Nine mistakes to avoid on your first self-assessment tax return

Self-assessment tax returns are due by midnight tomorrow. So, if this is the first time you've had to submit one, it's understandable if you're in a bit of a panic.

But, don't worry - there's still time to make sure your first tax return is a success.

From getting confused by expenses to choosing the wrong form, we explain common mistakes to avoid and help prevent you missing the deadline.

1. Not applying for a UTR number

In order to file your tax return, you need a unique tax reference (UTR) number. Like a National Insurance number, this stays with you for life, but you'll need to apply for it before your first tax return.

The bad news: if you haven't already applied, you won't receive it by the deadline. Generally, a UTR number will take 10 days to arrive by mail, and then you'll need to request an activation code for your account, which could take another 10 days.

Technically, you should have registered for self-assessment by 5 October last year, if you knew you had tax payable for 2017-18. But you generally won't be fine for registering late as long as you pay your bill on time.

If possible, work out how much tax you're likely to need to pay and submit your payment by 31 January, even without your tax return. You can create a payment slip usingthis HMRC form, and provide your NI number to help identify you.

In some cases, HMRC may give you an extended deadline of up to three months to send your tax return from when you register. Otherwise, you may still face a penalty for filing your return late. You'll be charged £100 on the first day, and an increasing amount after that, so submit it as soon as you can.

Find out more:our guide to UTR numbers

2. Overlooking key documents

Aside from your UTR number, you'll need your address details, date of birth and your National Insurance number.

If you're employed you'll need your P60, P11D if you receive one, P45 if you've moved jobs during the tax year, and possibly a PAYE notice of coding.

If you're paying off your student loan, you'll need your annual payment statement.

You'll also need specific details of bills and receipts if you're claiming expenses.

Once you're done with your tax return, don't throw your receipts, bills or other paperwork away; you're required to keep your records for a minimum of six years.

It may be worth keeping them for 20 years, as this is the limit for HMRC to launch an investigation if it suspects fraud.

Find out more:self-employed tax return

3. Getting confused about what taxes you pay

There are only certain taxes you'll need to worry about for self-assessment - and they are all related to how much you earn.

So, income tax, National Insurance, capital gains tax and dividend tax are all covered by your tax return, so you'll need to record and calculate how much you earned through these means.

But the likes of council tax, stamp duty land tax (or the Scottish and Welsh equivalents) and inheritance tax are paid in a different way/only in certain circumstances, so you don't need to worry about them.

Find out more:how much tax you pay

4. Filing the wrong self-employed supplement

If you're self-employed, you'll need to fill in the self-employment supplement (SA103) as well as the main self-assessment tax return (SA100).

If your turnover for the 2017-18 tax year was £83,000 or less, and you have no accounting complications, then you can fill out the short form.

However, if it's your first year of trading this may not be possible, as you can't use the short form if your accounting period (the dates you choose to prepare your accounts to each year) is different to your basis period (your business year, on which HMRC assesses your tax).

The chart below can help you decide which form you need:

Find out more:self-employed income tax

5. Missing out on your tax-free allowances

Maximising your allowances will mean you can essentially earn more money before being liable to pay tax.

Firstly, there's the personal allowance, which is £11,500 for the 2017-18 tax year. Earnings below this threshold are not taxed, unless you earn more than £100,000.

In this case, your personal allowance is reduced by £1 for each £2 you earn over £100,000.

If you're married or in a civil partnership and one person earns below the personal allowance, you may be entitled to the marriage allowance - when 10% of one spouse's personal allowance is essentially transferred to the other spouse.

You may also qualify for the personal savings allowance, which can be applied to interest earned on your savings (but not Isas, as they are tax-free).

Then there's the dividend allowance, which is £5,000 for 2017-18, and can be applied to any money you make from dividends and investments.

There are also property and trading allowances - both £1,000 each - that can be applied to money you make from your property and trading activities such as selling your stuff on eBay. If you're a landlord, however, you should weigh up carefully whether to use the property allowance or to claim expenses.

Used properly, these allowances could make a big difference to your tax bill.

Find out more:tax-free income and allowances

6. Failing to claim expenses

We recently reported that people who pay tax by self-assessment end up paying £355 too much tax due to failing to claim the expenses they're entitled to.

These can differ depending on when you're employed or self-employed.

If you're employed, you may be able to claim for things like the upkeep of your work uniform, expenses you've incurred during business travel and some costs of working from home.

You can also claim for working from home, as well as costs for running business premises, paying your employees, plus travel and accommodation costs for business trips and vehicle usage.

And while you'll need to show receipts for most of these claims, others - including vehicle expenses and working from home - can be claimed at a flat rate without showing receipts for every transaction.

Find out more:self-employed tax allowable expenses

7. Forgetting about payment on account

You may be prepared to pay your 2017-18 tax bill on 31 January. But did you know you'll also pay an installment towards your 2018-19 bill - known as payment on account?

The only exceptions are if your 2017-18 bill is less than £1,000, or you've already paid more than 80% of the tax you owe through your tax code or other deductions.

The instalment is generally half of the previous year's bill, and you'll then need to pay the other half on 31 July. Anything outstanding will then be settled up when you file your next return, on 31 January 2020 - or you might be owed a refund.

If you don't think your 2018-19 tax bill will be as high as this year's, you can ask HMRC to reduce your payments.

8. Missing the deadline

If you're still chasing up paperwork and it's unlikely to arrive before tomorrow's deadline, it's better to file on time and estimate the figures rather than filing your tax return late.

This is because late tax returns are automatically fined £100, and the penalty increases over time.

If you file on time, you'll avoid a fine, and can submit an amendment as soon as you know the correct figures.

Find out more:late tax returns and penalties for mistakes

9. Starting too late

To reduce your stress levels and the chance that you won't meet the deadline, it's a good idea to submit your tax return in plenty of time - so start earlier next year!

As soon as the 2018-19 tax year ends on 5 April, you'll be able to get your relevant receipts and other documentation together.

What's more, if you're employed and submit your online tax return before 30 December and owe less than £3,000 in tax, this can be taken via PAYE throughout the following year, so there's no need to make any large payments.

Find out more:paying tax - self-employed

Use the Which? tax calculator

Whether you're new to self-assessment tax, or you're a seasoned pro, the Which? tax calculator could make the whole process much easier.

The tool can tot up your expenses, work out your bill and submit it directly to HMRC.

Find out more:Which? tax calculator - submit your tax return today