A Simple Guide to Job Costing for UK Construction Businesses

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Published 9 March 2026 · 13 min read

Here is a question that most small builders in the UK cannot answer honestly: how much profit did you make on your last three projects?

Not roughly. Not "it was alright" or "we did OK on that one." Actual numbers. Revenue minus all costs. The real profit.

If you cannot answer that question with specific figures, you are not doing job costing. And if you are not doing job costing, you are almost certainly losing money on some of your projects without knowing it.

This guide explains how to set up a simple job costing system for a UK construction business, without needing an accounting degree or expensive software.

What Is Job Costing?

Job costing is the process of tracking all costs associated with a specific project, so you can compare total costs against total revenue and calculate the actual profit (or loss).

In construction, every project is different. A kitchen refit costs differently to a loft conversion, which costs differently to a new build extension. Job costing lets you see, at project level, where your money is going and whether each job is actually profitable.

Without job costing, you only see the big picture at the end of the year when your accountant tells you your overall profit. By then, it is too late to do anything about the three jobs that lost money and dragged your annual figures down.

The Five Cost Categories

Every construction project has five main cost categories. Track these consistently across all your jobs and you will have a clear picture of profitability.

1. Labour

This is your single biggest cost on most projects and the one most commonly underestimated. Labour includes:

• Your own time (yes, your time has a cost, even if you do not pay yourself a wage per project)
• Employed staff wages, national insurance and pension contributions
• Agency or day-rate labour
• Overtime

A common mistake is forgetting to cost your own time. If you spend three days on site plus two evenings writing quotes and ordering materials, that is five days of your time. At a typical builder's day rate, that has a real cost that should be charged to the project.

2. Materials

Every material purchased for the project, including:

• Bricks, blocks, cement, timber, steel
• Fixings, sealants, adhesives
• Finishes (tiles, paint, flooring)
• Deliveries and crane hire for material handling
• Waste disposal (skip hire, tip runs)

Keep every receipt. Photograph receipts with your phone so they are not lost. Record which project each purchase is for at the point of purchase, not at the end of the month when you cannot remember.

3. Subcontractors

All payments to subcontractors working on the project:

• Trade subcontractors (electricians, plumbers, plasterers, painters)
• Specialist subcontractors (structural engineers, party wall surveyors, building control)
• CIS deductions if applicable

Get written quotes from subcontractors before they start. Compare their final invoice against the quote. If they charge more than quoted, you need to understand why and decide whether it is legitimate.

4. Plant and Equipment

Hired or owned equipment used on the project:

• Scaffold hire
• Skip hire
• Excavator hire
• Power tool consumables (blades, discs, bits)
• Temporary electrics and water supply

5. Overheads

The indirect costs that keep your business running but are not directly tied to a specific project:

• Van running costs (fuel, insurance, maintenance)
• Public liability and employer's liability insurance
• Accountancy fees
• Phone and internet
• Marketing costs
• Tool replacement and maintenance
• Office or storage costs

Overheads need to be allocated across your projects. The simplest method is to calculate your total monthly overheads and divide by the number of active projects, or allocate proportionally based on project value.

How to Set Up Your Job Costing System

You do not need complex software. Here is a straightforward approach that works:

1. Create a cost sheet for each project - either a spreadsheet tab or a project in your construction app. List the five cost categories as rows.
2. Set a budget for each category - based on your original quote. If you quoted £15,000 for materials, that is your materials budget for the project.
3. Record costs as they happen - every purchase, every subcontractor payment, every day of labour. Do not wait until the end of the week. Record costs on the day they happen.
4. Review weekly - every Friday, compare actual spend against budget for each category. Are you on track? Over budget? If you are 20% through the project and 40% through your materials budget, you have a problem that needs addressing now.
5. Close out at the end - when the project is complete, total all costs and compare against total revenue. This is your actual profit. Record it. Learn from it.

Common Job Costing Mistakes

Even builders who attempt job costing often get tripped up by these common mistakes:

• Not costing your own time - if you do not include your own labour, every job looks more profitable than it actually is
• Mixing project costs - buying materials for two projects in one trip and putting it all against one project. Split costs accurately at the point of purchase.
• Forgetting small purchases - a packet of screws here, a tube of silicone there. These £5-10 purchases add up to hundreds over a project.
• Not tracking variations - client changes that add cost need to be tracked separately and invoiced. If they are absorbed into the original budget, they eat your profit.
• Only costing at the end - if you only calculate costs once the project is finished, you have missed every opportunity to course-correct during the build.

What Good Job Costing Looks Like

Here is a simplified example for a £45,000 kitchen extension in the South East:

This level of detail tells you exactly where money was won and lost. Labour overran because the groundworks took an extra two days. Materials came in under budget because you found a better supplier for the steel. That information is gold for pricing your next job more accurately.

Job Costing and Your Tax Return

Good job costing also makes your life significantly easier at tax time. If you are a sole trader or limited company, HMRC expects you to know your costs. Having project-level cost records means your accountant can work faster (and charge you less), and you can confidently claim all legitimate business expenses.

Under the Construction Industry Scheme (CIS), subcontractor payments require specific reporting. Accurate job costing records make CIS compliance straightforward rather than a scramble through old invoices in a carrier bag.

Getting Started Today

If you are not currently doing job costing, start with your next project. Do not try to retrospectively cost old projects. Just start fresh.

1. Create a simple cost sheet (spreadsheet or app)
2. List the five cost categories
3. Set budgets based on your quote
4. Record every cost, every day
5. Review every Friday

Within a month, you will know more about your business profitability than most builders learn in a year. And that knowledge is what separates the businesses that grow from the ones that stay stuck.