You priced the job. The rates look right, the total looks fair, and then two months in the budget's gone and you can't point to a single mistake. Nobody underpriced the brickwork. What got you was the stuff that never got its own line in the first place: the hidden costs in a construction budget that only show up once you're paying for them.
They're hidden for a reason. None of them appear as a neat quantity on the drawings. You have to go looking.
Prelims: the cost of simply being on site
Prelims are the usual one. Site set-up, welfare, supervision, scaffold hire for the length of the job, all the running costs that sit underneath the actual building work. Roll them into a single guessed figure and they quietly run over every time, because they scale with time on site, not with the work itself, and time on site is the one thing that never goes to plan.
Think of it this way. If the job slips by three weeks, not one extra block gets laid. But the scaffold's still up, the welfare cabin's still hired, the skips keep coming and someone's still supervising. A percentage added at the bottom of the tender doesn't know any of that. A prelims build-up priced week by week against the programme does.
Provisional sums: placeholders that become real
Provisional sums are the second trap. A number sat in the tender for something nobody has priced properly yet: ground conditions, an existing structure, a specification that's still being decided. Treat it as a placeholder and move on, and it turns into a real cost with no real number behind it the day it's confirmed.
The fix isn't to avoid provisional sums. Sometimes they're the only honest way to price something. The fix is to describe exactly what each one covers, make the allowance realistic rather than token, and make sure the client understands it will be adjusted once the work's known.
Access and logistics: the quiet one
Access is the quiet one. A tight site, a shared road, restricted working hours, a crane or lift that has to be hired because there's nowhere to store materials at ground level. None of it shows up in a rate for brickwork or first fix. It shows up in the plant hire invoice three weeks in.
Say you're pricing a terraced house refurb with no side access. Every bag of plaster, every length of timber and every skip of strip-out has to come through the front door and the hallway. That's extra labour on every material that goes in and every bit of waste that comes out, and none of it sits in a standard rate. Add a parking suspension on a busy street, or a neighbour who won't allow the scaffold on their side, and the access cost can easily outgrow some of the elements you spent hours measuring.
The lines that sink a budget are the ones nobody measured. If a cost has no quantity and no rate, it has no protection.
A checklist of hidden costs in a construction budget
Before any price goes out, run down this list and make sure every item is either priced on its own line, covered by a clear provisional sum, or written into your exclusions:
- Prelims against the programme. Site set-up, welfare, supervision, scaffold and skips, priced for the real duration, not a round percentage.
- Provisional sums with descriptions. Each one saying what it covers and how it will be adjusted.
- Access and handling. Restricted access, distance from road to work area, hoisting, storage and parking.
- Temporary works. Propping, shoring, temporary weathering and protection of existing finishes.
- Waste and muck away. Strip-out, excavation spoil and the disposal cost that goes with each.
- Making good. Where new meets existing, and the finishes that get damaged on the way.
- Statutory and service connections. Anything that depends on a utility company's quote or timescale.
That's seven places to look. On most jobs, at least one of them is where the margin quietly leaks, and it's usually the one you felt sure you'd covered in "sundries". It's the same set of gaps that shows up again and again in rushed tenders; there's more on that in what gets left off a tender and how to stop it.
Price them up front, not as they land
Each of these needs its own line, its own quantity and its own rate, not a percentage tacked onto the bottom of the tender. Measured properly, to NRM2, prelims and provisional sums sit where the client and you can both see them, so nobody's surprised when they move.
That visibility cuts both ways. It protects you, because when a provisional sum goes up, the client can see it was flagged from the start. And it helps the client, because they can see what they're paying for and where the uncertainty sits before they commit. Hidden costs in a construction budget cause arguments precisely because they were hidden. Put them on the page and most of the argument disappears.
We build the whole job this way. Send your drawings and we measure every element to NRM2, price it on current UK rates local to the job, and identify prelims and provisional sums as their own items rather than folding them into a total nobody can question. You get a client-ready estimate that shows exactly where the risk sits before you ever step on site, for a fixed fee agreed before we start.
The five-minute test
Open your last tender and find the prelims. If they're a single percentage, ask how many weeks on site that percentage actually pays for. If you can't answer, the budget had a hidden cost in it from day one, and on the next job it's worth pricing that line properly before anything else.