Self-assessment 2017/18 tax return guide: help on how to get your form right and avoid mistakes

Updated on 16 January 2019

Here's how to get prepared for and get help filling in your 2017/18 tax return.

Tax return deadlines

If you need to complete a Self-Assessment paper tax return for the 2017/18 tax year, time is running out to file it.

The deadline for completed paper tax returns is 31 October and if you fail to do it on time you will be hit with a £100 fine.

If you're filing your self-assessment online - with the deadline of 31 January 2019 - please go to our specialised guide.

What you need to fill in your tax return

To make sure you fill in the form correctly it’s important to have all the information you need to hand.

This will include details of all income earned in the 2017/18 tax year including self-employed earnings, income from investments and savings, rental income and capital gains.

You’ll also need your P60 and P11D documents, any interest statements from your bank or building society and information on dividends from shares as well as details of your deductions including Gift Aid and pension contributions.

You should also gather receipts and other evidence for any tax relief you want to claim.

If you’re self-employed and work from home, for example, you can claim towards heating, lighting and cleaning as well as other expenses necessary to run your business.

Where to get help filling in your return

You can authorise an accountant to deal with your affairs on your behalf. By authorising an accountant, they will be able to file your tax return if they are registered as a tax agent with HMRC. If you want to use this option, you should contact an accountant sooner rather than later.

But if you’re going to have a crack yourself there is plenty of help available.

For the main tax return (SA100) you should consult the guidance notes (SA150). This document is 12 pages long but should cover most of what you need to know.

You may have to fill in extra sections known as supplementary pages on your tax return ranging from SA101: Additional Information to SA105: UK Property, but these will also come with guidance notes to help you fill them in.

HMRC also provides a range of helpsheets as well as videos and webinars online.

But if you’re stuck you can also call HMRC on 0300 200 3310. Advisers are available 8am to 8pm Monday to Friday, 8am to 4pm Saturday or between 9am and 5pm on Sundays.

Get organised if your filing online

Make sure you don’t leave it to the last minute to complete your online return, especially if you’re filing online for the first time.

The deadline for filing an online tax return is 31 January.

The HM Revenue & Customs (HMRC) online system is designed to speed things up and make it easier for people to submit their tax return, but to do so you need a user ID and an activation PIN.

You can apply for these online. To do so you’ll need your 10-digit reference number, found on any tax statement, plus your National Insurance number.

Once you’ve registered, the ID and PIN are then sent to you in the post and could take up a week to arrive – that’s why it’s important to register as soon as possible if you want to be sure of getting your tax return filed by the 31 January deadline.

When you get the code, you need to log in and activate your account. You only have 28 days to do this before the code expires and you have to apply for another one.

Even if you’ve used the online system before it’s best to get your return completed as soon as possible as HMRC’s website always gets busy close to the deadline.

Paying your tax bill

The tax you owe can either be collected through your tax code or through a system called payment on account (POA).

POAs are made in advance to cover their tax liability for next year and are payable in January and July.

They are designed to ease cashflow and each of them is typically half of the previous year’s tax liability – although they can be reduced if you are earning less than previous years.


There is a £100 fixed penalty for submitting your paper tax return later than 31 October, even if you have no additional tax to pay. 

If the return is more than three months late there is a £10 daily charge for up to 90 days – so fail to file until the end of April and you’ll be fined £1,000 (£100 original fine + £900 daily charges).

The subsequent, further fixed penalties have been replaced by a sliding scale starting at £300, depending on the tax owed and the length of delay before payment is made.

After six months, a further penalty of 5% of the tax due or £300, whichever is greater, is levied and after 12 months, another 5% or £300 charge, whichever is greater. In serious cases, the penalty after 12 months can be up to 100% of the tax due.

There are also additional interest penalties for paying tax late: 5% of the tax unpaid at 30 days, six months and 12 months.

More on tax:

Pay less tax on your earnings

How to get a tax refund

What is payment on account?

How to make sure you’re on the right tax code


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