The online Self Assessment deadline for the 2025/26 tax year is not until 31 January 2027.
That does not mean you need to wait until January to complete it.
In fact, September can be one of the best times to get your tax return finished.
Filing early does not mean paying early
One common reason people delay their tax return is the belief that submitting it means the tax must immediately be paid.
That is not generally the case.
You can submit your 2025/26 Self Assessment return months before the deadline while the normal payment deadline remains 31 January 2027.
The difference is that you know exactly how much you need to find.
That can make tax planning considerably easier.
Give yourself time to budget
Suppose your return is completed in September and you discover that £8,000 will be payable in January.
You have several months to plan for it.
Discovering the same £8,000 bill on 25 January gives you less than a week.
The tax hasn’t changed, the difference is simply how much notice you have.
For many clients, knowing the number is far less stressful than wondering what the number might be.
Understand Payments on Account
Completing your return early can also identify whether Payments on Account will apply.
These can sometimes make the January bill considerably larger than expected.
This is particularly common for someone whose business profits have increased or who has recently moved from employment into self employment.
Your January payment may include both a balancing payment for the year already finished and the first Payment on Account towards the following year.
Knowing this several months beforehand makes a significant difference.
Find problems while there is time to solve them
Completing a tax return often identifies missing information.
Perhaps you are missing:
- bank interest figures;
- pension contribution details;
- invoices;
- dividend information;
- property expenses;
- employment information;
- details of asset purchases.
In September, there is plenty of time to locate them.
In late January, missing information becomes considerably more inconvenient.
Tax planning can happen earlier
Early accounts also provide an opportunity to consider what is happening in the current financial year.
If profits are changing significantly, you may need to reconsider how much tax you are saving.
If income has fallen, there could potentially be grounds to review Payments on Account.
If income has increased, putting additional money aside now may prevent a future cash flow problem.
January is busy enough
The Self Assessment deadline arrives shortly after Christmas.
Businesses are returning from the festive period, employees are returning to work and many people are already dealing with VAT, payroll and normal business commitments.
Adding an unfinished tax return into that period is unnecessary if the information was available months beforehand.
September gives you information
There is no prize for filing your tax return on 31st January.
Completing it early does not increase the tax payable.
It simply gives you more time to prepare for what is due by the 31st January.
For many business owners, that is the main advantage.
Know the number, know the deadline and make a plan.
