Tax & Expenses

Self Employed Builder Tax Return — Step by Step Guide 2026

Updated July 2026 · 10 min read · BuilderLog Team

Filing a self assessment tax return for the first time as a self employed builder is daunting — not because it’s difficult, but because nobody explains it in builder-specific terms. This guide walks through the actual process with real examples.

When is it due?

For the 2025/26 tax year, the deadline for online self assessment is 31 January 2027. The paper deadline is 31 October 2026. File online — it’s easier and you get longer.

File early. You don’t have to pay early — payment isn’t due until 31 January regardless of when you file. But the earlier you file, the earlier you know what you owe.

What you need before you start

• Your UTR number (on any HMRC letter)
• Your National Insurance number
• Your total building income for the year
• Your total business expenses broken down by category
• Your mileage log if claiming simplified mileage
• All CIS payment and deduction statements from contractors

If you’ve been tracking in BuilderLog, tap Report, select the tax year, and your totals are ready. If you’ve been using a shoebox of receipts, set aside a few hours.

Where builder expenses go on the form

Box 15 — Turnover

Your total income from building work. Include everything — cash, bank transfers, CIS gross amounts (before deductions). If you’re on cash basis, only include money actually received.

Box 17 — Cost of goods

Materials consumed on jobs: cement, bricks, timber, plaster, fixings, adhesives. Not your tools — those go elsewhere.

Box 24 — Travel costs

Your mileage claim at 55p/mile for the first 10,000 business miles, 25p after. Plus parking charges, congestion charges, and tolls. A builder doing 10,000 miles claims £5,500 here.

Box 25 — Phone, office costs

Business proportion of your phone bill (typically 50–75%). Plus accounting software, stationery, and admin costs.

Box 27 — Insurance

Public liability, professional indemnity, tool insurance, employers’ liability if applicable.

Box 29 — Other allowable expenses

This is the catch-all for builder-specific costs: CSCS card, skip hire, scaffolding hire, equipment hire, training courses, workwear and PPE, waste disposal, subcontractor payments.

Box 38/81 — CIS deductions

The total CIS tax deducted by contractors during the year. This is critical — these are advance payments of your tax. HMRC offsets them against your bill, and if they exceed what you owe, you get a refund.

Common mistakes

Not entering CIS deductions: Without these in box 38/81, HMRC keeps the money. Many builders are owed hundreds or thousands in CIS refunds they never claimed.

Forgetting mileage: 10,000 miles at 55p = £5,500 off your taxable profit. Most builders claim nothing.

Not claiming skip hire: Every skip is a deductible expense. On a busy year, this can be £2,000–£5,000.

Claiming mileage AND van costs: You can’t do both for the same vehicle. It’s one or the other.

Payments on account

If your tax bill exceeds £1,000, HMRC will ask for payments on account — advance payments towards next year’s tax. Each payment is half of the previous year’s bill, due on 31 January and 31 July. This catches many newly self employed builders off guard because in your second year, you’re paying this year’s bill plus half of next year’s estimated bill at the same time.

For example, if your first year’s tax bill is £4,000, you pay that £4,000 on 31 January, plus a £2,000 payment on account for next year. Then another £2,000 on 31 July. Budget for it from day one or it becomes a nasty surprise.

Cash basis vs traditional accounting

Most sole trader builders use cash basis accounting — you record income when you receive it and expenses when you pay them. This is simpler and is the default for most self employed people.

Traditional (accrual) accounting records income when you invoice and expenses when you’re billed, regardless of when money actually changes hands. This can benefit builders with large outstanding invoices at year end, but adds complexity.

Unless your accountant specifically recommends traditional accounting, stick with cash basis. It’s what HMRC expects for most sole traders and it’s what BuilderLog’s tax report uses.

National Insurance

As a self employed builder, you pay two types of NI: Class 2 (a flat weekly rate, currently £3.45/week) and Class 4 (9% on profits between £12,570 and £50,270, then 2% above that). These are calculated automatically when you file your return — you don’t need to work them out yourself.