On a complex building project, not every cost can be confirmed before work starts.
The customer may not have chosen their kitchen, bathroom fittings or flooring. Structural details may still need to be finalised. Some parts of the existing building may remain hidden until work begins.
These costs cannot simply be ignored or guessed.
At the HBXL Estimating Service, our expert estimators use allowances, Prime Cost Sums, Provisional Sums and contingencies carefully. Each has a different purpose and needs to be explained clearly to the builder and their customer.
What is an allowance in a building estimate?
An allowance is an estimated amount included for an item or area of work where the exact cost is not yet known.
It might be used for sanitaryware, flooring, finishes or another part of the specification that has not been fully decided.
The allowance should reflect the quality and standard the customer is expecting. An unrealistically low figure may make the initial quotation look more competitive, but it can lead to unexpected costs and difficult conversations later.
What is a PC Sum?
A Prime Cost Sum, usually shortened to PC Sum, is an allowance for the supply of a particular item that has not yet been selected (not the installation).
A building estimate might include a PC Sum for kitchen units, sanitaryware, taps, tiles or appliances.
PC Sums sometimes include ‘reasonable delivery costs’. Some contracts (especially Joint Contracts Tribunal (JCT) Home Owner/Occupier ones) treat delivery as part of the PC Sum.
What is a Provisional Sum?
A Provisional Sum is included for work that cannot yet be accurately priced. It can be either defined (scope broadly known) or undefined (scope genuinely unknown).
It may be needed for drainage alterations, structural work, groundworks or repairs that will only become clear once the building is opened up.
Unlike a PC Sum, which normally relates to the supply of a particular item, a Provisional Sum can include labour, materials, plant and subcontractor costs.
It should be based on the best information available, while making it clear that the final cost may change once the scope of work is known.
What is a contingency in a building estimate?
A contingency is not a provisional sum. It is a risk allowance, not tied to a specific item of work. It’s a separate amount included to help manage genuine project uncertainty.
You’ll know from experience that refurbishment, renovation and complex construction work can uncover unforeseen conditions.
The amount included will depend on the type of project, the quality of the available information and the level of risk involved.
A new-build project based on detailed drawings may need a different approach from an older property where the structure and condition of the building are less certain.
As a frame of reference – industry norms:
* New builds: 3-5% extra
* Renovations:10-15% depending on condition and risk
* Complex refurbishments: sometimes higher.
Why do these figures need to be discussed with the customer?
Allowances, PC Sums, Provisional Sums and contingencies should never be added without explanation.
Our estimators discuss them with the builder so we can understand:
- what has already been agreed with the customer
- which products and finishes are still to be selected
- where the scope of work remains uncertain
- what level of specification the customer expects
- where a sensible contingency may be appropriate
This allows us to include realistic figures and describe them clearly within the estimate.
It also gives the builder a stronger basis for discussing the price with their customer and explaining where the final cost could change.
Why use HBXL Estimating Service for complex projects?
Complex and costly building projects require more than a set of measurements and prices. They require judgement.
Our experienced estimators understand how to deal with missing information, incomplete specifications and uncertain areas of work. We work with the builder to make sure allowances are sensible, PC Sums are properly supported, Provisional Sums are clearly identified and contingencies are appropriate.
For time-poor builders, this means a more complete estimate, clearer customer conversations and less risk of important costs being overlooked.
Frequently asked questions
What is the difference between a PC Sum and a Provisional Sum?
A PC Sum usually relates to the supply cost of an item that has not yet been selected, such as tiles, taps or sanitaryware.
A Provisional Sum relates to work that cannot yet be accurately defined or priced, such as drainage, groundworks or structural alterations.
Does a PC Sum include installation?
The PC Sum itself normally relates to the supply of the selected item.
Is a contingency the same as a Provisional Sum?
No.
A Provisional Sum relates to a specific area of work that cannot yet be accurately priced. A contingency is a broader amount included to help manage unforeseen costs or project risks.
Why are realistic allowances important?
Realistic allowances help prevent the original estimate from appearing artificially low.
They also reduce the risk of significant price increases when the customer eventually chooses products or confirms the specification.
Can these figures change during the project?
Yes.
PC Sums may change when the customer selects the final product. Provisional Sums may change when the full scope of work becomes known. Contingencies may or may not be used, depending on what happens during the project.
The important point is that these figures are identified, explained and managed clearly from the start.
How does HBXL Estimating Service decide what to include?
Our estimators speak with the builder about the project, the customer’s expectations, the available drawings and specifications, and any areas of uncertainty.
We then use our estimating expertise to include appropriate figures and clearly identify where further decisions or information will be needed, whether that’s PC Sums or something else.
Send us your plans
Pricing an extension, loft conversion or residential new build?
Send your drawings and project information to HBXL Estimating Service. We’ll review what you’ve provided, ask the right questions and give you an accurate quote for preparing your estimate.
For complex or costly projects, expert input at the estimating stage can help you price with greater confidence, explain allowances clearly and protect your margin before work begins.