Business owners can claim the expense of using their home as an office, (as a business expense), based on the portion of rent, mortgage interest, council tax, water, fuel etc. used while working on the business at home.
There are 2 ways to calculate this:
Weekly Claim: Business owners spending small amounts of time, say, invoicing or doing minimal work relating to the business can claim £6 per week (or £26/month) without calculating the actual home expenses used.
Actual Expenses: If the owner spends, say, a few hours each day at home running the business, they can claim a proportion of actual home expenses. (Rent, rates etc. as above)
If you want to use the second method, HMRC provide examples online – but they are complex. I recommend that you ask your accountant to do the calculation.