Corporation Tax: a beginner’s guide

Corporation Tax is one of the cornerstones of the UK tax system. It touches nearly every incorporated business – from small local firms to global multinationals with UK operations. For accountants, it is a familiar but often evolving area of compliance, planning and advisory work.

This guide is designed to give you a thorough and practical overview of Corporation Tax. We’ve written it with accountants in mind, but with enough narrative and examples to help you explain Corporation Tax concepts to clients in simple terms. We’ll explore:

• Who has to pay Corporation Tax
• Current Corporation Tax rates and thresholds
• How and when to register to pay Corporation Tax
• How Corporation Tax is calculated
• Available tax reliefs and allowances (with practical scenarios) for Corporation Tax
• Corporation Tax filing and payment requirements
• Common Corporation Tax pitfalls and how to avoid them.

Who has to pay Corporation Tax?

Corporation Tax applies to limited companies on their taxable profits. If a client operates as a sole trader or partnership, Corporation Tax does not apply, and they’ll generally pay income tax and national insurance contributions through a self-assessment tax return instead. That said, it’s useful to understand the rules of Corporation Tax either way, as moving from self-employment to a limited company structure can change the tax position significantly.

Entities that pay Corporation Tax include:

• UK-registered limited companies
• Foreign companies with a UK branch or office
• Clubs, co-operatives and unincorporated associations (e.g. community sports clubs, trade associations).

The scope of Corporation Tax is intentionally broad. Essentially, any incorporated entity earning taxable profits in the UK is brought into the net and will pay Corporation Tax.

For accountants, this means you will often encounter Corporation Tax obligations even when advising charities with trading subsidiaries, not-for-profit clubs, or overseas groups setting up UK branches. Understanding the breadth of applicability – essentially, who has to pay Corporation Tax – is the first step to advising correctly.

Corporation Tax rates and thresholds

Companies that pay Corporation Tax are charged on taxable profits, not turnover. Profits include trading income, investments and chargeable gains. There are different Corporation Tax rates. The current system has three tiers:

• Small profits Corporation Tax rate – for companies with profits at or below a defined lower threshold, taxed at a reduced rate
• Main Corporation Tax rate – for companies above the upper threshold, taxed at the headline rate
• Marginal relief – for companies between the Corporation Tax thresholds, tapering the effective rate.

Why this matters in practice

Clients sometimes assume they pay Corporation Tax at a flat rate – either they “get the small rate” or they “pay the big one.” Walking them through marginal relief calculations (and how group structures affect thresholds) is one of the most practical teaching roles accountants take on.

How do you register for Corporation Tax?

Newly incorporated companies must register for Corporation Tax within three months of starting to trade. “Trading” is defined broadly – it can include employing staff, advertising or renting premises, not just buying and selling products and services.

The process involves:

1. Registering the company at Companies House via a business account
2. Receiving the Unique Taxpayer Reference (UTR) (you’ll need to register a business account with HMRC and create a username and password for this)
3. Creating a Government Gateway account and registering with HMRC for Corporation Tax.

In practice, many people choose to register with Companies House and HMRC at the same time and often use an accountant to help them far in advance of paying Corporation Tax. It might also be necessary to register for payroll with HMRC at this point.

Failing to register on time can trigger penalties, so it’s worth making this part of your client onboarding checklist.

How do you calculate Corporation Tax?

As part of clients’ compliance with Companies House, they’ll have to file a set of accounts which includes a profit and loss account, a balance sheet, notes and a directors’ report – as a minimum.

As their accountant, you’ll help explain that calculating Corporation Tax is not simply a matter of applying a rate to accounting profits. The Corporation Tax calculation involves:

1. Starting with accounting profit from the company’s statutory accounts
2. Making adjustments for disallowable expenses (e.g. client entertaining)
3. Claiming capital allowances, reliefs and deductions
4. Arriving at taxable profits
5. Applying the appropriate Corporation Tax rate.

Corporation Tax filing requirements

When it comes to Corporation Tax filing, companies must file annual accounts with Companies House. You’ll usually submit clients’ Corporation Tax return (known as a CT600) along with iXBRL-tagged accounts. The Corporation Tax return and payment are typically due nine months and one day after the end of the company’s accounting period (with exceptions for very large companies paying by instalments).

What are the deadlines for Corporation Tax?

Corporation Tax operates on strict timelines:

• Filing the CT600 – 12 months after the end of the accounting period
• Paying Corporation Tax – nine months and one day after the end of the period
• Large companies – may need to pay their Corporation Tax bill in quarterly instalments.

Missing Corporation Tax bill deadlines results in penalties and interest. Even minor lateness is penalised. Advising clients to plan ahead – and using software to set reminders – is one of the simplest ways to add value.

Is there any tax relief available for Corporation Tax bills?

Yes, there are several tax reliefs available, and Corporation Tax planning revolves largely around tax reliefs and allowances. These can reduce clients’ Corporation Tax liability significantly, but only if used correctly. Remember, businesses only pay tax on profit (not turnover) – and if they make losses in one year, they can be carried forward to offset profits in future years.

Capital Allowances

Capital Allowances are a type of tax relief designed to allow companies to deduct the cost of qualifying plant and machinery from taxable profits.

Example – A café upgrading equipment
A small café spends £12,000 on a new espresso machine and kitchen ovens. Under the Annual Investment Allowance (AIA), the café could deduct the full £12,000 from profits in the year of purchase. For a business with £30,000 profits, that deduction could reduce taxable profits to £18,000, slashing the Corporation Tax bill.

As an accountant, explaining the timing of purchases is key. Buying equipment just before year-end, rather than just after, can bring forward the corporate tax benefit.

Research and Development (R&D) relief

R&D tax relief rewards companies engaged in innovation by lowering their Corporation Tax liability. The definition of R&D is broader than many clients expect – it includes developing new processes, improving products, or solving technological challenges.

Example – A software start-up
A small tech company develops a bespoke algorithm to process client data more efficiently. Even if the project is not commercially successful, it qualifies as R&D. If it makes a loss, it may even receive a cash credit.

Your role is to help clients identify qualifying projects, as many underestimate their eligibility.

Loss relief

Companies making a trading loss can carry it forward to offset against future profits, carry it back to claim a refund, or in some cases surrender it to group companies.

Example – A new manufacturer
A company incurs £80,000 of losses in its first year due to high set-up costs. In its second year, it makes £120,000 profit. By carrying forward the loss, taxable profit falls to £40,000, ensuring the company stays in the small profits band. This not only reduces the Corporation Tax bill – it also stabilises cash flow in the crucial early years.

Pension contributions

Employer contributions to pension schemes are deductible for Corporation Tax purposes.

Example – A consultancy owner
A director-owned consultancy contributes £10,000 into the director’s pension. The payment reduces the company’s taxable profits by the same amount, lowering Corporation Tax while building retirement savings.

This is a straightforward example of tax planning that benefits both business and owner.

Other tax reliefs

• Creative industry tax reliefs (for film, TV, theatre, video games)
• Patent Box regime (reduced tax on profits from patented inventions)
• Group relief (surrendering losses within a group of companies)

As an accountant, you don’t have to memorise every tax relief or scheme. The value you can bring is to help clients spot when an activity might impact or reduce their Corporation Tax bill and then guide them through the claim process.

Common Corporation Tax pitfalls and how to avoid them

Corporation Tax compliance is full of small but costly traps. Clients can often see their accountant as the safety net, but that role can also become reactive if these pitfalls aren’t anticipated. Here are the areas where mistakes most often occur, and how you can help clients steer clear of them.

Confusing types of business profit

Many directors assume that the bottom-line figure on their business profit and loss account is the amount they will be taxed on. They don’t appreciate that Corporation Tax is calculated on tax-adjusted profits.

For example, a company might record £100,000 trading profit, but if £5,000 was spent on client entertaining (disallowable) and £15,000 qualifies for capital allowances, the taxable profit is £90,000, not £100,000.

How to avoid it: Walk clients through at least one example calculation each year, showing the adjustments. Even if they don’t remember every detail, they’ll grasp that the tax bill is not a straight percentage of the accounts.

Missing registration deadlines

New companies must register for Corporation Tax within three months of trading. The broad definition of “trading” means many directors miss the trigger – for instance, paying for adverts or hiring staff before they make their first sale.

How to avoid it: Build registration into your client onboarding checklist. If you offer company formation services, register for Corporation Tax at the same time as Companies House incorporation.

Overlooking reliefs and allowances

It’s surprisingly common for businesses to under-claim reliefs – particularly R&D, capital allowances, and pension contributions. Clients often assume these are only for “big” companies or tech firms, when in reality, many SMEs qualify.

Example: A small craft brewery improves its fermentation process and assumes it’s “just part of the job.” In fact, it may qualify for R&D relief.

How to avoid it: Encourage clients to describe projects or purchases in their own words. You can then translate their activity into tax terminology and spot opportunities.

Late filing and payment

Penalties for late filing for Corporation Tax start small but escalate quickly. Interest on late payments is another unnecessary cost. Even a one-day delay creates reputational headaches for clients.

How to avoid it: Use accounting software or practice management tools to set automated reminders for both you and the client. Position timely filing as part of good financial hygiene, not just compliance.

Inconsistent record-keeping

Disorganised records create headaches for both client and accountant. Missing invoices, unclear expense claims, or lump-sum entries make it harder to calculate accurate tax and risk overpaying or under-claiming reliefs.

How to avoid it: Encourage cloud-based accounting software, and train clients in basic habits like scanning receipts or tagging expenses. Position this as a way to save them money at year-end.

Misunderstanding loss relief options

Clients often fail to make the best use of trading losses. Some leave them unclaimed, while others don’t realise they can carry losses back for a refund.

How to avoid it: Proactively raise loss relief options when discussing year-end accounts. A short conversation could free up much-needed cash for a struggling business.

By anticipating these pitfalls, you move from being the person who “fixes mistakes” to the adviser who prevents them. That distinction often defines the strength of client relationships.

Corporation Tax as part of advisory work

Too often, clients think of Corporation Tax as an unavoidable tax bill that arrives once a year. As their accountant, you have the opportunity to shift this mindset – showing them that Corporation Tax can be a planning tool rather than a pure cost.

Positioning Corporation Tax in business strategy

Corporation Tax touches on almost every strategic decision: how to pay directors, whether to invest in equipment, when to expand, how to fund growth. By framing tax as part of these discussions, you help directors make choices that are both commercially sound and tax-efficient.

Example: A company debating whether to lease or buy vehicles will find the decision looks very different once capital allowances, cash flow and Corporation Tax rates are factored in.

Using Corporation Tax as a conversation starter

The annual CT600 is not just a filing obligation – it’s a chance to review the entire year. You can use the Corporation Tax return as a springboard for advisory conversations:

• Why were profits higher or lower this year?
• Did we make the most of available reliefs?
• Are there investments we should plan before the next year-end?
• How does the tax liability affect dividend planning?

These conversations deepen client relationships and often lead to broader advisory engagements.

Helping clients see the bigger picture

Clients often fixate on the size of their tax bill. Reframing the discussion can change their perspective:

• A higher tax bill means higher profits – a sign of growth
• Reliefs and allowances can reduce the amount of Corporation Tax paid, but the priority is always sustainable profitability
• Corporate Tax is not separate from the business – it reflects its success and direction

By helping clients interpret their Corporation Tax bill in context, you build trust and provide reassurance.

Building advisory services around Corporation Tax

Corporation Tax can underpin wider services, such as:

• Cash flow forecasting – factoring in tax liabilities to avoid surprises
• Business structuring – advising on group structures, associated companies, or incorporation
• Exit planning – preparing for disposals and managing chargeable gains
• Growth planning – modelling how expansion will impact tax bands and cash flow

Each area begins with Corporation Tax but extends into broader advisory support.

Technology and forward planning

Modern Corporation Tax software and cloud accounting tools mean that forecasting Corporation Tax is easier than ever. Accountants can produce “what if” scenarios in minutes, showing clients how decisions today affect their liability tomorrow.

For example, you might demonstrate how a £20,000 equipment purchase shifts taxable profit into the small profits rate, or how accelerating R&D spending this year creates a cash repayment. Visualising these outcomes makes tax planning tangible for directors.

From compliance to partnership

Ultimately, moving beyond compliance transforms your role. Instead of being the person who files the CT600, you become the partner who:

• Helps clients avoid pitfalls before they happen
• Frames Corporation Tax in the context of wider goals
• Shows opportunities to reduce the amount of Corporation Tax paid, reinvest or grow
• Uses each tax year as a chance to reflect and plan ahead.

This is where accountants add the most value – not just processing numbers, but making sense of them.

Corporation Tax can feel daunting to clients, but with the right guidance it becomes a manageable and even strategic part of running a business. For accountants, this is where technical knowledge meets client care: explaining rules clearly, identifying opportunities, and keeping businesses compliant.

Use this guide as a framework for conversations with clients. Walk them through who pays a company tax return, how their Corporation Tax liability is calculated, what reliefs are available and how Corporation Tax deadlines work. Share examples that reflect their own situation. And always remind them: the Corporation Tax bill is not just a number to pay – it is a number they can influence, with your advice.