Builder Overcharging: How Contractors Win on Price and Make It Back

How a low price wins the job

In a tender, the lowest price usually wins. Margins are thin: the biggest UK contractors averaged just over 3% before tax in the Construction Index Top 100 for 2026. So the winning price often left the most out, and the gaps are charged later. One way out of the race to the bottom is an open-book cost plus contract.

The 9 ways builder overcharging happens after a low price

None of these is fraud on its own. Each exists for a reason, and each has a fix you can put in place before you sign.

1. Provisional sums set too low

1. What happens. Undesigned work is priced as an allowance, at a low value. The quote looks cheaper.
2. Why it exists. Some work truly cannot be priced yet, so an allowance is fair.
3. Your fix. Ask how each sum was set, and test it against a realistic cost. Our guide to provisional sums in construction shows how.

2. Vague exclusions

1. What happens. A list at the back of the quote leaves out items you assumed were included.
2. Why it exists. Contractors limit risk on work they cannot see or control.
3. Your fix. Read every exclusion and put a value on it before comparing quotes.

3. Variations priced without competition

1. What happens. Before you sign, several builders compete. After you sign, one builder prices every change.
2. Why it exists. Changes disrupt the job, and you cannot easily walk away mid-job.
3. Your fix. Instruct every change in writing, and agree price and time before the work starts.

With us, variations are only agreed in writing and priced before the work is done. Our guide to variations in construction contracts shows how they are valued. On larger jobs, an employer's agent can run this for you.

4. Daywork with no cap

1. What happens. Unplanned work is charged by the hour, plus materials and a mark-up.
2. Why it exists. Some work cannot be measured until it is done.
3. Your fix. Agree the daywork rates in advance, sign the sheets daily and set a cap.

5. Delay and extended preliminaries

1. What happens. If the job runs longer, the contractor claims more for site management, scaffolding and welfare.
2. Why it exists. Those costs really do run by the week.
3. Your fix. Give decisions and information on time, and record the cause of every delay.

6. Loaded materials mark-up

1. What happens. Materials on daywork or changes carry a mark-up you never agreed.
2. Why it exists. Buying, handling and waste cost money, and a mark-up is normal.
3. Your fix. Agree the percentage in writing, and ask for supplier invoices on cost-based work.

7. Client-supplied items with no coordination

1. What happens. You buy the kitchen or the lights yourself, then pay for delays and refitting.
2. Why it exists. The contractor is not responsible for what it did not supply.
3. Your fix. Agree who coordinates, measures and fits each client item, and at what price.

8. An estimate dressed as a quote

1. What happens. You think the price is fixed. The paperwork says "estimate".
2. Why it exists. Builders avoid fixing a price when the scope is unclear.
3. Your fix. Ask for a written quote for a defined scope, and check the wording. Our guide to a builder asking for more money explains the difference.

9. The final account nobody checks

1. What happens. The last bill bundles every change, allowance and claim, and it is paid in a rush.
2. Why it exists. Everyone is tired, and the client wants to move in.
3. Your fix. Check that each provisional sum shows the actual cost, omissions are credited and daywork matches signed sheets.

Our guide to why construction projects go over budget explains how the quote becomes the final account.

Overcharging on materials and mark-ups

Clients often ask whether a builder can charge more for materials than it paid. On a fixed quote, the question does not arise: the materials are in the price, whatever the builder paid.

It matters on daywork, cost-plus work and changes. There, the contract should say what mark-up applies. If it says nothing, you have room to argue. Homeowners have a legal backstop too. Under the Consumer Rights Act 2015, where the price was not fixed, you pay a reasonable price and no more.

Delay and preliminaries

Preliminaries are the running costs of the site: management, scaffolding, welfare, security, insurance. They are often priced by the week.

When a job runs late, those weeks are claimed. Some delays are the contractor's fault. Many are caused by late decisions, late information or changes from the client side. A contractor that under-priced the job has every reason to claim for them.

Keep a simple record. When was each decision needed, and when was it given? That record is what decides a delay claim.

How to spot builder overcharging in a quote before you sign

Most of the money in this article can be seen in the quote, if you know where to look.

Our tips on choosing a commercial contractor cover the checks to make on the firm itself.

If you think you have been overcharged

Stay calm, and put everything in writing. Pay what is clearly due, and dispute only the part you question.

1. Ask for a breakdown. Request the extras line by line, against the quote and agreed changes.
2. Get an independent view. Another builder or a quantity surveyor can tell you whether the price is reasonable.
3. Negotiate. In our experience, many disputes settle once both sides see the same breakdown.
4. Escalate. Homeowners can use Citizens Advice and the small claims track, for claims up to £10,000.

If your builder is asking for more money right now, our step-by-step guide to a builder asking for more money covers what to do. Commercial clients have stricter rules on payment notices and pay less notices. Our guide to the construction adjudication process in the UK explains them.

Most overcharging starts with a price that was never complete. Want a builder whose price covers the whole job from the start? Book a site visit.