If I set up a massage therapy treatment room at home do I need a fire risk assessment? I would be the only therapist and clients would be there one at a time. 

You would need to for the part of your premises that was the workplace as oppose to the private dwelling but it would not need to be written unless a licensed activity.

You need nothing more complex than what's in this basic guide for the smallest type of business: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/14879/making-your-premises-safe-short-guide.pdf