A prime cost sum covers a thing rather than a piece of work. The drawings call for sanitary ware, ironmongery, tiles, light fittings or a kitchen, the choice has not been made, and the bill carries an allowance so that the contract sum can exist before the showroom visit does.
It sits alongside the provisional sum and is often confused with it. The distinction is worth holding onto: a provisional sum covers work of uncertain extent, and a prime cost sum covers an item of uncertain choice.
How the money is split
A PC sum is not a single number. It has three parts, and knowing that is what stops the argument later.
- The prime cost itself. What the supplier charges for the goods. This is the allowance, and it adjusts to what you actually buy.
- The contractor's profit, usually a stated percentage of the prime cost.
- Attendance. What the contractor does around the item: receiving it, storing it, providing access, unloading, protecting it, and in some cases fixing it. Priced separately, because it does not change much whether you choose the cheap tap or the expensive one.
The consequence people miss is that upgrading the item moves more than one of those numbers. Doubling the PC allowance for sanitary ware also doubles the profit line applied to it, and it may or may not touch attendance.
A worked example
The bill carries a prime cost sum of KES 180,000 for sanitary ware, contractor's profit at ten per cent, and attendance priced at KES 40,000.
Allowed: 180,000 + 18,000 + 40,000 = KES 238,000.
You go to the showroom and choose fittings costing KES 310,000.
Adjusted: 310,000 + 31,000 + 40,000 = KES 381,000. The contract sum rises by KES 143,000, of which KES 130,000 is the upgrade and KES 13,000 is the profit that came with it.
That is not a contractor being opportunistic. It is the mechanism doing arithmetic. But it is why the honest conversation to have in the showroom is about the adjusted figure and not the price on the tag.
What goes wrong
The allowance was never realistic. PC sums are sometimes set low to make a tender total look competitive, on the quiet understanding that the client will upgrade. The build then reads as over budget when it was under-allowed from the start. Ask what standard of item the allowance actually buys, and go and look at that standard before signing.
Choices are made too late. A PC item chosen after the programme needs it becomes a delay as well as a cost, and a delay carries preliminaries with it.
Nobody tracks the adjustments. Six PC sums each upgraded by a modest amount is a meaningful number that nobody ever sees, because each decision felt small on the day it was made. Track them on one running line with the provisional sums.