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

File your 2025-26 return online by 31 January 2027. Count every homeowner payment, card or cash, plus contractor pay before CIS as turnover, and put the CIS taken off in its own box.

Most plumbers have two kinds of income on one return: jobs paid in full by homeowners, and installs for builders or developers where a contractor took tax off first. This page follows GOV.UK as checked on 18 October 2026. Our general guide to a first return covers signing in and paying.

Homeowner jobs and contractor installs

CIS 340, HMRC's guide to the Construction Industry Scheme (CIS), says private householders are not contractors. A homeowner who pays for a boiler service, a leak or a new bathroom deducts nothing. CIS applies when a contractor pays you for construction work, such as installing a complete heating system on a new build. CIS 340 says repairs to a system, such as new taps, are not installations of systems, so on their own they are outside the scheme. Our guide to whether CIS applies to plumbers has the job-by-job detail.

If you are registered, a contractor deducts 20% from what it pays you, leaving out materials you paid for directly, and passes it to HMRC, or 30% if you are not. With gross payment status, nothing is deducted. GOV.UK says these deductions count as advance payments towards your tax and National Insurance.

Where a plumber's figures go

If your turnover was below £90,000, both forms let you put your total expenses in one box instead of filling in each line.

Card, cash and the merchant counter

The SA103S notes say turnover is the total received before taking off any expenses, and includes "all payments – cash, card, cheque or any other method", plus tips. A cash job for a neighbour counts the same as a card payment. GOV.UK lists till rolls, bank slips and sales invoices as proof, so keep a note of every cash job.

If your card reader pays out after its fee, the bank deposit is less than the customer paid. In our reading, turnover is the full amount, and the fee is a cost. Keep the provider's statements so you can show both.

Parts and materials from the merchant are a cost of goods: GOV.UK lists raw materials and goods for resale as allowable, but not goods bought for private use. Keep every merchant receipt and note the job it was for. Our plumber expenses guide lists more.

Your van, accounting method and National Insurance

For the van you claim either your actual costs or a flat rate per business mile. For 2025-26 the rate is 45p for the first 10,000 miles, then 25p. You cannot use the flat rate for a van you have claimed capital allowances on, or whose costs you already included. Once you use it for a van, you keep using it while that van is in the business.

Cash basis is the standard way for sole traders to keep books: you record money when it comes in or goes out. If you used traditional accounting instead, tick box 8 on the short pages.

For 2025-26, Class 4 National Insurance is 6% of profits between £12,570 and £50,270, and 2% above. If your profits were £6,845 or more and you are registered as self-employed, Class 2 is treated as paid.

Steps to file your first return as a plumber

1. Have your 10-digit UTR and your HMRC online sign-in ready. GOV.UK says the UTR usually arrives by post around 15 days after you register.
2. Gather every contractor's monthly payment and deduction statement. Contractors who take a deduction must give one within 14 days of the end of each tax month. Chase any that are missing.
3. Add up homeowner takings from card statements, bank transfers and your cash job notes.
4. Add contractor pay before deductions, then total the CIS taken off.
5. Total merchant receipts, van costs or mileage, insurance and other costs.
6. Complete the short or full pages, with gross pay as turnover and the deductions in the CIS box.
7. Submit and pay by 31 January 2027. You can save your entry and come back to it.

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

Deadlines, payments on account and penalties

If you started in 2025-26, you needed to tell 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.

Payments on account towards 2026-27 apply unless your 2025-26 bill was under £1,000 or more than 80% was paid outside Self Assessment. Each is half of that bill, due by 31 January and 31 July 2027. A late return 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% of the unpaid tax at 30 days, 6 months and 12 months, plus interest.

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. If it was over £30,000 in 2025-26, MTD starts for you on 6 April 2027, with the first quarterly update due by 7 August 2027.

A first year is often short, and HMRC scales a sole trader's income up to a full year if it has the information. Say you started on 6 October 2025 and took £10,500 from homeowners and £6,000 from a contractor before deductions. Six months' trading is doubled, so your £16,500 counts as £33,000, over the line.

Run the free MTD check to see your date, and visit the trades section for more guides for plumbers. Our tax plans start once our HMRC registrations are complete. Until then you can join the waitlist, and no payment is taken.