What Are Construction Preliminaries and Why Do They Add So Much to Your Estimate?

Preliminaries add so much to an estimate because they cover genuinely necessary costs that scale with project duration and complexity, not just project size. A project that takes twice as long to build carries roughly twice the weeks of site management, welfare provision, and temporary services, regardless of how much the direct building work itself has changed.

This is the importance of preliminaries in a nutshell: they’re the cost of keeping a site safe, staffed, serviced, and compliant for every single day it’s open, and those daily costs accumulate quickly across a typical construction project.

Site setup covers establishing access, a compound, and storage areas before any building work begins. Welfare facilities, toilets, rest areas, and washing facilities for the workforce, are a legal requirement on UK sites, not an optional extra, and are one of the more visible examples of preliminaries on a typical job.

On a small domestic project this might be a single welfare unit and a lockable storage container. On a larger site, it can extend to a full site cabin complex with separate management offices, canteen facilities, and drying rooms for wet-weather work.

Site management costs cover the site manager, supervisors, and any project management staff needed to run the job day to day, distinct from the tradespeople actually carrying out the building work. On larger projects, this can also include a dedicated quantity surveyor or commercial team tracking cost and progress throughout the build. UK contractors of all sizes need to budget for this layer of project management, since it doesn’t disappear just because a job is smaller.

Scaffolding, along with other access equipment needed to safely carry out the building work, is a classic preliminary item, since it enables the work rather than forming part of the finished building. The scale and duration of scaffolding hire scales directly with project size and programme length.
Access equipment isn’t limited to scaffolding either. Depending on the project, this category can include hoists, mobile access platforms, and temporary staircases, all priced and hired for as long as the relevant trades need them on site.

Site security, whether that’s fencing, cameras, or an on-site presence, protects materials, plant, and the site itself, particularly in urban areas or wherever expensive materials are stored on site for extended periods. Security costs are typically time-related, running for as long as the site remains open.

The right level of security depends heavily on location and what’s stored on site. A rural self-build with limited valuable materials on site overnight needs far less than an urban commercial project storing expensive plant and fittings behind a public-facing boundary.

Contract works insurance and public liability cover are generally included within preliminaries, since they protect the project and third parties for the duration of the build. These costs are a genuine part of running the project safely and shouldn’t be confused with the professional indemnity insurance a designer or consultant might separately carry.

Site signage and hoarding, covering health and safety notices, project information boards, and site boundary hoarding, are smaller preliminary items individually but add up across a project, particularly on sites with public-facing boundaries.

Planning and design costs, such as the administrative work of confirming planning permission and building regulations compliance before and during the build, sometimes sit adjacent to preliminaries in a cost plan, even though the fees themselves are often accounted for separately. It’s worth checking exactly how your own estimate has treated this boundary.

Preliminaries percentages vary, but UK industry commentary commonly places them somewhere between 5% and 15% of total project cost, with figures toward the middle and upper end of that range common on more complex or longer-duration builds. According to BCIS, the RICS-owned cost information service, contract percentages for preliminaries can be benchmarked against real UK project data segmented by project value, since smaller projects and larger ones tend to carry meaningfully different preliminary percentages.

There are two broad approaches to pricing prelims, and both are a form of cost estimation rather than guesswork. The percentage method applies a benchmarked percentage, informed by data such as BCIS contract percentages, to the estimated building works cost, which is a quick and practical approach during early cost planning. The detailed, itemised method prices each preliminary item and duration individually, staff costs, welfare hire rates, security costs, and so on, which gives contractors a more accurate figure once the project and its programme are better defined. A proper cost analysis will usually move from the percentage method early on to the itemised method as the entire project becomes better defined, and most experienced contractors will want to see both before agreeing a final figure.

Several factors push a project’s preliminaries percentage up or down. Project size matters, since smaller jobs often carry a higher percentage because fixed costs like insurance and site management make up a proportionally larger share of a smaller total. Site conditions, such as restricted access, an urban location, or a constrained site, also increase preliminaries, since they typically require more time and more careful planning to work around safely. Good construction planning at an early stage helps identify these factors before they become expensive surprises mid-build.

Project duration has one of the most direct effects on preliminaries, since so many preliminary items are time-related. A programme that overruns doesn’t just delay completion, it directly increases the cost of site management, welfare, security, and scaffolding hire for every additional week the site remains open, which is one of the clearest financial arguments for keeping a construction job on programme.

This is also why a realistic programme matters as much as a realistic budget when an estimate is first prepared. An optimistic timeline that later slips doesn’t just cause frustration, it quietly inflates the time-related portion of preliminaries well beyond what was originally allowed for.

Preliminaries look somewhat different depending on project type. Domestic projects tend to have simpler, smaller-scale preliminaries, a single welfare unit and basic site security, for example, while larger commercial or multi-unit projects carry more extensive site management structures, more substantial temporary infrastructure, and correspondingly higher preliminary costs in absolute terms, even where the percentage itself may be lower. Construction companies working across multiple project types generally hold separate benchmarks for each, rather than applying one flat assumption to every construction project.

Preliminaries and contingencies in construction serve different purposes and shouldn’t be blended together in an estimate. Preliminaries cover the known, ongoing costs of running the site. Contingency is a separate allowance for genuinely unforeseen costs. Keeping the two distinct, rather than folding risk management allowances into a vague preliminaries figure, gives everyone a clearer picture of where the budget is actually going.

Preliminaries typically appear as their own dedicated preliminaries section within a bill of quantities, itemised separately from the measured trade work sections that make up the rest of the document. Our guide to NRM2, the current RICS standard for measuring building works, covers how a bill of quantities should be structured in more detail, including how preliminary items are treated within it.

The importance of preliminaries comes down to this: without them, a building project simply can’t run safely, legally, or efficiently. The key components, management, welfare, access, security, and compliance, aren’t optional line items competing with your building budget, they’re what makes the rest of the budget deliverable at all.

Treating preliminaries as a genuine, carefully planned budget category, rather than a rough percentage bolted on at the end, is one of the clearest ways to avoid cost surprises once a project is underway.

A typical construction preliminaries breakdown includes site management staff, welfare facilities, temporary utilities, scaffolding and access equipment, site security, signage and hoarding, contract works insurance, health and safety compliance costs, and the administrative costs of running the site day to day. Many preliminaries in construction projects overlap across these categories, which is exactly why an itemised breakdown, rather than a single vague “prelims” line, matters so much when you’re reviewing a quotation. Not every project needs every item on this list, but most UK projects will include the majority of them in some form.