Self Assessment Deadline 2027: What Construction Businesses Need to Know

The deadline is closer than you think

The 2025/26 tax year ended on 5 April 2026. If you are self-employed, a partner in a business, a CIS subcontractor, or a company director with income that is not fully taxed through PAYE, you must file a Self Assessment tax return — and the deadlines are now just months away.

There are two deadlines to keep in mind:

• ◢31 October 2026 — the deadline for filing a paper Self Assessment return.
• ◢31 January 2027 — the deadline for filing online and for paying any tax you owe.

Most people now file online, so 31 January 2027 is the date that matters. But do not leave it to the last week — HMRC's online service slows down under the weight of last-minute filers, and if you need an accountant to prepare your return, they will be at their busiest from mid-January onwards.

Who must file a Self Assessment return

You must file a return if any of the following applied in the 2025/26 tax year:

• ◢You were self-employed as a sole trader and earned more than £1,000 from your trade.
• ◢You were a partner in a business partnership.
• ◢You were a CIS subcontractor and had tax deducted from your pay by a contractor — even if you expect a refund, you must file to claim it back.
• ◢You were a company director with income that was not taxed entirely through PAYE (for example, dividends, benefits, or a salary from another source).
• ◢You received untaxed income — rental income, tips, foreign income, or investment income above the relevant thresholds.
• ◢Your income was over £100,000 (even if it was all taxed through PAYE, because of the Personal Allowance taper).
• ◢You claimed Child Benefit and you or your partner had income over £60,000 (the High Income Child Benefit Charge applies).

If none of these apply, you generally do not need to file a return. But if HMRC has asked you to file one, you must — even if you believe you have nothing to pay.

What you need to have ready

Before you (or your accountant) can prepare your return, gather:

• ◢Income records — invoices, bank statements, and a summary of your turnover for the year.
• ◢Expense receipts — all allowable business expenses, including receipts and mileage logs.
• ◢CIS deduction statements — if you are a subcontractor, the statements from every contractor you worked for. These show how much tax was deducted, which is credited against your final bill (or refunded).
• ◢P60 or P45 — if you also had PAYE employment income during the year.
• ◢Dividend vouchers — if you trade through a limited company and took dividends.
• ◢Rental income records — if you let out property.
• ◢Interest and investment income — statements from banks and investment platforms.

The better your records, the faster your return is prepared — and the more tax you can legitimately save by making sure no allowable expense is missed.

Expenses construction businesses can claim

One of the biggest reasons construction sole traders and subcontractors overpay tax is missing legitimate expenses. You can claim for:

• ◢Materials and tools — anything you buy to do the job, from bricks to power tools.
• ◢Van and vehicle costs — either actual running costs or the simplified mileage rate (45p per mile for the first 10,000 miles, then 25p).
• ◢Workwear and PPE — protective boots, hi-vis, hard hats, gloves. (But not everyday clothes, even if you only wear them for work.)
• ◢Subcontractor labour — what you pay to other subcontractors, plus the CIS you deducted from their pay.
• ◢Scaffolding, plant and equipment hire — including short-term hires for specific jobs.
• ◢Office and admin costs — phone, stationery, accounting software, a portion of your home bills if you work from home.
• ◢Training — courses that relate to your current trade (but not training for a completely new trade).
• ◢Bank and finance charges — business bank fees and interest on business loans.

Keep receipts for everything. HMRC can ask to see them up to six years after the tax year in question.

Payments on account — the trap to watch for

If your Self Assessment tax bill is over £1,000, and more than 80% of your income is not taxed at source, HMRC will expect you to make payments on account — advance payments towards the following year's tax.

This means that on 31 January 2027, you may need to pay:

1. ◢Your balancing payment for 2025/26 (the tax you still owe for the year just ended).
2. ◢Your first payment on account for 2026/27 (half of last year's bill).
3. ◢Then your second payment on account is due on 31 July 2027.

For a first-time filer this can come as a shock — a tax bill that is effectively 150% of what you expected. If you know it is coming, you can budget for it. If you do not, it can be a serious cashflow problem.

If your income is falling (for example, a quieter year in construction), you can apply to reduce your payments on account. But be careful — if you reduce them too far and then underpay, HMRC charges interest on the shortfall.

What happens if you miss the deadline

HMRC's penalties for late Self Assessment are automatic and escalate quickly:

• ◢1 day late — an immediate £100 fine, even if you owe no tax or are due a refund.
• ◢3 months late — £10 per day, up to a maximum of £900.
• ◢6 months late — a further penalty of 5% of the tax owed, or £300, whichever is greater.
• ◢12 months late — another 5% or £300, whichever is greater.

On top of penalties, interest is charged on late tax payments from the date the tax was due. The interest rate is set by HMRC and reviewed regularly — it is not a small charge.

The £100 fine for being one day late applies even if you are owed a refund. This is the single most common reason subcontractors lose money — they assume that because HMRC owes them, filing late does not matter. It does.

How to get it done without the stress

The simplest way to avoid penalties and overpaying is to start early:

1. ◢Gather your records now — do not wait until January. The earlier you pull together your income and expenses, the more time you have to find missing receipts.
2. ◢Check your CIS statements — make sure every contractor who deducted tax has given you a deduction statement. If any are missing, chase them now, not in January.
3. ◢Use an accountant — a construction specialist will know the expenses you can claim, will handle the payments on account calculation, and will file for you well before the deadline.
4. ◢File online — it gives you until 31 January 2027, and the calculation is done for you.
5. ◢Budget for the payment — if you are on payments on account, know what January and July will cost you before they arrive.

How we help

As construction accountants we prepare and file Self Assessment returns for sole traders, subcontractors, partners and directors across London. We will:

• ◢Reconcile your income and expenses for the 2025/26 tax year
• ◢Claim every allowable expense, including CIS deductions and vehicle costs
• ◢Calculate your tax bill and any payments on account
• ◢File your return online, well before the 31 January 2027 deadline
• ◢Help you reduce your payments on account if your income has fallen

The deadline is fixed. The penalties are automatic. But with the right preparation, filing does not have to be stressful — and you will not pay a penny more tax than you owe.

If you have not started pulling your records together, book a free review now and we will tell you exactly what you need and what it will cost to have your return handled for you.

This article is for general guidance only and does not constitute formal tax advice. Self Assessment rules and thresholds change each tax year — always confirm the current position with HMRC or a qualified accountant.