If you’ve ever looked at a construction budget and wondered what “preliminaries” actually covers, you’re not alone. It’s one of those line items that sits near the top of every bill of quantities, often carrying a bigger number than people expect, and rarely gets explained properly before it lands on the page.

Preliminaries are one of the most underestimated parts of a construction budget, and understanding what they actually cover is a genuinely useful thing to know before you commit to a project, whether you’re a client trying to make sense of a quote or a contractor trying to price one properly.

What Preliminaries Actually Cover

Preliminaries are the costs of running the site itself, separate from the physical materials and labour that go into the building. Think of it as everything needed to keep a construction project operating safely and efficiently, rather than the building work you can see going up.

That includes site management and supervision staff, site accommodation and welfare facilities for the workforce, temporary power, water, and telecommunications, security and site hoarding, insurances and bonds, plant and equipment that services the whole site rather than one specific task, and health and safety provisions across the project.

None of it ends up visible in the finished building, but all of it is essential to actually delivering the project, and none of it is free.

Why the Number Is Often Bigger Than People Expect

Preliminaries commonly run between 10 and 20 percent of the total project cost, and on complex or long-duration projects, it can go higher. That surprises a lot of clients, particularly on smaller projects where it can feel like a disproportionately large chunk of the budget for work that isn’t, strictly speaking, “the building.”

But the logic holds up. A site running for twelve months needs a site manager on site for twelve months. A site with a compressed access point needs security and traffic management for the full duration of the works. A complex build with multiple trades on site simultaneously needs more coordination, more welfare capacity, and more supervision than a simple single-trade job. Preliminaries scale with the complexity and duration of the project, not with the size of the finished building.

Where Preliminaries Get Underestimated

The most common mistake is pricing preliminaries as a flat percentage applied automatically, rather than actually thinking through what the specific project needs. A straightforward site with easy access and a short programme needs a very different preliminaries allowance to a complex site with restricted access and a long build duration, and treating them the same way in the budget is where the numbers start to go wrong.

Programme extensions are another common trap. If a project runs longer than planned, whether from weather delays, design changes, or unforeseen site conditions, preliminaries costs keep accruing for every extra week the site stays open. A delay that seems manageable on the surface can quietly add a significant sum once the extended preliminaries cost is factored in, and it’s an impact that’s easy to underestimate until it actually happens.

Site-specific factors also get missed regularly. A tight urban site with restricted access, limited storage space, and strict noise or working-hour restrictions from the local authority will always carry higher preliminaries than an open rural site, even for a building of similar size and spec.

Why This Matters for Your Budget

Preliminaries aren’t a padding item added on top of the “real” costs. They’re a genuine, necessary part of delivering the project safely and to programme, and they deserve the same careful, project-specific thinking as every other line in the budget.

Getting this figure right matters for two reasons. Underestimate it, and you’re heading for a budget shortfall that has nothing to do with the physical building work and everything to do with simply keeping the site running. Overestimate it without understanding why, and you risk pricing yourself out of a tender you were otherwise well placed to win.

The way to get it right is to have preliminaries calculated specifically for your project, based on its actual duration, complexity, and site conditions, rather than a generic percentage carried over from a different job. That level of detail is exactly what a proper cost plan should provide, and it’s a core part of the estimating and cost management work we do for clients at Swiftcore.

If you want to understand exactly what your project’s preliminaries should look like, rather than guessing at a percentage, get in touch with Swiftcore and we’ll help you build a budget that reflects the real cost of running your project, not just the building itself.