Exclusive Which? research has found more than three in 10 people who do their own tax return don't know which expenses can be claimed - putting them at risk of paying too much or too little.
In a survey of almost 1,300 Which? members, almost a third of respondents said they didn't know what could be included as an expense claim when they were faced with a list of possible bills, items and services.
Not knowing which expenses you can apply to your return could mean you'll end up paying more tax than you need to - while claiming expenses you're not entitled to could land you with a fine from HMRC.
Here, we explain which of your expenses are eligible to offset your tax bill, and how these differ for employed and self-employed workers.
If you've had to pay for certain things that are necessary for your job, you may be able to claim tax relief on what you've spent.
This means that the tax you would usually be charged on an item is removed; it doesn't mean you'll get the item for free.
If you're a basic-rate taxpayer, you could get a 20% refund on the cost, while higher-rate taxpayers may be able to get up to 40% back.
Generally, you can only claim relief on things that are used 'wholly and exclusively' in relation to your job, but there are a few exceptions. For instance, in cases where you use your personal car for work, or use where you live to work from home.
You'll need to keep records of your expenses in case HMRC asks for proof of what you've spent. While nine in 10 of the people we surveyed said HMRC had never asked for additional information regarding their expenses claims, one person said they had once had to provide seven years' worth of all income and expenses - so be prepared to back up your claims.
The expenses you can claim are different depending on if you're employed or self-employed.
If you're self-employed and pay tax by self-assessment, your tax bill is based on your annual profit.
By claiming expenses, you can deduct the cost of certain purchases, which will then lower your profit and your tax bill.
If you use an office or other business premises, you can claim for some of the costs of keeping it running.
This includes the costs of heating, lighting, cleaning, water, rent, business rates and general maintenance.
It's a little more complicated to claim expenses for working from home, as you won't be using the space solely for business purposes.
As a result, you can only claim for a proportion of your bills, depending on how much of your home is used for work, and how much of your time you spend working.
For instance, say you exclusively carry out your work in the study - a room which takes up 10% of your home's total surface area.
You'd be able to claim for using 10% of the heating, but then reduce this further to show the proportion of time you've spent using it.
So, if you work for 35 hours a week, out of a total 168 hours, that's just under 21%. That means you'd be able to claim for 21% of the initial 10% slice of the bill.
You may be able to claim expenses for some work-related travel costs, which include:
If you own the vehicle privately, you can only claim for the proportion of what you spend when it's being used for work.
You can't claim for the cost of travelling from home to work, for buying a vehicle to do this journey, or for your meals - although you may be able to claim a 'reasonable amount' for breakfast and evening meals when on an overnight trip.
There are some employee expenses you can claim back for, including:
If you're employed, you can only make an expenses claim if your employer hasn't already reimbursed you - or if you've had to pay tax on the reimbursement.
But if you usually pay tax by self-assessment, or your expenses claim is worth more than £2,500, you'll have to submit a tax return.
If you work for an employer from your home, it's possible to claim tax relief on a proportional amount of expenses for things such as heating, lighting and telephone costs.
If your claims exceed £4 a week, or £18 a month, you must be able to provide evidence of what you've spent.
Your employer may choose to contribute to your expenses - but they don't have to. The same payment threshold applies, and you won't have to pay income tax or National Insurance on the tax relief you receive from your employer.
You can't make any claims if you're working from home voluntarily.
Any items or services you have to pay for while on a business trip could be eligible for tax relief. These include things like:
You can also claim for mileage, whether you use your private car or a company car. HMRC has an approved mileage allowance in this case:
You can't claim for your usual commute between home and work.
If your profession isn't listed, you may still be able to claim a standard annual amount of £60.
You can't claim for lifetime membership subscription, or for any fees or subscriptions your employer has already paid for.
The deadline to submit your 2018-19 self-assessment tax return is looming. You have until midnight on 31 January to send your return to HMRC - failing to send it on time could mean you'll face a late penalty.