HMRC’s Business Income manual (at BIM47825, Example 4) apportions fixed costs on the basis of business hours used as a proportion of 24 hours in a day, whereas Example 6 further down does the same apportionment using business hours as a proportion of total hours used (i.e. business plus domestic), which of course gives a much higher proportion. In both cases, the business room is also used for domestic purposes as well as business. Am I missing something here? 
 
Arthur Weller replies: 
The difference between Example 4 and Example 6 is that in Example 4 the room is available for business use for only four hours a day (i.e. between 8am and 12am), so 4/24 of the fixed costs are claimed. In Example 6, the room is available for eight hours a day (i.e. between 9am and 5pm) out of total use of the room of 10 hours a day, so 8/10 of the fixed costs are claimed. You are correct that this is a contradiction because Example 4 looks at a total of 24 hours, and Example 6 looks at the number of total hours the room is used in the day (i.e. 10 hours).