If you use the Rent a Room Scheme you don’t have to keep a record of your expenses since you don’t claim relief for them under the scheme. You will need to keep a note of any capital improvements to mitigate any Capital Gains Tax liability on disposal of the property though.
From 6 April 2016 if the rent you receive is more than £7,500 (or £3,750 if the property is jointly owned) you can opt to pay tax on your profit in the normal way by deducting your expenses from the rent received.
For the tax year from 6 April 2015 to 5 April 2016 the threshold was £4,250, or £2,125 if the property was owned jointly.
Useful resource: //www.gov.uk/government/publications/hmrc-property-rental-toolkit