Your builder has sent a quote for £80,000. At the bottom, one line: "15% deposit required on acceptance." That's £12,000 before anybody picks up a shovel.
Every instinct says this is wrong. You search "should I pay builder deposit" and get a wall of warnings. Don't pay too much. Don't pay in cash. Reputable builders don't need deposits. Your money is at risk. The forums are full of horror stories: deposits paid, builders vanished, £10,000 gone.
All of that is real. Builder deposit fraud happens. But the advice stops at "be careful" and never answers the question that's actually keeping you awake: is this specific deposit, on this specific quote, reasonable?
The only way to answer that is to do the maths.
What Your Builder Spends Before Earning a Penny
A builder starting a single-storey extension doesn't turn up with a wheelbarrow and a bag of sand. On day one of my extension, the builder arrived with a 1-tonne mini digger on a trailer, a labourer, and a plan to excavate strip foundation trenches for a 55.5m² rear and side-return extension. By the end of that first week, there was a concrete pour.
The key driver of first-week cost is your foundation perimeter: the total length of new external wall. A 30m² extension is roughly 6m × 5m with three new sides, giving about 17 metres of new wall. An extension priced at £80,000 in 2026 will typically run 22-26 metres of perimeter (larger footprint, an L-shape adding a side return, or deeper foundations in expansive clay).
Here is how the numbers work. The example below uses 22 metres of new wall at 0.9 metres foundation depth, typical for clay soils across much of southern England. These are 2026 trade prices for south England outside London.
| Item | How it is calculated | Quantity | Unit cost | Week-one cost |
|---|---|---|---|---|
| Ready-mix concrete (trench fill) | 22m × 0.6m wide × 0.9m deep | 11.9m³ | £123/m³ | £1,465 |
| Concrete pump hire | Fixed (needed when truck cannot reverse in) | 1 | £375 | £375 |
| Blockwork to DPC, both leaves | 22m × 2.2 blocks/m × 3 courses, inner and outer leaf | 293 blocks + mortar + DPC | £2.00/block avg | £690 |
| Mini digger (1-tonne) | Week hire plus delivery and collection | 1 week | £400 | £400 |
| Spoil removal | Trench spoil: 22m × 0.6m × 0.9m = 11.9m³ | 2-3 grab lorry loads | £215/load | £430-645 |
| General labourer × 2 | Excavation, levelling, directing the pour | 2 workers × 5 days | £175/day | £1,750 |
| Bricklayer labour | Blockwork from concrete to DPC | 2 bricklayers × 3 days | £250/day | £1,500 |
| Site welfare | Portable toilet, legal requirement on construction sites | 1 week | £50 | £50 |
| Total | £6,660-6,875 |
Scale to 26 metres of perimeter (a larger L-shaped extension or deeper clay foundations), the concrete and blockwork lines add roughly £370 combined, pushing the first-week total past £7,000.
A note on spoil removal: many builders use a grab lorry rather than a skip for foundation excavation. A grab lorry arrives, lifts the spoil over your fence in a single visit, and leaves in an hour. At £150-250 per load (5-6 tonnes each), two loads runs £300-500. A hired skip does the same job at £200-350 but requires a council permit if placed on the pavement and sits on your drive for two weeks. Both work. Builders doing volume work tend to use grabs because they clear the site faster.
First-week cost components for a single-storey extension, with the 15% deposit line shown for comparison.
The first-week total for a single-storey extension in southern England sits at £6,700 to £7,200. Against a 15% deposit on an £80,000 quote (£12,000), the numbers hold. The deposit covers genuine first-week outlays with meaningful headroom for the builder's slot-booking cost and overhead. It is not free money.
Real Numbers: What Happened on My Build
I paid a 20% deposit of £9,100 on a £45,500 quote for a 55.5m² extension in Oxfordshire (a rear addition with a side return, roughly L-shaped). I paid by bank transfer. Use a credit card if you can, for the Section 75 protection.
Several months passed between paying the deposit and work starting. During that gap, the deposit functioned as a slot-booking payment. My builder was finishing another job that had overrun (they always overrun). The deposit locked in my place in his schedule.
When he did arrive, the spend was immediate. The pre-mix concrete for the foundation pour came to £2,650. Blockwork to DPC cost £6,450. That's £9,100 in materials and labour within two weeks of starting, almost exactly matching the deposit.
The £2,650 concrete figure is higher than the worked example above implies for a standard rear extension. The reason is perimeter. An L-shaped build with rear and side return runs around 34-36 metres of new external wall. At 35m perimeter and 1.0m depth, the concrete volume is roughly 21m³, nearly double the 11.9m³ in the 22m worked example. At 2021 SE prices with pump hire, that produces a figure very close to £2,650.
The builder had quoted £9,000 for the foundations-to-DPC stage. The actual cost came in at £10,450. The deposit didn't cover it.
Was 20% too much? No. A 15% deposit (£6,825) would not have covered the concrete pour plus the first blockwork payment (£2,650 + £4,000 = £6,650). On a project with 35m of new wall rather than 22m, the 15% benchmark in the worked example doesn't hold. The deposit maths always comes back to one question: does the percentage match the first-week spend profile for a project of your size?
My build was priced in 2021. Construction costs, both materials and labour, rose sharply through 2022 and 2023 and have not retreated. A comparable structural package from the same builder today would sit closer to £65,000-£80,000, which is why the worked example uses that range.
Why the "Reputable Builders Don't Need Deposits" Line Is Half-True
You'll read this everywhere. Established builders have trade accounts at merchants with 30 to 60 day credit terms. They can order blocks, cement, and timber without paying upfront. So why do they need your money?
The answer is that trade credit doesn't cover everything.
Ready-mix concrete is usually paid on delivery. Plant hire companies want payment on collection. Labourers and subcontractors are paid weekly. A builder starting your job is out of pocket for those costs the moment the digger arrives on site, and their merchant credit terms don't help with any of it.
A builder running two or three jobs simultaneously, each with a different payment schedule, might have £15,000 to £25,000 in outstanding costs at any given time. That's normal working capital for a small construction firm. A deposit from a new client isn't a sign of financial distress. It's how the business model works.
That said, the line is half-true for a reason. A builder who needs 30% or more upfront, specifically for "materials," is either taking on work their cash flow can't support, or funding another project with your money. Both are problems.
The Benchmarks
Where your builder's deposit request falls on this scale tells you something.
10% or less. Conservative and safe. The FMB (Federation of Master Builders) recommends members charge no more than 10%. At this level, the deposit barely covers first-week concrete and labour costs on a large extension. The builder is absorbing the cash-flow gap themselves.
10 to 20%. Market standard. Most extension builders in England sit here. Citizens Advice considers up to 25% "not unusual," but the practical consensus clusters at 10 to 20. This is the range where the deposit aligns with genuine mobilisation costs plus a reasonable slot-booking element.
20 to 25%. Needs justification. If your builder is pre-ordering expensive items (structural steelwork, bespoke glazing, specialist materials with long lead times), a higher deposit can make sense. But you should see an itemised list of what's being ordered and when. "Materials" as a one-word explanation is not enough.
Above 25%. Red flag. Walk away unless there is an extraordinary, documented reason (rare specialist imports, for example). The community data is unanimous on this. Across eleven forum threads analysed for this post, every cautionary tale about lost deposits involved an upfront payment above 25%.
Any builder requesting 50% or more upfront is either in severe financial difficulty or running a scam. This is not an exaggeration. One forum post that appeared to be a legitimate 50% deposit request turned out to be a fraudster who had hacked the real builder's email account. The actual builder confirmed he never takes advance payments. Verify deposit requests through a phone call to the builder's known number, not by replying to an email.
The Question to Ask
Don't just ask "how much is the deposit?" Ask this:
"Can you break down what the deposit covers?"
A builder who can answer with specifics (concrete delivery for the pour, digger hire for excavation, first week's labour, skip) is giving you a deposit that maps to real costs. A builder who can't, or who says "it's just how we work," is asking you to fund their cash flow on trust alone.
You can go further. Ask whether you can pay for certain items directly. Some homeowners order the concrete themselves, arrange the skip hire, and pay the plant hire company directly. This reduces the deposit to a labour-and-booking amount. Not every builder will agree, but it's a legitimate negotiation, not an insult.
Protect Yourself Anyway
Even when the deposit amount checks out, protect the payment.
Pay by credit card where possible. Section 75 of the Consumer Credit Act 1974 makes the card provider jointly liable on purchases between £100 and £30,000. The catch for extensions: the threshold applies to the total contract value, not the deposit. Most extension contracts exceed £30,000, which technically puts them outside Section 75 coverage. One workaround is asking the builder to invoice in separate elements (foundations, walls, roof) so each falls under the threshold. Even without Section 75, a credit card gives you chargeback rights that a bank transfer does not.
Get a receipt. A proper receipt showing the date, amount, what it covers, and the conditions for refund. Not a text message saying "cheers, got the money."
Have a written contract before you pay anything. The contract should include the deposit amount, the payment schedule for the rest of the build (tied to completed milestones, not dates), and what happens if either party cancels.
Tie your payment schedule to building control milestones, not calendar dates. Foundations approved, walls to roof height, roof weathertight, first fix complete, second fix complete, final inspection. Each payment releases only when the milestone is verified. This gives you a third-party check (the building control officer) before you release funds.
The Bottom Line
A 10 to 20% deposit on an extension is not a red flag. It's a normal part of how small construction firms manage cash flow on jobs where the first week's costs run into thousands before any income arrives.
The red flag is not the deposit itself. It's a deposit you can't trace to real costs. If the number matches the first-week spend profile for a project of your size, the builder can explain what it covers, and you're paying by credit card with a proper contract, the deposit is doing exactly what it's supposed to do: getting your project started.