As with everything in life this is not a straightforward answer and depends on whether you’re employed (including Directors of Limited companies) or self-employed and whether you have to work from home on a regular basis or choose to.
Your employer may reimburse your additional costs, if this isn’t the case then you’ll have to claim tax relief either through filing a tax return or completing a form P87.
Only the increase in costs incurred can be reimbursed, costs that would be the same whether or not you work at home cannot be included, these are costs such as mortgage interest, rent, council tax and water rates.
From 6 April 2020 your employer can pay you up to £6 a week (£26 a month) to cover your additional costs if you have to work from home. For previous tax years the rate is £4 a week (£18 a month) without you having to keep any records. If a higher level is needed these need to be agreed with HMRC and detailed records kept.
If you’ve agreed with your employer to work at home voluntarily, or you choose to work at home, you cannot claim tax relief on the bills you have to pay.
If you’re a sole trader or partnership there are two ways you can calculate business expenses for working from home. You can use simplified expenses https://www.gov.uk/simpler-income-tax-simplified-expenses/working-from-home or calculate your expenses by working out the actual costs, you’ll have to keep detailed records with the latter.