, if sustained over the remaining programme, would overspend its tender allowance by approximately 11 percent.

This is the gap that sits inside the preliminaries section of almost every BOQ — not at tender stage, where most QS professionals price it carefully, but during construction, where it drifts quietly while attention is focused on the measured works. Understanding what BOQ preliminaries contain, how they are calculated, and how their costs actually behave on a live project is the foundation of keeping them under control.

What Are Preliminaries in a BOQ — and Why They Sit Separately from the Measured Works

In a standard BOQ structure, the document is divided into three principal sections: preliminaries, measured works, and provisional sums. The measured works section contains everything that becomes part of the finished building — concrete, reinforcement, brickwork, finishes, mechanical and electrical installations, each item measured and described with a quantity and a rate. Provisional sums cover defined and undefined work that cannot yet be fully measured or specified.

Preliminaries sit before all of that — as a separate section covering everything required to organise, manage, and support the construction process without forming part of the completed structure. They are sometimes called prelims or site overheads. They represent the cost of making construction possible rather than the cost of construction itself.

The reason preliminaries appear as a separate BOQ section rather than being distributed across the measured works is that they cannot be meaningfully allocated to individual work items. The cost of the project manager on site for twelve months cannot be attributed to a cubic metre of concrete or a square metre of blockwork — it serves the entire project. The temporary electricity supply powers every trade on site simultaneously. The performance bond covers the contract sum as a whole. These costs exist at project level, not at work item level, and the BOQ structure reflects that by containing them in their own section.

For the QS preparing the BOQ, the preliminary section is also where the client gets visibility of all the contractor's site-running costs in one place. A well-structured preliminary section tells the client exactly what operational infrastructure the contractor is committing to provide — and gives both parties a clear baseline against which cost changes can be assessed if the programme extends or the scope expands.

Fixed vs Time-Related Preliminaries — The Distinction That Changes How You Price Them

The single most important concept in pricing preliminaries accurately is the distinction between fixed costs and time-related costs. Every item in the preliminary section falls into one of these two categories — and understanding which is which determines how the tender allowance should be calculated and how cost changes are assessed when the programme moves.

Fixed Preliminary Costs

Fixed preliminary costs are incurred once regardless of how long the project runs. They are associated with specific events — mobilisation, demobilisation, contract formation — rather than with the passage of time. A performance bond is issued at contract award and its cost is determined by the contract sum, not the programme duration. The cost of connecting a temporary electricity supply from the distribution network has a fixed connection charge that applies whether the project runs for six months or eighteen. The initial delivery and erection of a tower crane involves fixed mobilisation costs that do not increase if the programme extends.

Fixed costs in the preliminary section should be priced as lump sums — a single amount that covers the activity regardless of duration. When the programme changes, these items do not change with it. A programme extension does not increase the performance bond cost or the scaffolding erection cost.

Time-Related Preliminary Costs

Time-related preliminary costs accumulate continuously for as long as the project runs. Project staff salaries are the clearest example — the project manager, site manager, site engineer, and QS are employed for the full construction programme, and every week the programme extends adds another week of their combined cost. Site office rental is charged weekly. Security patrols run every night. Temporary power standing charges accumulate monthly. Scaffolding rental is calculated per week.

Time-related costs should be priced by calculating the weekly or monthly rate and multiplying by the programmed duration. This is where the connection between the preliminary section and the construction programme becomes critical. A preliminary allowance calculated for a 36-week programme that actually runs for 44 weeks will be underfunded by the cost of 8 weeks of full site operations — which on a medium-size project can represent a very significant sum.

BOQ Preliminaries Full List — With Real Project Examples and Cost Drivers

The table below covers the preliminary items that appear in most commercial construction BOQs. Each item is described with a real project example, its cost type, and the specific factor that drives its cost — because understanding the cost driver is what allows accurate pricing at tender and accurate monitoring during construction.

The cost driver column in this table is the most commercially useful information for a QS managing a project. When the programme extends, time-related items increase proportionally. When the scope expands and additional trades are brought in, staff and security costs increase. When the client instructs additional work that requires the scaffolding to remain in place longer, the scaffolding rental period increases. Each of these changes has a preliminary cost implication — and that implication should be assessed and recorded as a variation at the time the change is made, not discovered in a cost report three months later.

How to Calculate Preliminaries in a Construction BOQ — The Method That Works on Real Projects

Calculating preliminary costs for a BOQ requires a more structured approach than simply applying a percentage to the measured works total. The percentage method — typically cited as eight to fifteen percent of the overall project cost — is a useful order-of-magnitude check, but it is not a reliable pricing method for a competitive tender. Projects with short programmes and large measured works values will have a lower preliminary percentage than projects with long programmes and complex site management requirements. The percentage tells you where to look — the detailed calculation is what gives you a defensible number.

Step 1 — Establish the Construction Programme

Before any preliminary cost can be calculated, the construction programme must be agreed in draft. The programme defines the duration against which all time-related costs are multiplied. If the programme has not been developed at tender stage, the QS must develop a realistic duration estimate based on the scope of work and available resources — pricing against an optimistic programme and then running over it is one of the most consistent sources of preliminary budget failure.

Step 2 — List Every Required Preliminary Item

Work through the project requirements systematically and identify every operational item the project needs — site establishment, staffing levels, temporary services requirements, safety obligations, administrative requirements, and any contractual obligations such as bonds, insurances, and warranties. Use the contract documents and the employer's requirements as the primary reference — some clients specify particular preliminary items that must be provided, and omitting them at tender stage creates a cost the contractor must absorb.

Step 3 — Separate Fixed and Time-Related Costs

For each preliminary item, determine whether it is fixed, time-related, or a combination of both. Scaffolding, for example, has a fixed erection and dismantling cost and a time-related weekly rental. The site office has a fixed delivery and installation cost and a time-related monthly rental. Separating these components for each item ensures that the preliminary allowance responds correctly to programme changes — fixed costs stay constant, time-related costs adjust with duration.

Step 4 — Apply Programme Duration to Time-Related Costs

Multiply each time-related weekly or monthly rate by the programme duration. Add a programme risk allowance — typically five to ten percent of the time-related total — to reflect the realistic possibility that the programme will extend beyond the tender estimate. Projects that run exactly to tender programme are the exception rather than the rule, and a preliminary allowance that has no tolerance for programme movement will be underfunded on most projects.

Step 5 — Cross-Check Against the Construction Programme

With all preliminary costs calculated, review them against the programme to confirm that the costs are aligned with the planned sequence. Site establishment costs should peak at the start. Major temporary works costs should align with the structural programme. Commissioning and testing allowances should be positioned against the M&E programme. Costs that are inconsistent with the programme sequence indicate either a pricing error or a programme that does not reflect the actual planned construction approach.

For a detailed guide on how the preliminary section connects to the broader BOQ structure — and how the BOQ as a whole is prepared accurately under project pressure — see our article on How to Prepare a BOQ Faster Without Losing Accuracy.

Why Preliminary Costs Drift During Construction — and How to Catch It Early

Returning to the fit-out project from the opening — the QS who joined in week three found a preliminary budget that was already drifting because changes had been made operationally without being captured commercially. This pattern is not unusual. It is, in fact, one of the most consistent patterns in construction cost management across project types and sizes.

Preliminary costs drift during construction for three main reasons. The first is programme slippage — the project falls behind its planned sequence and the site team makes operational decisions to recover time without formally assessing the preliminary cost of those decisions. Additional staff are brought in, extended working hours are agreed, temporary works are upgraded. Each decision is justified by the programme pressure. None of them are recorded as cost changes.

The second reason is scope growth in the preliminary section itself — items that were included in the tender at one level of provision are upgraded during construction because the project requires more than was anticipated. The site office specified for six people ends up accommodating twelve because the client's team is working from site. The security specification that assumed daytime-only patrols needs upgrading to 24-hour cover because of material theft incidents.

The third reason is the absence of a formal preliminary monitoring routine. On projects where the QS is focused primarily on the measured works — valuations, variations, subcontract management — the preliminary section can run for months without a formal cost review. By the time the overrun is visible in a monthly report, the deviation has been accumulating for weeks and is no longer manageable through efficiency improvements alone.

The monitoring habits that prevent preliminary cost drift:

•       Weekly preliminary cost review: A short review of all time-related preliminary items every week — actual cost versus budget rate, current programme duration versus tender duration, any items where the provision has changed

•       Programme-linked cost update: Whenever the programme is updated — whether at the contractor's own initiative or following a client instruction — the time-related preliminary costs are recalculated against the new duration and the change is assessed as a potential variation

•       Operational change capture: Any operational decision that affects the preliminary section — additional staff, upgraded services, extended temporary works — recorded at the time of the decision with a cost assessment, not discovered in a monthly report

•       Preliminary variation register: A separate variation register for preliminary changes — programme extension entitlements, instructed changes to the preliminary specification, and unforeseen additional requirements — maintained alongside the measured works variation register

The connection between programme management and preliminary cost control is one of the most direct relationships in construction commercial management. For a detailed guide on how programme and cost decisions interact and where the most significant overruns originate, see our article on Construction Project Management Mistakes That Blow Your Budget.