Split every line into fixed price, prime cost allowance or provisional sum, and tie each stage payment to something the customer can see on site. AI can draft the wording and then attack it for gaps, but you fill in every figure and leave compliance claims to your engineer and building control.
A provisional sum is an allowance for work too uncertain to price at survey, adjusted up or down against what the work really costs. It is not a hidden extra, and it works only when each one carries a plain description, an assumption and an adjustment rule. Have the final wording checked by a lawyer or compared with your trade body's contract before it goes to a customer.
Three buckets for every line: fixed, prime cost or provisional
The fixed price covers what the survey, the drawings and the engineer's design define. A prime cost allowance, often shortened to PC sum, covers an item the customer has not chosen yet, such as roof windows, a staircase or a shower room suite. It is the supply cost only, with fitting priced inside the fixed price. A provisional sum covers work whose extent cannot be known until the roof is open.
Construction guidance describes a provisional sum as an allowance for work not yet defined well enough to price accurately. It sits inside the contract price, nobody expects the round figure to be paid unchanged, and the final account replaces it with the value of the work actually done, so the total can rise or fall. A prime cost sum, in the same guidance, is an allowance for goods or work from a supplier the client picks. Keep those two words apart in your document, because customers read "estimate" as "rough guess" and consumer guidance treats a quote as a fixed price.
| Loft item | Bucket | Why | Wording to use |
|---|---|---|---|
| Steel beams to the engineer's drawing | Fixed | Drawn and calculated | "Supply and fit three steel beams as drawing S1" |
| Extra steel if the final calculations need more | Provisional sum | Depends on the engineer's final design | "Allowance for steel beyond drawing S1, at supplier's invoice price" |
| Roof windows | PC sum | Customer picks the model | "PC sum for supply of four roof windows; fitting included" |
| Staircase | PC sum for supply, fixed for fitting | Style not yet chosen | "PC sum for staircase supply; fitting included" |
| Rotten rafters found after strip-out | Provisional sum | Unknowable until opened | "Allowance for up to 6 metres of timber repair" |
| New electrical circuits | Fixed | Defined by the specification | "New circuits for lighting, sockets and shower" |
| Upgrading the existing consumer unit | Provisional sum | Depends on what the electrician finds | "Allowance for consumer unit upgrade only" |
| Shower room suite and tiles | PC sum | Customer chooses | "PC sum for suite and tiles; fitting included" |
A sanity check keeps this honest. Add up your provisional sums and divide by the contract price. My rule of thumb, and it is only that, is to stay near a tenth or below. In the quote below the figure is 8.7 per cent. If yours is a quarter, either the survey was too thin or the customer is being asked to carry too much of your risk, and a homeowner comparing three quotes will see it.
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Buy nowA rear dormer quote, filled in
All figures here are illustrative: a rear dormer conversion with a bedroom and shower room. The summary page of the quotation looks like this.
| Section | What it covers | Amount |
|---|---|---|
| A. Fixed price | Scaffold, steel and new floor structure, dormer shell and roofing, insulation, fitting of roof windows and stairs, first and second fix electrics and plumbing, plastering, joinery, first coat of paint | $54,600 |
| B. Prime cost allowances | Roof windows $1,400; staircase $2,600; shower room suite $1,900 | $5,900 |
| C. Provisional sums | Roof timber repairs $1,500; extra steel $2,200; consumer unit upgrade $1,200; re-routing of existing services $900 | $5,800 |
| Contract sum (A + B + C) | $66,300 | |
| D. Fees at cost | Structural engineer $1,100; building control $1,350 | $2,450 |
| Total including fees | $68,750 |
Three of the four provisional sums deserve full wording, because wording is where disputes start. Each follows the same pattern: what it covers, what it assumes, how it is adjusted and who approves.
Provisional sum 1: roof timber repairs, $1,500. Covers replacement of defective rafters, purlins or wall plates found when the roof is opened. Assumes up to 6 metres of timber in total. Adjusted to the actual cost of timber and labour at the day rate in this quotation. We will photograph the defect and obtain your written approval before any work goes above the sum. If no repair is needed, the full sum is deducted.
Provisional sum 2: additional steelwork, $2,200. The fixed price includes three beams to the engineer's drawing S1. If the final calculations need more or heavier steel, the extra is charged at the supplier's invoice price, which we will show you. If none is needed, the full sum is deducted.
Provisional sum 3: consumer unit upgrade, $1,200. Allows for upgrading the existing consumer unit only, if our electrician finds it cannot serve the new circuits. Rewiring of existing circuits is excluded. Adjusted to the electrician's invoice. If the existing unit is suitable, the full sum is deducted.
A prime cost item reads differently, because it is about choice, not discovery: "Prime cost sum, roof windows, $1,400 for supply of four windows. Fitting, flashings and making good are in the fixed price. If you choose windows that cost more or less than the sum, the difference is added to or deducted from the final account at our cost." Notice the last phrase. Say whether you pass on savings in full and whether you add a handling margin to increases. Either answer is fair; silence is not.
Stage payments tied to things a customer can see
Consumer guidance on building work advises paying in stages, being clear which points in the work trigger each payment, avoiding deposits where possible and not agreeing to a deposit of more than a quarter of the price. A builders' trade body's contract templates likewise end with a payment schedule. A good stage table satisfies both.
| Stage | Share | Amount | Trigger the customer can check |
|---|---|---|---|
| 1. Deposit | 10% | $6,630 | Signed quotation; covers the engineer's fee, building control application and made-to-order items |
| 2. Site set up | 10% | $6,630 | Scaffold up and strip-out complete |
| 3. Structure | 20% | $13,260 | Steel fitted and new floor boarded; where building control inspects this stage, the inspection passed |
| 4. Watertight | 20% | $13,260 | Dormer roofed and roof windows in |
| 5. Services and stairs | 15% | $9,945 | Cables, pipes and staircase in, insulation fitted |
| 6. Plastering | 10% | $6,630 | All new walls and ceilings plastered |
| 7. Completion | 12% | $7,956 | Fittings finished and the final inspection requested |
| 8. Retention | 3% | $1,989 | Released eight weeks after completion once snagging is done |
| Total | 100% | $66,300 |
Notice the deposit. Ten per cent is well inside that quarter, and it is sized to the money you must spend before the first day on site: the engineer, the building control application, and any windows or steel that are made to order. If your own pre-start outlay is larger, the better answer is usually to ask the customer to pay for the made-to-order item directly, not to raise the deposit. Some contracts hold back a small retention until a defects period ends. If you use one, state the percentage, the date it is released and who decides that snagging is finished.
The triggers do the real work. "Structure complete" is an opinion; "steel fitted and new floor boarded" is something a customer can see from the landing. Where a third party inspects, name them in the trigger so the customer is not relying on your word. To turn each stage into an invoice, see stage payment requests written with AI, and setting up quotes, variations and stage payments in Tradify if you want job software to do it.
Engineer, building control and neighbour costs: who pays
These are the lines customers most often assume are included. Trade guidance tells homeowners to work with a structural engineer, says a loft conversion always needs building regulations approval whether or not planning permission is required, and says a building control officer inspects at stages and issues a completion certificate. Check what applies where you work, then state in the quote who appoints and pays for each item.
| Cost | Typical arrangement | Wording to put in the quote |
|---|---|---|
| Structural engineer's design and calculations | Fixed fee quoted by the engineer before you price | "Engineer's fee of $1,100 for design and calculations is included at cost; a site inspection visit is excluded unless stated" |
| Building control application and inspections | Fees paid at cost against the invoice | "Allowance of $1,350 at cost; receipts supplied; we arrange inspections; the completion certificate goes to you" |
| Neighbour notices and any surveyors | Usually the owner's responsibility, often excluded | "Notices to neighbours and any surveyors' fees are excluded; we will supply the drawings you need" |
| Planning application, if one is needed | Owner's cost, with drawings from a designer | "The price assumes no planning application; if one is required, fees and design time are extra and the programme moves" |
Shared walls deserve one extra sentence in the survey notes. If the house shares a wall with a neighbour, neighbour notices may be needed, and a neighbour who objects can add cost and delay that neither of you controls. Say so in writing on day one, not after the steel is ordered.
Exclusions that prevent the arguments
An exclusion list is not defensive. It is the other half of the scope, and it is what lets a customer compare your quote with a cheaper one on equal terms. Consumer guidance asks for a breakdown of work and materials, how long the price is valid and when it can increase. This list is a starting point; edit it for your own contracts.
- Decoration. "Decoration beyond one coat of paint on new plaster is excluded."
- Floor finishes. "Carpets, flooring and floor tiling outside the shower room are excluded."
- Furniture and fittings. "Fitted wardrobes, curtain poles, blinds and furniture are excluded."
- Heating capacity. "Radiators are included; any upgrade of the boiler or hot water cylinder is excluded."
- Hazardous materials. "Work stops if asbestos is suspected; testing and removal are priced separately."
- Defects elsewhere. "Damp, rot or structural defects outside the loft area are excluded."
- Access costs. "Parking suspensions, scaffold licences and skip permits are excluded."
- Contents. "Clearing the loft of belongings is the owner's job before strip-out."
- Later requirements. "Work required by building control that is not on the approved drawings is a variation."
- Validity. "This quotation is valid for 30 days. After that, material prices may be reviewed."
Add one line about variations: any change to the work needs a written price and the customer's written agreement before it starts. Consumer guidance says a trader can charge more than the quote only for extra work the customer asks for or agrees to pay for, so this line protects both of you.
A prompt to draft the wording from your survey notes
An assistant is good at turning survey bullets into consistent paragraphs, and at tirelessly repeating the same structure for each provisional sum. It is bad at your prices. The prompt below makes it ask for what it lacks and checks its own sums.
You are drafting a loft conversion quotation for a homeowner. I will give
you survey notes, the figures, the engineer's fee and a stage table.
Write in plain English with our usual spelling.
Rules:
- Use only figures I give you. If a figure is missing, write [TBC] and list
every [TBC] at the end.
- Sections: scope of fixed-price work (numbered), prime cost allowances,
provisional sums, fees paid at cost, exclusions, stage payments,
validity and variations.
- Every provisional sum needs a one-sentence description, the assumption it
rests on, how it will be adjusted, and a line saying the customer
approves in writing before work goes above the sum.
- Every stage trigger must be something a customer can see or a certificate
they can ask for.
- Do not write that work is guaranteed, fully compliant or under a warranty.
Do not state any legal rights.
- Check that stage percentages total 100 and that contract sum equals fixed
price plus allowances plus provisional sums. Report any mismatch.
Survey notes: [paste]
Figures: [paste the summary table]
Stage table: [paste]A draft from a prompt like that reads well, and it can still need five fixes. Look for these slips every time.
- A compliance promise. The introduction may say "all work is guaranteed to comply with building regulations". Replace it with "work will be built to the approved drawings and inspected by building control". You can promise a process, not an outcome you do not control.
- An invented guarantee. Drafts sometimes add a "10-year workmanship guarantee" that you never supplied. Delete anything you do not actually give.
- An unobservable trigger. Stage 3 may read "when the structure is complete". Rewrite it as "steel fitted and new floor boarded, with the inspection passed where building control inspects this stage".
- A drifting term. A paragraph may call provisional sums "estimates". Keep "provisional sum" throughout, for the reason given above.
- Arithmetic. If a draft's stage table totals 102 per cent, the final instruction in the prompt should make the assistant say so. Fix the table yourself; do not assume the assistant will.
A suitable pairing is the worked example of a builder's quote written with ChatGPT, which shows the plain-quote version without stages.
A second prompt: read it as a sceptical customer
Drafting and checking are different jobs, so use a fresh conversation for the second one, where the assistant has not seen your intentions. Ask it to attack the document.
Read the quotation below as a sceptical homeowner who has a friend in the
building trade. List:
1. Sentences that could be read two ways.
2. Provisional sums missing an assumption, an adjustment rule or an
approval step.
3. Stage payments whose trigger a customer could not check.
4. Costs a homeowner would expect to be included that are neither included
nor excluded.
5. Arithmetic that does not add up.
For each finding give the line, the problem and a one-sentence rewrite.
Do not rewrite the whole quotation.
[paste the quotation]A reply to the dormer quote might contain findings like these, illustrative and shortened.
| Line | Problem | Suggested rewrite |
|---|---|---|
| "Making good included" | Making good what? | "Plaster repairs and one coat of paint to the new stair opening and landing" |
| "Allowance for electrical works $1,200" | No assumption stated | "Upgrade of the existing consumer unit only; rewiring of existing circuits excluded" |
| "Scaffold included" | No duration or licence terms | "Scaffold for up to [TBC] weeks; extra weeks charged at [TBC]; licences excluded" |
| "Payment 5 due at first fix" | Jargon the customer cannot check | "Due when cables, pipes and the staircase are in and insulation is fitted" |
| "Insulation to current standards" | Which standard? | "Insulation to the thickness on the approved drawings" |
Here is what to fix by hand. On the scaffold line an assistant may fill the gap with "up to 12 weeks", a number you never gave it, which is why the rewrite above says [TBC]; supply the figure yourself. A critic assistant also over-flags: it may complain that "plastering" is vague when your drawings define it. Take the findings a real customer would raise, and ignore the rest.
Settling provisional sums once the roof is open
The quote earns trust or loses it at the final account. When a provisional sum is about to be exceeded, tell the customer before the work, in writing, with a photo and a figure. A message like this does it, and it uses the same language as the quotation so nothing is a surprise.
Hi [first name], when we opened the rear slope this morning we found rot in three rafters (photos attached). Provisional sum 1 allows $1,500 for timber repairs on the basis of up to 6 metres of timber. This will need about 9 metres, which I expect to cost $2,300 at our day rate: $800 over the sum. I need your written reply before we go ahead; an email saying yes is fine. The repair will add one day. If you would like a second opinion first, tell me today and I will keep the roof covered. [your name]
At completion, the final account sets every allowance against what was actually spent. Using the example, with figures illustrative again:
| Item | Allowance | Actual | Adjustment |
|---|---|---|---|
| PS 1 roof timber repairs | $1,500 | $2,300 | +$800 |
| PS 2 additional steel | $2,200 | $0 | -$2,200 |
| PS 3 consumer unit upgrade | $1,200 | $1,200 | $0 |
| PS 4 re-routing of services | $900 | $1,350 | +$450 |
| PC roof windows | $1,400 | $1,650 | +$250 |
| PC staircase | $2,600 | $2,350 | -$250 |
| PC shower room suite | $1,900 | $2,300 | +$400 |
| Net adjustment | -$550 |
The contract sum of $66,300 becomes $65,750. Payments one to seven were invoiced at their original amounts, a total of $64,311, so the final payment is the $1,989 retention less the $550 net credit: $1,439. The point of showing it is that provisional sums cut both ways. In this job the customer ends up $550 better off, and a customer who sees the credit arrive without being chased is a customer who recommends you.
Twelve checks before the quote leaves your hands
- Recalculate every total in a spreadsheet, not in the document.
- Stage percentages total 100 and every stage amount matches its percentage.
- Each provisional sum has a description, an assumption, an adjustment rule and an approval step.
- Each prime cost sum says it is supply only and what the fixed price includes.
- Provisional sums total roughly a tenth of the contract sum or less.
- Each stage trigger is something a customer can see or a certificate they can ask for.
- The quote says who appoints and pays the engineer, building control and any neighbour or planning costs.
- Exclusions cover decoration, floor finishes, hazardous materials, defects elsewhere and access costs.
- Validity is stated, with what happens after it.
- Variations need a written price and written agreement.
- No sentence promises compliance, a guarantee or a legal right that you cannot back.
- A lawyer or your trade body's contract has been compared with your terms, including the cancellation wording for work agreed at the customer's home.
For contracts that follow an accepted quote, pricing variations and change orders with AI picks up where the final account leaves off, and drafting allowances and exclusions for a custom home proposal shows the same allowance logic on a bigger job. When a quote touches the rules, answering building code questions with AI safely sets out a routine for checking what an assistant says.
Provisional sums and stage payments: common questions
Is a quote with provisional sums still a fixed price?
Consumer guidance describes a quote as a fixed price and an estimate as a rough guess. A quotation with provisional sums is fixed for everything defined, with clearly labelled allowances for the rest. Say that in the heading, keep the allowances small and give each one an assumption and an adjustment rule.
How large can the provisional sums be?
I know of no official ceiling, so this is a rule of thumb: keep the total near a tenth of the contract price or below. Much higher suggests the survey was thin or the customer carries too much risk, and a homeowner comparing three quotes will notice which firm priced the unknowns.
Can AI work out the prices for me?
It can add up figures you give it and flag mismatches, but it does not know your costs, your suppliers or what the engineer will specify. Prices come from your rate sheet and supplier quotes. Tell the assistant to write [TBC] wherever a figure is missing rather than guessing one.
Does the same template suit a hip-to-gable or a mansard?
The structure does: fixed price, allowances, provisional sums and stages. The items and triggers change, because a hip-to-gable rebuilds the end of the roof and a mansard changes the whole roof shape. Write the provisional sums afresh for each type instead of copying the dormer list.
Further reads
- Loft Conversion Leads: Screen Roof Type, Head Height and Budget With AI — Screen enquiries before the survey so the quote goes to the right people.
- Bathroom Renovation Scope of Work: Draft Room-by-Room Lines With AI — Room-by-room scope lines you can borrow for the shower room.
- How to Write a Method Statement for a Small Build, With AI Help — The safety paperwork that sits alongside a loft job.
- Can AI Draft a Remodelling Contract Template in Plain English? — Turning the accepted quote into a plain-English contract.
- Remodelling Proposal Template: Scope, Allowances and Terms With AI — Another allowances-and-terms layout to compare with this one.
- Construction Estimating With AI: Turn Supplier Quotes Into a Rate Sheet — Build the rate sheet that your quote figures should come from.
- AI for Electrical Estimating: Set Fitting Allowances and Provisional Sums — Fitting allowances versus provisional sums in a domestic quote: how to set the amounts, who chooses, how to settle overspend, and where AI drafts the wording.
- Demolition Estimate Template: Scope, Waste and Disposal Lines With AI — A demolition estimate template for a rear extension: scope, asbestos and services exclusions, tonnes by waste stream, and skip and disposal lines.
- Garage Conversion Leads: Qualify by Door Size, Floor and Planning With AI — A seven-question intake, a six-photo checklist and green, amber, red sorting that tells a garage conversion firm which enquiries deserve a survey visit.
- Insulation Proposals: Explain Options With AI Without Promising Savings — Option-by-option wording for insulation proposals, with a plain-English R-value explanation and a claim check that keeps savings promises out.
- Insulation Quote Template: Material, Thickness and Area Lines With AI — A line-item insulation quote with thickness, area and exclusion wording, plus a prompt that flags missing measurements instead of inventing them.
- Kitchen Installation Quote Template: Scope and Exclusions With AI — Write a fitting-only kitchen quote with a clear scope, a trade-boundary table and exclusions, using AI for the first draft and a worked quote with totals.
- Kitchen Remodel Estimate Template: Line Items Written With ChatGPT — A kitchen remodel estimate by phase, a two-pass ChatGPT prompt that finds forgotten lines, and a worked estimate with allowances, contingency and totals.
- Plastering Quote Template: Skim, Render and Patch Lines With AI — What a plastering quote should list for skim, render and patch repairs, with a worked quote, a rate build-up and an AI drafting prompt.
- Renovation Quote Template: Room-by-Room Scope Written With AI — A room-by-room renovation quote with stage grids, allowances and exclusions, plus prompts that draft clear wording from your survey notes.
- Window Installation Quote Template: Specs, Lead Times, Exclusions With AI — A window quote built from a worked eight-window schedule: spec lines, U-value wording, lead-time dates, making-good exclusions and an AI prompt with checks.
- AI Tools and AI Development: The Complete 2026 Guide — the AI hub, including every tutorial in the AI-for-business series.
Sources: construction guidance on provisional and prime cost sums; consumer-advice guidance on quotes, deposits and stage payments; builders' trade-body guidance on loft conversions and payment schedules (read 1 November 2026). Not legal advice.