What Are Preliminaries (Prelims)?

A complete prelims schedule covers far more than a site shed and a set of amenities. It typically groups costs under site establishment, management and supervision, temporary services, shared access and plant, statutory and insurance costs, and demobilisation. A typical prelims section contains:

• Site establishment — temporary fencing, hoarding, signage, site sheds, amenities and power/water connections.
• Supervision and management — site manager, foreman, leading hand and project administration time for the duration of the works.
• Scaffolding, hoists and cranage — shared access and lifting equipment used across multiple trades.
• Temporary services — power, water, site telephone/data, and temporary lighting during construction.
• Insurances, permits and statutory costs — contract works insurance, public liability, council permits and inspection fees.
• Cleaning and demobilisation — progressive site clean-up, waste removal and final de-establishment at completion.

Preliminaries split into two cost behaviours, and mixing them up is a common pricing error. Fixed prelims are incurred once regardless of how long the job takes — mobilisation, demobilisation, connection fees and one-off permits. Time-related prelims recur for every week or month the site remains open — supervision wages, shed hire, temporary fencing and site insurances.

This distinction matters because it determines how a delay affects cost. If the program blows out by four weeks, the fixed items do not change, but every time-related item keeps accruing — another four weeks of supervision, shed hire and insurance. That is also why an approved extension of time on a contract usually carries a corresponding adjustment to prelims, not just an extended completion date.

• Fixed prelims — mobilisation, demobilisation, connection and disconnection fees, one-off permits and approvals.
• Time-related prelims — supervision, site facilities hire, temporary services and insurances, calculated for the duration on site.

Prelims are best built up item by item from the construction program, not estimated as a flat percentage of the job. An estimator lists every prelim item, decides whether it is fixed or time-related, and prices time-related items against the number of weeks or months the program shows the site will be open.

Because the build-up depends on program duration, an accurate program is the single biggest input to an accurate prelims price — a program that is optimistic by even a few weeks understates every time-related line. Rules of thumb still have a place: many estimators cross-check their build-up against a percentage of the total job value as a sense check, but the percentage should confirm the build-up, not replace it.

Because most prelims are time-related, the program and the prelims price are locked together for the life of the job. A tight, realistic program keeps prelims lean; a program with slack that then gets used up quietly inflates the real cost of running the site beyond what was priced.

This link cuts both ways during construction. If the client causes a delay — a late decision, a variation that adds time, or restricted access — the builder has grounds to claim the extra time-related prelims that delay caused, on top of any extension of time. Keeping a clear program and an itemised prelims build-up from day one makes that claim far easier to prove.

• Client-caused delay or a variation that adds time to the program.
• An extension of time approved under the contract.
• Practical completion reached later than the original program date, through no fault of the contractor.

On larger projects a quantity surveyor or senior estimator typically prices prelims as its own exercise, working closely with the project manager who will actually run the site, because they best understand staffing levels, site logistics and the shared equipment multiple trades will need.

On smaller and mid-sized jobs, the builder or head contractor prices their own prelims as part of preparing the tender — often the part of the estimate that gets the least time, simply because trade quantities feel more concrete to measure. Software like My Trade Hub helps close that gap by generating a structured prelims schedule alongside the measured Bill of Quantities, so site-wide costs get the same rigour as the trade items.

It helps to be precise about what prelims are not, since the term is often used loosely alongside similar-sounding costs:

• Preliminaries vs overheads — prelims are project-specific costs tied to running this site; overheads are the builder’s general business costs (office, admin, company-level insurances) recovered across all jobs, usually via a margin.
• Preliminaries vs margin — margin is the builder’s profit and risk allowance added on top of cost; prelims are an actual cost of running the site, not a markup.
• Preliminaries vs provisional sums — a provisional sum is an allowance for undefined trade work; prelims are defined, site-wide running costs that apply regardless of the final trade scope.

Most prelims blowouts trace back to a small set of recurring errors:

• Pricing prelims as a flat percentage without a build-up, so an unusually long or short program is not reflected in the price.
• Under-resourcing supervision on a complex job because the prelims schedule was copied from a simpler previous project.
• Forgetting to reprice time-related items when the program changes during tendering or after award.
• Not claiming prelims on approved extensions of time, leaving genuine additional running costs unrecovered.