Self Employed - how to calculate tax relief while working at home

RonSwanson

I work 45 hours per week from my home office. There are three rooms in my home and the home office is used for work 90% of the time.
HMRC says I can claim expenses on Council Tax, Mortgage Interest, Insurance, Electricity, Heating and Broadband. All of which add up to roughly £7,000.
Do I just work out 90% of 1/3 of all my bills? Or do I have to factor in time spent in the office as well? 

Jeremy535897

Are you employed or self employed?

Grumpy_chap

Self employed as per the thread title.

Jeremy535897

See https://www.gov.uk/expenses-if-youre-self-employed