Do I still need to complete a Self Assessment if I am under CIS?

At Towerstone Accountants we provide specialist CIS accountancy services for contractors, subcontractors, and construction businesses across the UK. We created this webpage for people working in construction who want clear guidance on CIS, including registration, deductions, refunds, and common compliance tasks, without jargon. Our aim is to help you stay compliant with HMRC, avoid costly errors, and keep your records in good order.

This is one of the most common and most important questions I am asked by subcontractors working under the Construction Industry Scheme. Many people assume that because tax is already being deducted from their payments under CIS there is nothing more to do at the end of the year. Others are told by contractors that CIS takes care of everything, so a tax return is no longer needed. Unfortunately that assumption is one of the biggest causes of missed deadlines, unexpected HMRC letters, and avoidable stress.

I have dealt with CIS from every angle for many years, preparing Self Assessment tax returns for sole traders, advising limited companies, and resolving HMRC queries where returns were never filed because someone believed CIS replaced the need for one. In this article I want to answer the question properly, clearly, and in real world terms. Do you still need to complete a Self Assessment if you are under CIS, when is it mandatory, when it might not be, and what happens if you get it wrong.

The short answer and why it needs explaining

The short answer in most cases is yes, if you are working under CIS as a subcontractor you still need to complete a Self Assessment tax return. CIS does not remove that obligation.

However the reason this question keeps coming up is because CIS feels like a final tax. Money is deducted, statements are issued, and HMRC already has the information. It is understandable that people assume the process ends there.

The reality is that CIS deductions are only advance payments of tax. They are not a final calculation of your tax liability. Self Assessment is still required to work out the true position, taking into account your income, expenses, personal allowances, and other factors.

Once you understand the purpose of CIS and the purpose of Self Assessment separately the answer becomes much clearer.

What CIS actually does from HMRC’s perspective

CIS exists to collect tax upfront from subcontractors in the construction industry. HMRC recognised long ago that construction work is often temporary, mobile, and fragmented, with people working for multiple contractors in a single year. Collecting tax only at the end of the year created a risk of non payment.

Under CIS contractors deduct tax from subcontractors and pass it to HMRC monthly. HMRC then credits those deductions to the subcontractor’s tax record.

What CIS does not do is calculate your final tax bill. It does not consider your expenses, your personal circumstances, or other income. It simply ensures tax is collected during the year.

That is why CIS on its own is never enough to close the loop.

What Self Assessment is designed to do

Self Assessment exists to calculate how much tax you actually owe for the year. It brings together all sources of income and allows you to deduct allowable expenses and claim reliefs.

For someone under CIS Self Assessment is where you

• Declare your gross income

• Deduct allowable business expenses

• Apply personal allowances

• Offset CIS tax already deducted

• Calculate whether you owe more tax or are due a refund

Without a Self Assessment return HMRC has no way of knowing whether the CIS deductions taken during the year were too much, too little, or just right.

Sole traders under CIS and Self Assessment

If you are a sole trader working under CIS you almost always need to complete a Self Assessment tax return.

This is because as a sole trader you are taxed on your profits, not your turnover. CIS deductions are based on labour payments and ignore your expenses entirely.

In practice this means

• CIS may deduct far more tax than you actually owe

• CIS may deduct less tax than you actually owe

• CIS will almost never match your final tax position exactly

Self Assessment is the only way to reconcile this.

For example if you have significant expenses such as tools travel vehicle costs insurance or accountancy fees your taxable profit may be much lower than the income CIS deductions were based on. Without a tax return HMRC would never know this and any refund would never be issued.

Equally if you have other income such as rental income or employment income CIS deductions may not be enough and additional tax could be due.

Limited companies under CIS and filing obligations

If you operate through a limited company under CIS the picture changes slightly but the principle remains the same.

A limited company does not complete a Self Assessment tax return for its trading income, but it does still have filing obligations. The company must prepare accounts and submit a Corporation Tax return, and any CIS deductions suffered by the company are offset against its Corporation Tax or PAYE liabilities.

However directors of CIS companies often still need to complete a personal Self Assessment return, particularly if they take dividends, have other income, or have CIS related adjustments.

The idea that CIS removes filing obligations is therefore incorrect whether you are a sole trader or operating through a company.

When people think they do not need to file and why that is risky

There are a few common situations where people believe they do not need to complete a Self Assessment under CIS.

One is where all income has had CIS deducted and there is no other income. Another is where deductions feel high and people assume HMRC has already taken more than enough tax.

I understand why this belief exists but it is risky. HMRC still expects a return if you are registered for Self Assessment or meet the criteria for filing. Failing to file can trigger penalties even if no tax is ultimately due.

I have seen cases where CIS deductions exceeded the tax liability but the subcontractor failed to file and ended up with late filing penalties that wiped out most of the refund.

HMRC rules on who must file a Self Assessment

HMRC requires you to file a Self Assessment tax return if you are self employed and earn more than a small threshold from trading. CIS subcontractors almost always fall into this category.

You must also file if you are already registered for Self Assessment unless HMRC formally withdraws that requirement.

Being under CIS does not automatically remove you from Self Assessment and HMRC does not assume that deductions mean no return is needed.

If you stop trading you still usually need to file a final return to close things down properly.

What happens if you do not file under CIS

If you do not file a Self Assessment return when required HMRC will issue penalties automatically.

These start with a £100 late filing penalty and can escalate quickly if the delay continues. Interest and further penalties can apply even where no tax is owed.

In addition any CIS tax deducted will sit on your account but cannot be refunded without a tax return. This is one of the most frustrating situations I see, people knowing HMRC holds their money but being unable to access it because returns are missing.

In more serious cases repeated non filing can trigger compliance checks or estimated tax assessments which are often higher than the true liability.

When you might not need to file under CIS

There are limited situations where someone under CIS might not need to complete a Self Assessment, but these are far less common than people think.

For example if you are paid under CIS through a limited company and you personally have no other income and take only salary taxed through PAYE, HMRC may not require a personal tax return.

Similarly if you are employed and CIS does not apply to you at all then the question is irrelevant.

However these cases depend on HMRC confirmation. You should never assume you are exempt without explicit confirmation from HMRC or advice from a professional.

How CIS deductions are included in the tax return

When you complete a Self Assessment tax return under CIS you must include the total CIS tax deducted during the year.

This figure usually comes from CIS deduction statements provided by contractors. It is essential to include the correct amount because this directly affects your tax calculation.

HMRC cross checks these figures against contractor returns so accuracy matters.

Once included the CIS tax is treated as tax already paid and offset against your liability.

This is the mechanism that leads to refunds where deductions exceed the tax due.

Why many CIS subcontractors receive refunds

It is very common for CIS subcontractors to receive tax refunds after filing their Self Assessment return. This often surprises people who assumed the deductions were final.

Refunds typically arise because

• Expenses reduce taxable profit

• Personal allowance was not fully used

• CIS deductions were higher than the actual tax liability

Without a Self Assessment return these refunds would never be issued.

I have helped many subcontractors recover significant sums simply by filing correctly and on time.

Self Assessment deadlines still apply under CIS

Being under CIS does not change the standard Self Assessment deadlines.

The tax year ends on 5 April. Returns must be filed by 31 January following the end of the tax year if filed online. Any balancing payment is also due by that date.

Missing these deadlines triggers penalties regardless of CIS deductions.

I always encourage CIS subcontractors to treat Self Assessment deadlines as non negotiable, even if they believe no tax is due.

Common mistakes I see with CIS and Self Assessment

There are several recurring mistakes I see in practice.

One is assuming CIS replaces the need to file. Another is failing to include CIS deductions on the return which leads to overpayment of tax. I also see missing CIS statements and estimates being used instead of accurate figures.

Some people also forget to register for Self Assessment in the first place because they believe CIS handles everything.

All of these issues are avoidable with the right understanding.

How to know for certain whether you need to file

If you are unsure whether you need to complete a Self Assessment under CIS the safest approach is to assume you do until HMRC confirms otherwise.

You can check your HMRC online account or contact HMRC directly. An accountant can also review your situation quickly and give clear guidance.

What matters most is not guessing.

My experience advising CIS subcontractors

From my experience the subcontractors who feel most confident about CIS are those who understand that it is only part of the tax process.

Once Self Assessment is seen as the final reconciliation rather than an extra burden the process feels logical rather than punitive.

The stress tends to come from uncertainty rather than the tax itself.

Final thoughts from experience

So do you still need to complete a Self Assessment if you are under CIS. In most cases yes you do.

CIS does not replace Self Assessment, it feeds into it. It ensures tax is collected during the year but Self Assessment is where everything is finalised.

If you are a CIS subcontractor filing a tax return is not optional, it is how you reclaim overpaid tax, settle any balance due, and keep HMRC satisfied.

The key takeaway is simple. CIS deductions are not the end of the story. Self Assessment is where the story is finished properly.

Taking that approach avoids penalties, unlocks refunds, and gives you control over your tax position rather than leaving it to assumption or chance.

You may also find our guidance on how to claim your cis refund from hmrc and how do CIS tax refunds work for subcontractors helpful when dealing with related CIS questions. For a broader overview of CIS rules, compliance, and support, you can visit our cis guidance hub.