Nobody runs out of money at the deposit stage. The builder's deposit is the payment everyone researches and haggles over, and on a typical kitchen extension it turns out to be one of the smaller sums you'll transfer. The month that actually hurts arrives about a year later, when a kitchen balance, a stage payment and a screed invoice all fall due within a few weeks of each other and your account needs tens of thousands in it, cleared and ready.
That crunch has a shape, and the shape is predictable. A real 30m² kitchen extension in Oxfordshire, documented invoice by invoice, shows exactly when the money left: a modest deposit, a steady drumbeat of stage payments, then a steep late ramp where the kitchen, the glazing and the finishing trades all concentrate. Plan your extension cash flow against that curve and the expensive months stop being surprises.
Where the money comes from is a separate question. Savings, a remortgage, borrowing money for an extension in some other form: financing your extension walks through the options. Everything below is the other half of the planning problem, the half that rarely gets written down: when the money leaves.
The Shape of the Spend
Three headline numbers anchor everything. On the documented build, construction excluding the kitchen came to £120,480, roughly £2,190/m². The kitchen itself added another £52,420. All-in: £172,900, about £3,143/m².
Put those numbers on a calendar and the curve appears. Professional fees trickled out months before anyone dug: £2,604 of architect fees through spring 2021, an £882 building control full-plans fee that August. Planning was granted in May 2021 and the builder started that October, so nearly half a year passed with only small cheques leaving. Then came fourteen months of active site work, paid in stages. And then a final stretch in which the kitchen, the replacement glazing, the screed and the finishing trades stacked up, concentrating roughly a third of the entire project cost into the last four or five months.
One caution on totals before the timeline. The builder's quote was £45,500. The all-in spend reached £172,900, because the quote never included the kitchen, the glazing, the professional fees, the meter moves or the landscaping. Plan your float against everything you intend to buy, and treat the builder's number as one line item among many.
The documented build also ran 35 months end to end, which is an outlier: a steel substitution saga and a supplier insolvency stretched it badly. A well-managed kitchen extension takes 9-15 months. The shape of the spend holds either way; only the horizontal axis stretches.
How Much Deposit for an Extension?
Between 10% and 20% of the contract value. That's the normal band for a reputable builder on an extension-sized job. A deposit of 20-25% needs a good justification. Above 25% is a red flag, and the reasons why (plus what a legitimate deposit actually pays for) are covered in builder deposits and stage payments.
The documented build paid 20% of the £45,500 quote: £9,100 by bank transfer, at the top of the acceptable band. The builder had spent almost exactly that sum within two weeks, on the concrete pour and the blockwork to damp-proof course, which is precisely what a deposit is for: materials for the opening stage, in the ground before the first stage payment falls due. If you want the arithmetic behind that, the worked deposit example runs it line by line.
Some builders take no deposit at all and simply invoice when the first milestone completes. That model exists and it's fine. Don't read a zero deposit as a warning sign or a 15% one as greed; read the payment schedule as a whole.
Stage Payments for Building Work
A sensible extension contract splits the price across stages that match visible, completed work. A common structure: 10% deposit, 15% at foundations, 20% when the walls are up, 20% at roof complete, 25% through fit-out, and 10% at completion. Each payment falls due when you can stand in the garden and see the stage finished. Agree that schedule in writing before anyone starts. Contracts and payment schedules covers what a good one looks like, including the 2.5-5% retention you hold back from each payment until the defects period ends.
Reality is lumpier than the model. The documented build's main contractor issued 29 invoices totalling £51,007 across fourteen months of active site work, roughly one every two to three weeks. Smaller payments than six clean stages suggest, but relentless. Groundwork also overran on cue: the foundations-to-DPC stage was quoted at £9,000 and came in at £10,450, because early stages carry the most unknowns and the ground hides them until the digger finds them.
The practical consequence for your cash flow: through the middle of the build you need a reliable few thousand pounds available every fortnight, on top of whatever lump is coming next. The combination is what catches people, and the lumps deserve their own section.
The Lumps: Kitchen, Steels, Glazing
Stage payments are predictable. The lumps are what break budgets, and three dominate a kitchen extension.
The kitchen is the biggest single lump of the entire project. £52,420 on the documented build, more than five times the builder's deposit. Kitchen suppliers commonly ask for around 25% when you order and the balance shortly before delivery; that's a market convention reported consistently by homeowners, not a rule any regulator sets. The balance-before-delivery payment is the one that blindsides people. It lands mid-build, often with a week's notice, at exactly the point your account has been drained by months of stage payments. The deposit never hurts. The balance does.
Steels arrive in the middle and grow. Supply and fabrication on the documented build ran to roughly £5,000-£7,000 across the timeline, pushed upward by a ridge beam substitution that had to be reversed and a rafter redesign. Steel is the line most likely to move after the contract is signed, because it's specified late and re-specified whenever building control asks questions.
Glazing wants a big deposit early. Bifold and sliding doors are ordered months ahead, and suppliers ask for serious money up front. The documented build paid £10,339 for its original bifold order by credit card in May 2022. The supplier went into liquidation before delivering, and the full amount came back through a Section 75 claim. Replacement doors were ordered late in 2022 at £16,743. Pay any supplier deposit between £100.01 and £30,000 by credit card: Section 75 of the Consumer Credit Act makes the card provider jointly liable, and on this build that one habit recovered £10,339 that would otherwise have gone down with the supplier.
The smaller late lumps deserve a line each. Liquid screed: £2,622 for 80m², one invoice near the end of the build. Plumbing and underfloor heating: £10,400, spread across first and second fix but weighted late. Moving the gas and electric meters: £1,830, small money with long lead times, so the booking and the payment happen months before the meters actually move.
The Real Curve, Month by Month
Here is where every payment above sits on the documented build's calendar. Dates shift on your build; the sequence barely does.
| When | What happened | Cash out |
|---|---|---|
| Spring 2021 | Architect drawings and planning application | £2,604 in fees |
| August 2021 | Building control full-plans application | £882 |
| October 2021 | Build starts; builder deposit paid (20% of £45,500) | £9,100 |
| Winter 2021-22 | Foundations and blockwork up to damp-proof course | £10,450 (quoted £9,000) |
| Through 2022 | Stage payments against completed work: 29 invoices over 14 months on site | £51,007 builder total |
| Mid 2022 | Steel supply and fabrication, spread across several invoices | £5,000-£7,000 |
| Mid 2022 | Failed utility trench; separate drainage and soakaway contractor | £3,250 |
| Mid 2022 | Gas and electric meter relocations, booked months ahead | £1,830 combined |
| May 2022 | Bifold doors ordered by credit card | £10,339 (recovered in full under Section 75 after the supplier failed) |
| Late 2022 | Replacement bifolds ordered elsewhere | £16,743 |
| September 2022 | Kitchen delivered; supplier balances concentrate here and in the weeks around it | Most of the £52,420 kitchen total |
| November 2022 | Liquid screed poured (80m²) | £2,622 |
| December 2022 | Kitchen fitting starts; plumbing and decoration balances | Final trade invoices |
| January 2023 | Practical completion; the curve flattens | Retention the only money still owed |
Read the right-hand column again and one thing stands out: the most dangerous stretch is autumn 2022, when the kitchen balance, the replacement glazing, the screed and ongoing stage payments all overlap. Your build will have its own version of that season. The point of the curve is to know which season it is before you're in it.
Contingency Gets Spent on Real Dates
Hold 10-15% of your total budget as contingency and assume you will spend it. Budgeting and contingency covers how to size it against your own risks. What the percentage never tells you is when it goes, and on the documented build the drawdowns had dates attached:
- £3,250 in cash to a separate drainage contractor when a utility trench failed mid-build.
- A steel bill that finished thousands higher than the first estimate after the ridge beam and rafter rework.
- Roughly £6,400 of net extra cost on the replacement bifolds, even after the Section 75 recovery.
Notice what's missing from that list: horrors discovered behind a wall. Most contingency on a real build goes on rework, substitutions and delay, and delay carries its own running costs (storage, a temporary kitchen that operates for extra months, trades returning for split visits). Homeowners on forums describe watching contingency drain into storage fees during an overrun, and that matches the documented experience. Treat contingency as a cash reserve for the second half of the build, and keep it somewhere you can draw on within a week or two.
Track the Curve While You're On It
Your bank balance lies to you mid-build, because it says nothing about what you've already committed. Track three numbers every week: what you've actually paid, what you've committed but haven't yet been invoiced for (the kitchen balance, the ordered steels, the glazing), and what you forecast for the remaining stages. The gap between your balance and the first two numbers is your real position. Tracking your build costs turns this into a working weekly habit.
Committed-but-not-invoiced is the number that saves you. When the kitchen balance exists as a known figure with a rough due date months in advance, the scariest payment of the build becomes a diary entry.
When the Spending Actually Stops
The documented build reached practical completion in January 2023: kitchen fitted, family using the space, cash curve flat. The formal building control completion certificate didn't arrive until a year later, held up by a subcontractor's delayed Gas Safe certificate. Fourteen months of paperwork tail, and not one payment fell due in it. "Done" and "signed off" are different milestones; only one of them costs money.
The genuinely last payment is the retention release: the 2.5-5% held back from the builder, paid out once the defects period ends, typically 6-12 months after completion. Put it in the diary at practical completion so it doesn't arrive as a surprise bill from a builder you'd mentally archived.
Cash timing is one thread of project managing a kitchen extension. The same calendar has to carry building control visits, kitchen design deadlines and trade sequencing, and each of those has its own money moment. The full kitchen extension task tree lays every task out in build order, and the structure is free to browse. The Access Pass (£49) unlocks the working detail on all of them, including the payment schedule and cost tracking guidance that sits behind the curve above.
Frequently Asked Questions
How much deposit does a builder take for an extension?
Between 10% and 20% of the contract value is normal. A deposit of 20-25% needs a good reason, such as expensive materials the builder must order before starting. Above 25%, ask hard questions or walk away. Some builders take nothing up front and invoice at the first completed milestone instead.
When do stage payments fall due?
At completed, visible milestones: typically foundations, walls, roof, fit-out and completion. During active work that means money leaving every few weeks; the documented build paid 29 invoices over fourteen months, roughly one every two to three weeks. Payments should follow finished work, never calendar dates.
How much contingency do you need for a kitchen extension?
10-15% of your total budget, held as accessible cash, and assume you will spend it. On real builds contingency goes on rework, substitutions and the running costs of delay far more often than on dramatic hidden defects.
What are the biggest single payments in a kitchen extension?
The kitchen balance is usually the largest single sum: the documented build's kitchen came to £52,420 all told, with the balance concentrated shortly before delivery. Glazing orders and structural steel follow. The builder's deposit, the payment most people worry about, was £9,100 on the same build: a fraction of the kitchen spend.