Your builder is due to start in six weeks. Then eight. Then nine. When he finally arrives, he's on site every day for a fortnight, then gone for most of a week, then back. You text. He apologises. He'll be there Thursday.
He's not being difficult. He's managing cashflow.
Every small extension builder runs multiple jobs simultaneously. Not because they're disorganised, but because construction payment schedules make single-project working almost impossible to sustain.
The Gap That Drives Everything
Building contracts pay in stages. The homeowner doesn't wire money daily; they release funds when milestones are reached. The builder, on the other hand, spends money daily. Labour is weekly. Materials arrive before work starts. Subcontractors want settling within seven to fourteen days of completing a stage.
The result is a structural gap between spending and receiving. On a typical kitchen extension, the pattern looks something like this: the builder starts groundworks, spends £3,000 to £5,000 on concrete, hardcore, and labour in the first two weeks, then invoices for the foundations milestone and waits seven to fourteen days for payment. He's carried that spend entirely from his own resources or trade credit. Once the foundations payment lands, he's back in positive territory, but for perhaps two to three weeks he was operating at a deficit.
Now the walls go up. Block delivery, bricklayer day rates (£200–300/day), scaffolding, the labourer humping materials (£150–200/day). The next stage payment doesn't land until the roof is on and weathertight. That's six to ten weeks of spending against a milestone that hasn't been reached.
That gap, the period between money going out and the next milestone payment coming in, typically runs thirty to sixty days. On a £55,000 extension, the builder might be carrying £8,000 to £15,000 of unreimbursed spend at any given point during the structural phase.
Construction insolvency rates in the UK are the highest of any sector. Nearly 4,000 construction firms went under in the twelve months to August 2025, representing 17% of all business collapses nationally. The primary cause isn't bad workmanship. It's cashflow: firms running out of operating capital because their spending runs consistently ahead of their receipts.
A builder running one job at a time would need to fund those gaps entirely from savings or a business credit line. Most small builders carry neither. Running two or three jobs simultaneously solves the problem: when Job A is in a waiting period, Job B generates an incoming stage payment that keeps the business liquid. The multiple-site model isn't a choice, it's the operating mechanism.
Dave's Year: A Worked Scenario
The following is a fictitious but realistic year in the life of a small extension builder. Dave runs a two-person operation with himself, a labourer who also helps with groundwork, and occasional subcontractors for specialist stages. His annual turnover sits around £240,000, above the VAT threshold, from three to four extension projects per year at £55,000 to £70,000 each. His net profit target, after overhead, materials, labour, and VAT, is 3 to 5%. That's around £7,000 to £12,000 after costs on a good year.
His monthly overhead runs roughly £2,500: van finance and fuel, tools and equipment, public liability insurance, employers' liability, scaffold hire that isn't included in job quotes, accountant, and phone. That overhead runs whether he's on site or not. Whether it rains in January or not.
Here's how the year unfolds across four overlapping jobs.
Job A: The Harris kitchen extension (30m²)
Dave starts January with Job A already two months in. The Harris family in Guildford hired him in November for a 30m² single-storey rear extension, quoted at £54,000. He collected a 15% deposit (£8,100) in November, started groundworks in December, and received the foundations milestone payment of £10,800 (20% of quote) in early January when BCO signed off the foundations.
| Month | Event | Money In | Money Out | Running Position |
|---|---|---|---|---|
| Jan | Foundations BCO approved; stage payment received | +£10,800 | Labour + materials (blockwork): £6,200 | +£4,600 |
| Feb | Walls rising; blockwork, scaffold, steels | nil | Materials £7,800 + labour £4,400 | -£7,600 |
| Mar | Roof structure, tiles, watertight reached | +£10,800 (watertight payment) | Roofer £3,100, carpenter £2,200, materials £2,400 | -£4,100 |
| Apr–May | First fix; electrician and plumber (client-procured); plasterboard | +£10,800 (first fix payment) | Plasterboard, hardware, Dave's labour: £4,800 | +£1,900 |
| Jun | Plastering, second fix beginning | nil | Plasterer £2,400, labourer £1,600 | -£2,100 |
| Jul | Second fix complete; snag and sign-off | +£8,100 (second fix) + £5,400 (final, less 5% retention) | Dave's labour £2,800, snag remedials £600 | +£7,500 |
The retention (5% of £54,000 = £2,700) sits held until the defects liability period ends, typically six months after practical completion. Dave will collect it in January of the following year if the Harris family are happy. If they're not, he'll be chasing it.
Job B: The Okafor garage conversion (smaller job)
Dave picks up a garage conversion in Woking in February. Quoted at £22,000. Simpler work than a full extension, but valuable because the cashflow profile is faster: shorter stages, quicker milestone payments.
He collects a 10% deposit (£2,200) in February and starts in March. The whole job runs ten weeks. Three stage payments, final payment in May. Total money in over ten weeks: £22,000 minus retention (£1,100).
This job's incoming payments land during the worst cashflow dip on Job A: February and March, when Dave is carrying £7,600 to £4,100 of unreimbursed spend on the Harris extension. Job B's deposit and first stage payment directly fund the gap.
Job C: The Patel extension (35m², starting April)
In April, Dave accepts a 35m² extension in Reigate. Quoted at £62,000. 15% deposit (£9,300) collected on acceptance. Work doesn't start until June, but the deposit lands in April, improving Dave's cashflow position during the first-fix gap on Job A.
Job C runs June through November. This is Dave's most complex job of the year: the Patel build hits a steel delivery delay in August (six weeks from the fabricator), and BCO holds up the watertight milestone inspection by two weeks. The delay compresses Dave's cashflow position heading into autumn, which is why he needs Job D.
Job D: A small extension in Dorking (25m², starting September)
Dave books this job in July, deposit in August, starts in September. The Dorking client, call them the Robinsons, are getting a 25m² utility room and WC addition at £38,000. Dave runs this alongside the tail end of Job C.
The Robinson deposit (£5,700) lands in August, right when the Patel steel delay has left Dave with six weeks of payroll and overhead and no incoming payment from Job C. Without Job D's deposit, Dave would have had to draw down on a business overdraft facility.
What the year looks like, summed up
| Month | Approx Money In (all jobs) | Approx Money Out (all jobs) | Net Monthly |
|---|---|---|---|
| Jan | £10,800 | £10,600 | +£200 |
| Feb | £2,200 (Job B deposit) | £14,200 | -£12,000 |
| Mar | £12,200 | £11,800 | +£400 |
| Apr | £9,300 (Job C deposit) + £8,800 (Job B stage) | £13,600 | +£4,500 |
| May | £11,000 (Job B final) | £12,100 | -£1,100 |
| Jun | £10,800 (Job A second fix) | £14,800 | -£4,000 |
| Jul | £10,200 (Job C foundations) | £11,200 | -£1,000 |
| Aug | £5,700 (Job D deposit) | £9,800 | -£4,100 |
| Sep | £12,400 (Job C watertight) | £14,600 | -£2,200 |
| Oct | £10,000 (Job D foundations) | £11,600 | -£1,600 |
| Nov | £14,000 (Job C second fix + final) | £10,800 | +£3,200 |
| Dec | £9,200 (Job D watertight + Job A retention) | £8,400 | +£800 |
| Total | ~£116,600 | ~£143,500 |
The total income figure of £116,600 appears lower than Dave's annual turnover because not all jobs start and finish within the calendar year. Job A started in the previous November; Jobs C and D will finish in the following year. Annual turnover of £240,000 accounts for the full revenue from each job across its lifetime, not just the portion of payments that landed in this twelve-month window.
What the monthly table shows clearly is that Dave runs at a negative monthly cashflow position for seven of twelve months. He's never genuinely flush. The positive months in April and December are largely driven by deposits and retention release from the previous year, not by profitable months' trading. His net margin on the year is approximately £8,000 to £10,000. On a six-day week.
Why This Explains So Much
The slow start. In a real extension project in southern England, the builder started one month late, citing "overrunning jobs." That's not an excuse. That's Job B or C on a different client's property still generating the cashflow that funded the previous month's overhead. The delay was structural, not personal.
The disappearing act. Builders are most intensively present around invoiceable milestones. Once the foundations BCO sign-off lands and the stage payment is collected, there is often a period of lighter attendance: the builder is on another site generating the next incoming payment from that project while this project's materials are on order or waiting for a subcontractor. Homeowners read this as loss of interest. It's cash management.
The "next week" reply. Labour runs weekly. If Dave is carrying two weeks of unreimbursed spend on the Patel job and the Robinson's deposit has just covered payroll, the Harris extension's second-fix phase has to wait until next week because that's when the money will be there.
The variation pricing. Small builders tend to price variations fast and high, not because they're taking advantage, but because unexpected scope on a fixed-price contract can tip a marginally profitable job into a loss. A net margin of 3 to 5% means a single unexpected week of delay or one unplanned material substitution can wipe the profit on that job entirely.
What to Do With This Knowledge
Understanding the mechanics doesn't mean accepting delays passively. It means managing the relationship with better information.
Ask about the schedule when you book, not when problems start. Ask your builder directly: how many other live projects do you have running when mine starts? What would cause you to need to pause my job? What's your usual pattern of site visits during the structural phase? A good builder will answer honestly. Evasiveness is a flag.
Tie payments to inspections. Stage payments linked to building control milestones mean your builder's incentive to complete each stage runs parallel to your protection. When the BCO signs off foundations, that's your confidence check before releasing the next payment. This structure also forces visibility: you both know what the next milestone is and roughly when it's expected.
Understand the gap. When your builder is absent mid-project, ask what the next milestone trigger is and what's holding it up. Is he waiting for a BCO inspection appointment? Waiting for steel delivery? On another site because this project is in a materials wait? The answer tells you whether the delay is something you can influence (chasing BCO, confirming steel order) or something structural.
Don't push for rushed milestone sign-offs. It's tempting to accelerate payment milestones to show goodwill and maintain momentum. Resist it. Milestone payments exist to protect you. Releasing money ahead of completed work gives your builder less incentive to return, not more.
For a detailed breakdown of how deposits work and what first-week costs actually look like, see Justifying the Builder Deposit: A Worked Example with Real Numbers.
The Margin Reality
Dave's 3 to 5% net margin puts him in line with the construction industry as a whole. Residential extension builders average 3 to 5% net after all costs. Large commercial contractors average closer to 1.7%. The FMB's State of Trade survey for H2 2025 found that 51% of construction SMEs reported lower-than-expected profits or losses, and 20% feared their business viability was under threat.
Those figures are the context for every invoice, every deposit request, and every "next week" message you receive. Your builder is not running a fat business. He's running a lean one on thin margins with expensive fixed costs, lumpy income, and structural cashflow gaps that require careful management across multiple jobs to keep the whole operation moving.
Knowing this doesn't make delays acceptable when they're excessive or unexplained. It does make it possible to have a straightforward conversation about them rather than an adversarial one.
The single most effective thing you can do to keep your builder on your site is to release stage payments promptly when milestones are achieved. A builder who knows that reaching the watertight milestone means payment within three days has a genuine financial incentive to prioritise your roof. One who expects to wait three weeks has less reason to rush. Fast payment creates fast work.
A Note on Small Builder Turnover
The scale difference between small and larger builders matters when thinking about resilience. A builder turning over £85,000 operates below the VAT threshold and is typically a sole trader or one-person operation. Three bad months and the business is done.
A builder turning over £400,000 has more buffer: more projects running simultaneously, more revenue to absorb a problem job, and the resources to hold trade credit across multiple merchant accounts. But the fundamental mechanism is the same. Multiple jobs running at once, each plugging the cashflow gap from another. The bigger the operation, the more projects needed to maintain the same structural balance.
When you're selecting a builder, the turnover question is partly a resilience question. A very small operation (below the VAT threshold, one or two jobs per year) has almost no buffer. Any delay, late payment, or problem on another client's job can directly affect your build timeline. It's not a reason to avoid small builders, many of the best extension builders are small operations, but it is a reason to ask how their schedule looks and whether they have the capacity for your job without it representing their entire workload.