How to Price and Agree Extra Building Work in the UK

To price extra building work, start with the scope your customer has already accepted. Identify what's changing, calculate the cost of additions and any credits for work being removed, explain the effect on the schedule, and agree how the change will be paid for before carrying it out wherever practicable.

A change that sounds minor can involve another delivery, a subcontractor's return visit or making good finished work. A clear additional-work quotation helps you recover legitimate costs without leaving the customer wondering why the final bill is higher.

What counts as extra building work?

In construction, a variation is a change to the work originally agreed. On a domestic building or landscaping job, it might be:

• An addition: the customer asks for another drain, socket, garden step or area of paving.
• A specification change: they choose a more expensive finish or different materials.
• An omission: they decide not to proceed with part of the original work.
• An unforeseen condition: hidden rot or damaged pipework is uncovered and the proposed solution changes the agreed work.

An unexpected problem does not automatically mean you're entitled to charge more. Check the accepted quote, drawings, specification and any relevant contract terms to establish who is responsible for the work and the risk.

For example, installing an extra drainage run requested after a patio quote was accepted may be additional work. Completing preparation that was already included in that quote isn't. Nor should you treat correcting your own defective workmanship as a paid variation merely because it takes extra time.

Check allowances as well. If the original price included a provisional sum for an uncertain item, follow the agreed method for adjusting it. Make the original allowance and the replacement amount visible so the customer isn't charged twice.

How to price extra building work step by step

Your starting point is the additional selling price, not just the cost of materials. Consider all the resources the change requires and use any agreed contract rates or valuation method where applicable.

1. Work out the extra labour

Break the change into tasks: protection, setting out, removal, installation, testing, cleaning and making good. Include any extra visit or collection time caused by the change.

Estimate the hours realistically, then multiply by the appropriate labour rate. For example, 12 hours × £45 = £540. This is an illustrative rate, not a recommended industry rate. Check what your labour rate already includes: adding overheads or profit again when they're built into the rate would double-count them.

2. Add materials, plant, waste and subcontractors

Use current supplier prices rather than relying on the figures from the original quote. Remember delivery charges, realistic material waste, consumables, plant hire, disposal and any additional trips.

If another trade is involved, get their price for the changed scope. Check who supplies materials, who makes good and whether their visit needs rescheduling. Those small details often decide whether the variation is profitable.

3. Allow for overheads and profit

A variation may create administration, vehicle and site-running costs beyond the direct labour and materials. Account for these where your other rates haven't already covered them, then add an appropriate profit allowance.

Markup and margin are different. If a job costs £100 and you add a 20% markup, you charge £120. The £20 difference is a 16.7% gross margin on that selling price, before any costs not included in the £100. Don't assume 20% markup means 20% margin.

There isn't one correct markup for every variation. Price according to the work, your costs, the risk and any rates already agreed in the contract.

4. Explain uncertainty before agreeing a figure

Sometimes you can't see how deep a pipe is or how much damaged timber needs replacing until work starts. Rather than guessing a fixed price, you could agree a separately priced investigation, or a suitable daywork arrangement with labour and plant rates, daily records and an agreed spending limit.

State assumptions clearly. If the price covers one day's excavation and excludes unexpected contaminated waste, say so before the customer accepts it. Any later adjustment still needs a proper contractual or agreed basis.

How much should you credit for omitted work?

Extra building work isn't always an addition. A customer might remove a gravel border, reduce the area being tiled or decide against a fitted feature. The omission may reduce the price, but the correct credit depends on the contract and the stage the project has reached.

Look at the original priced item, the labour and materials no longer needed, and whether any hire or subcontract costs can be cancelled. Also consider legitimate, evidenced costs that cannot now be avoided, such as materials already ordered or an agreed cancellation charge.

Don't assume you can retain every committed cost. Equally, don't assume the full original line-item price must always be credited. Follow the contract's valuation rules or reach a clear agreement with the customer, explaining how you've arrived at the figure.

Be particularly careful when work is being removed so another contractor can undertake it. Whether the customer can do that under a variation clause depends on the agreement. Construction law firm Fenwick Elliott explains why omissions and reassignment of work can be contentious.

Worked example: additions, credits and the revised total

Suppose you've agreed a garden refurbishment for £12,000 excluding VAT. The homeowner then requests an additional drainage run and agrees to omit a decorative gravel strip. You've checked that these are changes to the accepted scope, and the following figures are illustrative.

This is an internal costing breakdown used to calculate the selling price. The customer-facing quotation below shows the agreed scope and selling price without listing your overheads or profit separately.

The separately agreed credit for the omitted gravel strip is £350, after reviewing the original scope and any relevant committed costs.

The net increase is £850 excluding VAT (£1,200 − £350).

What about VAT?

Only for this example, assume the contractor is VAT-registered and all the original and varied work is standard-rated at 20%:

Show domestic customers the full amount they'll pay, not just an ex-VAT figure. Real construction jobs can qualify for different VAT treatments, and some qualifying business-to-business supplies are subject to the domestic reverse charge. Check HMRC's VAT Notice 708 and its construction reverse-charge guidance if relevant. If you're not VAT-registered, don't add VAT to your quote.

Variation quotation example: what to send the customer

A useful additional work quotation identifies the original job, exactly what will change, the price difference, timing and payment arrangements. It should be clear enough that someone who wasn't at the site meeting can understand what was approved.

Here's a completed fictional example, using the figures above. The assumed dates and VAT treatment are illustrative, not instructions for your own job.

Additional-work quotation EX-01 — Garden refurbishment
Original quote: Q-104 (accepted)
Issued: 5 November 2026
Customer: Example homeowner
Location: Rear garden; drainage route as marked on drawing EX-01A
Reason: Customer-requested drainage addition and removal of the gravel border from the accepted scope.

Work added: Install approximately 6 metres of additional drainage, including excavation, pipework, connection, backfilling and making good, to the agreed drawing and specification.
Addition: £1,200 excluding VAT.

Work omitted: Decorative gravel strip described in the original quote.
Agreed credit: £350 excluding VAT.

Net change: +£850 excluding VAT (+£1,020 including VAT at the assumed 20% rate).
Revised project total: £12,850 excluding VAT / £15,420 including VAT.

Timing: The change adds one working day. Subject to timely materials delivery and no further changes, the agreed target completion date moves from Friday 20 November to Monday 23 November 2026.
Payment: Add the £1,020 VAT-inclusive net change to the next payment stage under the original agreement. All other agreed payment terms remain unchanged.
Acceptance: Please reply by email confirming acceptance of EX-01, its scope, price and revised target date, and authorising the change to proceed, subject to any instruction requirements in the original contract.

On a real quotation, replace the example references and scope with precise details. Resolve VAT treatment, exclusions, payment terms and any uncertain dates before asking the customer to approve it. If your contract prescribes a particular variation form or approval process, use that process.

How to agree variations with customers

A verbal conversation on site is easy to misremember, especially when a project takes several weeks. Aim to get the scope, price and timing agreed in writing before ordering materials or starting the extra work.

A practical process is to show what has changed, issue the additional-work quotation, explain any effect on other work, and ask the person authorised under the contract to give clear approval. Keep the reply with the original quote and the relevant drawings or photos.

You might write:

Subject: Please confirm additional drainage work — EX-01
Following our discussion, I've attached quotation EX-01 for the additional drainage and removal of the gravel strip. The net change is £1,020 including VAT, bringing the agreed project total to £15,420 including VAT. The proposed target completion date is now Monday 23 November 2026.

Please reply to confirm you're happy with the scope, price and revised target date and would like us to proceed. I'll hold off on the additional work until we've confirmed the change.

An email or clear message can be useful evidence, but don't assume a text overrides a contract requiring a formal instruction. Silence isn't a reliable form of approval. Keep proposed and disputed amounts separate from agreed changes.

What if you uncover an unexpected problem?

Imagine lifting a floor and finding rotten joists. Photograph the issue where safe, stop the affected work if necessary, and explain what you've found. Distinguish the cost of investigating from the cost of repairing the problem, and check the contract before deciding who should pay.

If there's an immediate safety concern, take appropriate protective action within your competence and involve a qualified specialist when needed. Record what happened. Whether the protective work is chargeable is a separate question.

If the change affects subcontractors, delivery dates or a previously agreed finish date, explain the knock-on effects. You may need to agree a new completion date or give an update date where an essential delivery remains unconfirmed.

What are the UK rules on charging for extra building work?

There isn't one approval rule that covers every building contract. Your position depends on the original agreement, who instructed the change and whether the contract sets a procedure for pricing it. Some formal commercial contracts allow a properly authorised change to be instructed before the price is finally assessed. Smaller domestic jobs often depend on an agreed change to the original quotation or contract.

For consumer work, Citizens Advice explains that a trader generally cannot charge above an agreed quote simply because their own costs have risen. Extra work that the customer has agreed to pay for is a different matter.

The Consumer Rights Act 2015, section 50 addresses when information given by a trader is binding and the effect of expressly agreed changes. Section 51 deals with reasonable prices in certain consumer service contracts where no price, or method for fixing it, has been agreed. It isn't a general right to set a new price for extra work after the event.

The Competition and Markets Authority's guidance on fair consumer contracts also warns against terms that effectively give a business a blank cheque to impose higher costs without consent.

For a substantial or disputed change, check your own contract and get appropriate professional advice. Some new agreements made at a customer's home or at a distance may also involve consumer cancellation requirements; the details depend on how and what was agreed.

Keep the original quote and a running record

Don't overwrite the original accepted quote and lose the evidence of what was first agreed. Give each change a reference, keep its quotation and approval, and maintain a simple list of proposed, agreed and disputed adjustments.

For the worked example, your running record would show the original £12,000, an approved £1,200 addition, an agreed £350 credit and a revised £12,850 value, all excluding VAT. Record payment-stage and completion-date changes alongside those figures so they aren't forgotten at the final invoice.

Quote Forge can help you prepare and save a separate, clearly labelled quote for additional work, calculate totals and VAT from the figures entered, and download a branded PDF. Keep the customer's written approval and your running record of agreed changes alongside the quote PDF. A separate quotation doesn't, on its own, establish that a contract change has been accepted.

If the valuation involves complicated contract rates or several overlapping changes, a quantity surveyor may help. For disputed entitlement, customer rights, suspension or termination, seek suitable legal advice before acting.

Frequently asked questions

Can a builder charge for extra work the customer hasn't approved?

Not simply because the builder has carried it out. Payment needs a contractual or other legal basis, which might include a valid instruction under the agreed contract. Check the facts and keep records; don't assume a customer's silence is consent.

Does every variation need a signed form?

Not necessarily. The contract may require a signed or formally authorised instruction, but there isn't a universal rule that every extra on every job needs the same form. A clear written exchange is useful evidence where it is sufficient under the agreement.

Can I increase a fixed price because materials cost more?

Don't assume you can. A supplier price increase by itself generally doesn't justify charging a domestic customer more than an accepted fixed quote. Check whether a valid adjustment clause applies or agree an appropriate change with the customer.

Must I refund the whole quoted amount for work removed from a job?

Not automatically. Follow any omission and valuation provisions, or agree a fair credit based on the original scope and the circumstances. Explain any non-recoverable costs with evidence rather than deducting them without discussion.

What if the customer disputes the extra work after completion?

Compare the original scope with the change quotation, instructions, photos and approval. Explain the calculation in writing, keep disputed items separate from agreed payments and follow the contract's dispute process. Get professional advice if the issue cannot be resolved.

The simple way to avoid arguments over extras

Check the original scope, price the change rather than the whole job again, show additions and credits clearly, explain any effect on the finish date and get the right approval. A short written agreement before the work starts is usually much easier than defending an unexpected charge after the job is finished.