What Are Preliminaries in Construction? A Plain-English Guide

If you've ever read through a Bill of Quantities or a tender document and hit a section simply labelled "Preliminaries" — or just "Prelims" — you've found the part that confuses most people. It's not bricks, it's not labour for a measured task, and yet it's often one of the biggest single sums on the whole document.

Preliminaries are the costs of running the site as a whole, rather than the cost of building any one part of it. They're real, unavoidable costs — and under-pricing them is one of the fastest ways to turn a profitable job into a loss.

What Are Preliminaries, Exactly?

Preliminaries are the project-wide costs that aren't tied to a single measured item of work. Laying a brick has a clear rate per square metre. But the welfare cabin the bricklayer uses, the site manager who supervises the work, the scaffold everyone stands on, and the skip the offcuts go into — none of those belong to one measured item. They support the entire project.

So instead of trying to bury those costs inside the rate for every brick and board (which would be guesswork), the BOQ pulls them out into their own section. That's preliminaries.

Why Are Prelims Separated Out in a BOQ or Tender?

Separating prelims keeps the measured rates clean and comparable. When a client receives three tenders, they can line up the measured work item by item — but only if the site-running costs are pulled out into their own section. Otherwise every contractor would spread those costs differently and no two quotes would be comparable.

It also makes the project easier to manage once it's running. Because prelims are itemised separately, both the client and contractor can see exactly what the site setup, management, and overheads are costing — and, crucially, how those costs behave if the programme changes.

What's Included in Preliminaries?

The exact list varies by project, but a thorough prelims section usually covers the following:

Not every project carries every line. A small domestic extension won't need a tower crane or a security guard, but it will still need scaffold, a skip, welfare, insurance, and supervision. The principle is the same at any scale — these are the costs of being on site.

Fixed vs Time-Related Preliminaries

This is the distinction that catches people out, and it's the most important thing to understand about prelims.

Fixed (one-off) prelims

These are one-off costs that don't change if the job runs long. You pay them once regardless of how many weeks the project takes. Examples include delivering and removing the welfare cabins, setting out the site, the performance bond, and the final clean. Whether the build takes 10 weeks or 14, you set up once and clean down once.

Time-related prelims

These costs accrue every week the site is open. The site manager's wages, the scaffold hire, the cabin rental, temporary power, insurance — all of these keep ticking for as long as the job runs. If the programme slips from 10 weeks to 14, the time-related prelims go up by roughly 40%, even though no extra measured work has been done.

How Much Are Prelims as a Percentage of Contract Value?

There's no universal figure, but prelims commonly land somewhere between 7% and 15% of contract value. The key driver is the relationship between project size and duration: small, short jobs carry proportionally heavier prelims because the fixed setup costs are spread over a smaller pot of measured work. Large, long projects dilute those fixed costs and tend to sit at the lower end.

Treat these as a sense-check rather than a rule. The right way to price prelims is bottom-up — work out the actual cost of each item over the real programme — and then compare the result against these benchmarks to make sure nothing's been missed.

Prelims vs Overheads & Profit — What's the Difference?

It's easy to lump prelims in with Overheads & Profit (OH&P), but they're different things.

Preliminaries are the cost of running this specific site — its own scaffold, its own manager, its own welfare. They exist because the project exists, and they disappear when it finishes.

Overheads are the cost of running the business behind the project — the head office, the accounts team, the estimators, the vans, the directors. A slice of those company costs is recovered on every job, usually as a percentage added on top.

Profit is the margin the contractor earns for taking on the work and its risk — again, typically a percentage on top.

So a tender total is roughly: measured work + preliminaries + overheads + profit. Prelims sit with the measured cost of the project; OH&P is the markup the business applies to the whole lot.

Common Mistakes With Preliminaries

Under-pricing prelims to win the tender. Because prelims are a single, visible section, they're an easy target when someone wants to shave a quote down. But the scaffold, the manager, and the insurance still have to be paid — cutting the prelims figure doesn't cut the cost, it just moves the loss onto your own margin.

Forgetting time-related costs when the programme slips. The classic error is pricing prelims as one lump sum without splitting fixed from time-related. When the job overruns, the time-related costs keep running but there's no mechanism to recover them — and you eat the difference.

Burying prelims in the measured rates. Spreading site-running costs across every brick rate makes your quote impossible to compare and your variations impossible to price. Keep prelims in their own section where they belong.

Copying last job's prelims. Prelims are programme-driven. A job that's the same value but twice as long carries very different time-related costs. Price them against the actual programme every time.