Cash Flow Forecasting for Construction Subcontractors

Most construction subcontractors manage cash flow the same way: a spreadsheet, a rough idea of what is owed, and a hope that payments arrive on time. When you are running three or four projects at once, each with different payment cycles, retention percentages, and client habits, that hope starts to feel like a gamble.

Cash flow forecasting gives you something better than hope. It takes the payment applications you have submitted, the ones you are about to submit, and the retention that is due for release, and projects them forward so you can see what is coming in over the next 30, 60, or 90 days. If there is a gap, you see it before it becomes a crisis.

According to the Federation of Small Businesses (2023), late payment is the single biggest cause of cash flow problems for small construction firms. A cash flow forecast does not stop late payments, but it does give you the visibility to prepare for them.

What Is Cash Flow Forecasting in Construction?

A cash flow forecast is a forward-looking view of money coming into and going out of your business over a defined period. In construction, cash inflows come from two main sources: payment applications and retention releases.

Unlike a profit and loss report, which tells you what has already happened, a forecast tells you what is likely to happen next. For subcontractors running multiple projects, this distinction matters enormously. You might be profitable on paper but still unable to pay your suppliers because the cash has not arrived yet.

The key difference between a general business forecast and a construction-specific one is the structure of payments. Construction payments follow contractual timelines set by the Construction Act, with payment due dates, notice periods, and retention deductions that all affect when money actually lands in your account.

Why Subcontractors Need Cash Flow Forecasting

Cash flow problems do not usually happen because a business is unprofitable. They happen because the timing is wrong. You finish work in January, submit your application in February, and the payment does not arrive until April. Meanwhile, your wages, materials, and plant hire are due every month.

A forecast helps you in three specific ways:

• Spotting shortfalls early. If you can see that April is going to be tight, you can chase outstanding invoices, delay non-essential spending, or arrange an overdraft before it becomes urgent.
• Timing retention releases. Retention can represent a significant sum. On a £200,000 contract at 5%, that is £10,000 sitting in your client's account. Knowing when it is due for release, and including it in your forecast, gives you a more accurate picture.
• Planning across multiple projects. Each project has its own payment cycle. A forecast combines them all into one view, so you can see your total expected income week by week or month by month.

The Three Inputs That Drive Your Cash Flow

A construction cash flow forecast is built from three things: payment applications, retention releases, and payment terms. Understanding how they interact is the key to an accurate forecast.

Payment applications

Each submitted application has an expected payment date based on your contract terms. A typical JCT contract gives the client 14 days to issue a payment notice and a further 14 days to make payment. So an application submitted on 1 March might have a payment due date of 29 March. For more on how these timelines work, see our guide to Construction Act payment terms.

Retention releases

Retention is money your client holds back as insurance against defects, typically 5% of each payment application. It accumulates over the life of the project and is released at specific milestones, usually practical completion and the end of the defects liability period. For a detailed explanation, read our guide to retention in construction.

Payment terms and client behaviour

Contract terms set the theoretical payment timeline. Client behaviour determines the actual one. Some clients pay on the dot. Others routinely pay 14 or 30 days late. A good forecast accounts for this reality, not just the contractual ideal.

A Worked Example: Three Projects, One Cash Flow

Let's say you are running three projects simultaneously:

• Project A: £80,000 contract, 5% retention. Application #3 for £18,000 submitted, due 15 April.
• Project B: £150,000 contract, 5% retention. Application #5 for £25,000 submitted, due 28 April. Retention release of £3,750 due 1 May (practical completion reached).
• Project C: £60,000 contract, 5% retention. Application #2 for £12,000 being prepared, expected submission next week, due mid-May.

Without a forecast, you know you are owed money but you do not know exactly when it will arrive or whether it covers your outgoings. A forecast maps this out:

• April: £43,000 expected (£18,000 from Project A + £25,000 from Project B)
• Early May: £3,750 retention release from Project B
• Mid-May: £12,000 from Project C (if submitted on time)

Total expected over the next 6 weeks: £58,750. Now compare that to your outgoings. If your monthly costs are £30,000, you can see that April is covered but early May might be tight before Project C's payment arrives. That is the kind of insight a forecast gives you.

Common Cash Flow Mistakes Subcontractors Make

Even experienced contractors fall into these traps:

• Forgetting retention. Your application says £20,000, but after 5% retention you only receive £19,000. Over a full project, this adds up to a significant amount sitting in someone else's account.
• Assuming payment on the due date. According to the Chartered Institute of Building (CIOB, 2020), the average payment time in UK construction is 43 days from invoice, well beyond most contractual due dates. Build a buffer into your forecast.
• Not updating the forecast. A forecast is only useful if it reflects reality. When a payment is received, an application is delayed, or a new project starts, the forecast needs updating. A static spreadsheet forecast goes stale within a week.
• Looking at projects in isolation. Each project might look fine on its own. It is the combined picture that reveals problems, when two large payments are due out in the same week but nothing is coming in until the following month.

How BuildQS Forecasts Your Cash Flow

BuildQS generates a 90-day cash flow forecast automatically from your project data. There is nothing to set up or configure. It pulls from two sources:

• Payment applications: Every submitted application with a payment due date appears in the forecast. When the application is marked as paid, the forecast updates to show actual vs expected.
• Retention releases: Scheduled retention releases (6+6, 12-month, or custom) are included with their expected release dates. For more on how retention release schedules work, see our guide to retention release.

The forecast groups expected income by month, giving you a clear picture of what is coming in and when. It updates automatically as you submit new applications, receive payments, or adjust dates. No manual recalculation needed.

If you want to model different outcomes, BuildQS also offers scenario planning. You can create what-if scenarios to see the impact of a late payment, a delayed project start, or a change in retention terms. This is available on the Professional plan and above.

Frequently Asked Questions

How far ahead should I forecast?

For most subcontractors, 90 days is the right window. It is long enough to spot problems before they happen but short enough that the data is still reliable. Beyond 90 days, too many variables change for the forecast to be accurate.

What about outgoings?

BuildQS focuses on the income side, payment applications and retention releases. For a complete cash flow picture, you would compare the forecast against your outgoings tracked in your accounting software (such as Xero). The BuildQS forecast gives you the income half of the equation, which is the part most subcontractors find hardest to track.

Does the forecast include VAT?

The forecast shows net amounts from your payment applications. VAT handling depends on your VAT scheme and is managed through your Xero integration. For more on VAT in construction, see our VAT for construction guide.

Can I share the forecast with my accountant?

Yes. You can invite your accountant as a team member with viewer access. They will see the same forecast you do, without being able to modify your project data. This is particularly useful for quarterly reviews or when discussing finance facilities with your bank.

Sources

• Federation of Small Businesses (2023). "Late Payment Report: The Impact on Small Firms."
• Chartered Institute of Building, CIOB (2020). "The Real Face of Construction: Payment, Performance and Productivity."