Preliminaries explained: the site costs most builders under-price

Preliminaries are everything it costs to run the site rather than to build the thing. Scaffold, welfare, skips, supervision, the phone calls. On commercial jobs they get their own priced section; on domestic quotes they usually get forgotten, which is why so many "profitable" extensions pay out like a modest wage.

What preliminaries actually are

Split any job's cost in two. There is the work itself: bricks laid, boards fixed, pipes run. And there is everything that has to exist so the work can happen: somewhere to stand, somewhere to put waste, someone keeping the job organised, insurance if it goes wrong. The second pile is the preliminaries, prelims for short.

The defining feature of prelims is that they scale with the running of the site, mostly with time, rather than with the quantity of work. A wall twice as big needs twice the bricks; a job twice as long needs twice the welfare hire, twice the supervision, and usually a second scaffold inspection cycle. That distinction matters enormously when jobs overrun, and we come back to it below.

On a bill of quantities, prelims sit as their own section, before or after the measured work. That is not bureaucracy, it is visibility: when they are a section, you can price them, review them, and point to them when a client asks why your quote is 12 percent above the bloke who "doesn't charge for scaffold".

The list most domestic quotes miss

Run down this list against your last quote. Every unpriced line here was paid for by your margin:

The biggest and most invisible line is supervision. If you spend ten hours a week running the job, ordering, phoning, tidying, walking the architect round, that is a real cost at your charge-out rate, every week, for the whole programme. Builders who "never seem to make what the quote said" are usually donating exactly this.

Fixed versus time-related: the split that saves you on overruns

Professional estimators split prelims in two, and it is worth copying because it changes how you handle delays.

Fixed prelims happen once regardless of duration: scaffold erection and dismantle, site setup, connection of temporary services, the initial protection. Time-related prelims run by the week: scaffold hire, welfare hire, supervision, small plant. On a typical domestic job the split is roughly even, but the time-related half is the dangerous one, because every week of overrun buys another week of all of it.

This split is also your honest answer when a client delay hits the job. If their kitchen choice arrives six weeks late and the site stands, the cost is not vague inconvenience, it is your weekly time-related prelims figure times six, and you can show it. Builders without that number end up absorbing the delay; builders with it have a conversation. It is the same logic that makes programme length quietly change your price.

How to price prelims properly

Do it in three passes, and resist percentages until the end.

1. List the physical items against real prices: scaffold quote, welfare hire per week, skip count from the demolition and waste in the job. These are ordinary lines like any other.
2. Price the time. Estimate the programme in weeks, decide honestly how many supervision hours per week the job needs, and multiply by your charge-out rate. Add the weekly hire items times the same programme.
3. Sanity check as a percentage. Divide your prelims total by the job total. If a twelve-week extension shows 3 percent, you have forgotten something; if a two-week bathroom shows 25 percent, check whether fixed costs are doing the damage and whether the job is simply too small to carry them, which is useful to know before you sign it.

Then keep the section visible in the quote. Some builders fear itemised prelims invite nitpicking; in practice the opposite happens, because the client sees a firm that knows what running a safe, tidy site involves. It reads as competence, exactly like the rest of a professionally structured quote.

What happens when you skip them

The arithmetic is brutal and worth spelling out. Suppose a £50,000 extension carries true prelims around 12 percent, call it £6,000. A builder targeting a 20 percent margin who forgets them still incurs them, so the £10,000 of expected profit quietly becomes £4,000, a 60 percent cut in take-home for the same three months of work and all of the same risk. Nothing went wrong on site. The job was simply priced without its own running costs.

This is one of the seven leaks we cover in estimating mistakes that kill margin, and it is the most fixable. Build the checklist into every quote. If you would rather not maintain that discipline by hand, QS Quoter includes a prelims section in every bill it generates, scaffold, welfare, waste, supervision and the rest, priced from your rates and fully editable, so the running of the site is never accidentally free again.