How do I file a Self Assessment tax return for the first time as a builder?

File your 2025-26 return online by 31 January 2027. Enter your CIS pay before deductions as turnover and the total deducted in the CIS box, so the tax contractors took counts towards your bill.

A builder's first return is the same Self Assessment return everyone files, with the self-employment pages added. The difference is the Construction Industry Scheme (CIS): you need your contractors' statements, and the figures go in set boxes. This page follows GOV.UK as checked on 18 October 2026. Our general guide to a first return covers sign-in and paying in more detail.

Where a builder's figures go

Short or full pages

GOV.UK's SA103S notes say to use the short self-employment pages if your turnover was less than £90,000, or would have been had you traded for a full year. Use the full pages (SA103F) if it was £90,000 or more, or your business is more complicated. Both versions have a CIS box, so a subcontractor under £90,000 can usually use the short pages.

The notes also say to check that turnover includes the full amount before CIS deductions.

Getting your CIS figures right

A contractor who takes a deduction must give you a payment and deduction statement within 14 days of the end of that tax month. With gross payment status, nothing is taken off, so there is no CIS figure. Check each statement against your bank account.

If a statement is missing, ask the contractor. If you cannot get it, for example because the firm has stopped trading, GOV.UK says to write to HMRC with your name, address and UTR, the contractor's details, the dates you were paid and why you have no statements.

Check that no deduction was taken from materials. GOV.UK says contractors will not take CIS deductions from materials you have paid for directly, or from VAT or plant hired for the job. Every deduction still counts towards your tax.

Over-deducted CIS is claimed back through your tax return. The sooner after 5 April you file, the sooner HMRC can repay you.

Costs, allowances and National Insurance

The cash basis has been the standard way for sole traders to keep their books since 2024-25, with no turnover limit. You record money when it comes in and goes out, and tools and vans you buy are usually claimed as expenses in the year you pay. You can choose traditional accounting instead and tick the box to say so.

Typical builder costs include materials for jobs, tools, protective clothing such as safety boots and hard hats, and van costs. For the van, you claim either actual running costs or a flat rate per business mile. For 2025-26 that is 45p for the first 10,000 miles, then 25p. You choose per vehicle, but not the flat rate for a van whose cost you have already claimed. Once you use the flat rate, you keep it while that van is in the business. Our builder expenses guide lists more.

Instead of claiming costs, you can take the £1,000 trading allowance. You cannot then deduct any expenses.

For 2025-26, Class 4 National Insurance is 6% of profits between £12,570 and £50,270, and 2% above. If your profits are £6,845 or more, Class 2 is treated as paid. Below that, you can choose to pay it voluntarily to protect your State Pension.

Steps to file your first return

1. Make sure you are registered and have your 10-digit UTR. GOV.UK says it usually arrives by post around 15 days after you register.
2. Set up your sign-in for HMRC online services and prove your identity if asked.
3. Collect every payment and deduction statement for 2025-26 and chase any that are missing.
4. Add up gross CIS pay, homeowner income and total deductions.
5. Total your costs, and decide between flat-rate mileage and actual costs for each vehicle.
6. Complete the short or full self-employment pages, with gross pay as turnover and the deductions in the CIS box.
7. Check the bill, submit and pay any tax owed by 31 January 2027.

Deadlines, payments on account and penalties

If you started in 2025-26, you should have told HMRC by 5 October 2026. If you register after that, HMRC gives you 3 months from its letter or email to file, but the tax is still due by 31 January 2027.

If your bill after CIS deductions is £1,000 or more, and tax taken at source, such as CIS, covered 80% or less of your tax, payments on account apply. Each is half of that bill, due by 31 January and 31 July. Filing late costs £100 at once, then £10 a day after 3 months, up to £900, with more at 6 and 12 months. Late payment adds 5% at 30 days, 6 months and 12 months, plus interest. GOV.UK says new penalties apply to everyone who submits a personal Self Assessment return from April 2027, and current penalties still apply to earlier years.

What this means for you

This return also sets your Making Tax Digital for Income Tax (MTD) date. Qualifying income is your self-employment and property income before expenses. In our reading, CIS work counts at its gross figure, because that is the turnover on your return. If the total was over £30,000 in 2025-26, MTD starts for you on 6 April 2027, with four updates a year and the first due by 7 August 2027.

If you started part-way through the year, HMRC scales a sole trader's income up to a full year if it has the information. Six months' trading is doubled, so a short first year can still put you over £30,000.

Run the free MTD check to see if MTD starts for you in April 2027, and visit the trades section for more on CIS. Our tax plans start once our HMRC registrations are complete. Until then you can join the waitlist, and no payment is taken.