For anyone signing a building quote
Provisional Sums in Construction: The Gaps Hiding in Your Quote
Sviatoslav JerdetchiiCEO & Founder of Craftex. Updated , 5 min read
The short answer
Provisional sums in construction are allowances for work that can't be priced yet. They are a placeholder, not a price, and the final cost replaces them.
- A low provisional sum makes a quote look cheaper and moves the risk back to you.
- Defined sums include the knock-on time and site costs; undefined sums do not.
- A prime cost sum is an allowance for a specific item, such as a kitchen.
- Exclusions and 'by others' lists shift cost to you in the same way.
Every quote has numbers that are prices and numbers that are guesses. The guesses are called provisional sums. They look exactly like prices on the page. They are not.
This guide explains provisional sums in construction from the builder's side. We look at this from inside a London contracting firm. Any quote, ours included, may carry allowances; ask how each one was set. We will show you how to read them, and why a low one is often the reason a quote looks cheap.
Want a builder who prices the whole job before you sign? Book a site visit.
This is general information for England and Wales, not legal advice. Your contract decides the detail.
Provisional sums, prime cost sums and contingency
Three kinds of allowance turn up in building quotes. They do different jobs, and they move risk in different ways.
| Provisional sum | Prime cost (PC) sum | Contingency | |
|---|---|---|---|
| What it is for | Work that isn't designed or known yet | A specific item you haven't chosen yet | Risks nobody can name yet |
| Typical example | Drainage repairs after a survey | Kitchen, sanitaryware, ironmongery | Something unexpected on site |
| Who sets the figure | Usually the client or designer, sometimes the builder | Usually the builder or designer | The client |
| What happens at the end | Replaced by the real value of the work | Replaced by the real cost of the item | Spent only if needed |
| Inside the contract price | Yes | Yes | Usually no |
A provisional sum is an allowance, not a price. Whatever the work really costs replaces it at the end. So the number in your quote is only as good as the thinking behind it.
Defined and undefined provisional sums
This distinction decides who pays for the knock-on effects. It is the most important line in this guide.
- Defined. The quote states what the work is, roughly how much, where it goes and any limits. The contractor is then treated as having allowed for it in the programme and site costs.
- Undefined. The quote just names the work and a figure. The contractor has not allowed for the time or site costs it causes. Those can be claimed on top.
Construction law guidance on provisional sums sets out this rule. It also notes a rule in the standard method of measurement (NRM2): provisional sums are exclusive of overheads and profit. So the builder's margin on that work comes on top of the allowance.
How a low provisional sum makes a quote look cheaper
Here is the trick that is not really a trick. The builder who sets the lowest allowances shows the lowest total. Nothing is hidden. It is written in the quote. But the risk has moved to you.
Two builders quote £250,000 for the same refurbishment. Each includes two provisional sums.
| Line | Builder A | Builder B |
|---|---|---|
| Fixed price for the defined work | £225,000 | £205,000 |
| Provisional sum: rewiring | £15,000 | £35,000 |
| Provisional sum: structural repairs | £10,000 | £10,000 |
| Quoted total | £250,000 | £250,000 |
| Rewiring actually costs | £34,000 | £34,000 |
| Likely final total | £269,000 | £249,000 |
Both quotes said £250,000. Builder A's low rewiring allowance means you pay £19,000 more by the end. If that sum was undefined, add the extra weeks of site costs too.
Why do builders do it? Partly to win. A lower total wins more tenders. Partly because honest guesses vary. Without a survey, nobody knows the condition of old wiring. Either way, the fix on your side is the same: check every allowance against the real work.
For the bigger picture on comparing prices, see our guide on why builders' quotes vary so much.
Exclusions and "by others"
Exclusions do the same job as low allowances, in smaller print. An excluded item is not in the price at all. A "by others" item is something the builder assumes someone else will do and pay for.
Common ones in London projects:
- Scaffolding, hoarding or skip licences.
- Party wall awards and surveys.
- Building control and planning fees.
- Utility connections and meter moves.
- Asbestos surveys and removal.
- Decoration, floor finishes or loose furniture.
- Making good after other trades.
Each of these will still need doing. If the builder excludes it, you pay someone else, often at short notice. Ask for every exclusion to be priced, or to have a named owner and budget.
How to check the allowances in your quote
You don't need to be a quantity surveyor to test a quote. Work through this list with the builder before you sign.
The last point matters most. Every allowance you turn into a fixed price before signing is a risk you no longer carry. That usually means a survey, an opening-up visit or picking your kitchen early.
How provisional sums are settled at the final account
When the work behind a provisional sum is done, it is valued like a variation. Contract rates apply where they fit, otherwise fair rates, daywork or an agreed quote. The real value then replaces the allowance.
At the final account, all the allowances are added back and the real values put in. That is often the moment a client sees the gap for the first time. Track each sum through the job instead. Ask for its real value as soon as the work is priced, not at the end.
Provisional sums in a JCT design and build contract
On a design and build contract, the contractor completes the design. So in theory, there should be fewer provisional sums. In practice, they still appear for things like below-ground work, surveys not yet done and items the client hasn't chosen.
Watch for two things. First, a provisional sum covering design the contractor should own. That moves design risk back to you. Second, sums described so loosely that they are effectively undefined.
Our guide to the JCT Design and Build Contract explains how the documents fit together. For how procurement routes change who carries this risk, see design and build vs traditional procurement.
Provisional sums are not your contingency
A provisional sum covers known work. Contingency covers the unknown. You need both, and they should never be the same money.
For contingency, we usually advise 5% to 10% of construction cost, depending on complexity. Keep it outside the contract price.
If your contingency is quietly spent topping up low allowances, there is nothing left when a real surprise arrives. Our guide on how much contingency a construction project needs explains how to size it by stage.
Most clients only find out what their provisional sums were worth at the final account. You can find out before you sign. Planning a project and want fewer guesses in the price? Book a site visit.
Your provisional sum checklist
- List every provisional sum, prime cost sum and exclusion in one place.
- Ask what each allowance is based on: a survey, a supplier quote or a guess.
- Ask whether each provisional sum is defined or undefined.
- Compare each allowance across all the quotes you have.
- Raise any allowance that looks low to a realistic figure before you compare totals.
- Agree how each sum will be valued, and when you will see the price.
- Turn allowances into fixed prices wherever a survey or a choice can settle them.
